Diatoms are the most important part of marine and freshwater plankton. The microscopic organism of diatoms is a single cell covered with a silicon shell. There are colonial forms of this group of algae, they often form a gray, green-brown or brown gel-like coating on various surfaces. Diatoms play a huge role in ecosystems, even as small as an aquarium in a room. Algae create a significant amount of biomass, which has already attracted the attention of producers of organic substances and fighters for the purity of water bodies.

Department of Diatoms

The international systematic name for the group of diatoms is Bacillariophyceae. Combine in this vast group - according to various sources - from 20,000 to 200,000 species living in salt and fresh water, even under the ice cover of the Arctic and Antarctic. Diatoms are predominantly planktonic organisms, which implies the absence of organs for active movement in the water. The smallest representatives of this group, which are able to move independently, have been studied, for this they have numerous adaptations.

The surface of solid valves of unicellular and colonial organisms is covered with a thin layer of organic compounds. There is a pattern of ribs, strokes, by which diatoms can be distinguished. The photo of one of these microscopic creatures, located below, gives an idea of ​​the shape of the cell, the structure of its surface. The color of diatoms varies from yellow to brown, which is due to the presence of a mixture of chlorophylls, xanthophylls and carotenes - there are about 9 different pigments in total.

The external structure of diatoms

One of the main distinguishing characteristics of diatoms is the hard shell of the cell, formed by silicon dioxide with impurities of compounds of aluminum, magnesium, iron, and organic substances. The cover consists of two halves of different sizes, which are closed like a casket or box. One leaf is larger than the other, both have a slightly convex or flat shape. The larger half of the shell covers the smaller half like a lid. There are additional structures - septa. They divide the cell into separate chambers.

Numerous signs of internal and external structure distinguish diatoms. A photo under a microscope allows you to carefully study the features of the cell membrane and internal contents. The outer wall is not homogeneous throughout its length, it is permeated with many holes through which the exchange of substances with the environment takes place. There are outgrowths on the silica shell that help colonial forms create associations, resist waves and currents.

The structure of a diatom cell

The cytoplasm forms a thin parietal layer, there is a kind of bridge in which one diploid nucleus with nucleoli (1-8 pieces) is located. Almost all of the internal cell space is occupied by the vacuole. Chromatophores are mainly located along the walls. These are small but numerous discs or larger platters. Heterotrophic diatoms do not contain pigments. The chromatophores of autotrophic diatoms contain colored plastids:

• a- and c-chlorophylls (green);
• β- and ε-carotenes (orange);
• xanthophylls (diadinoxanthin, diatoxanthin fucoxanthin and other yellow pigments).

Photosynthesis reactions in diatom cells produce lipids, not carbohydrates, as in most land plants. In addition to the molecules of fats necessary for life, grains of volutin and chrysolaminarin are present as inclusions and reserve substances in the cells.

colonial diatoms

The successive division of one diatom cell results in the appearance of an entire colony. Separate tiny organisms are held together thanks to a variety of outgrowths of the shell, in the form of hooks, horns, and spikes. Diatoms in the form of shapeless mucous lumps and films do not have an ordered structure. If there is no common mucus, then the cells are held together by solid valves, forming ribbons, chains, bushes, or have the shape of a star, a fan.

Reproduction of diatoms

Vegetative reproduction of diatoms is associated with mitotic division of the nucleus in the cell. There are two protoplasts, each of them eventually leaves one solid valve. The second is being completed, after which the new organisms of the colonial forms of diatoms remain connected, and the unicellular ones are separated. The result of vegetative reproduction is a decrease in cell size from generation to generation. A second one, smaller in size, is being completed to the already existing valve, therefore, such diatoms become smaller over time.

Representatives of a number of groups of diatoms undergo conjugation during sexual reproduction, like unicellular green algae. In centric diatoms, an oogamous process takes place, when some individuals produce eggs, while others produce flagellated spermatozoa. Fertilization occurs, resulting in a zygote covered with a pectin membrane, then an auxospore is formed.

The main systematic groups of diatoms

The Bacillariophyta division unites over 10,000 modern species. Representatives of biological science claim that there are actually several times more of them. The taxonomy of diatoms has undergone several changes over the past centuries. The debate is mainly around the number of classes. The most common classification:

Class Centric diatoms (Centrophyceae) - contains 5 orders. Unicellular and colonial diatoms belong to this group. Species: Cyclotella meneghiniana, Melosira arctica and others. Representatives of centric diatoms form the basis of plankton; they cannot actively move in the water column.

Class Pennate, or Cirrus diatoms (Pennatophyceae). Has 4 orders.
In this group, unicellular species that are able to actively move, and colonial forms are widely represented. Among cirrus diatoms, benthic organisms that inhabit lakes and seas predominate. They prefer the surfaces of various substrates, cover them with a mucous film or lumps. The feathery diatoms belong to such genera as Synedra, Fragilaria, Tabellaria, Diatoma, Navicula, Cymbella.

Origin of the division Bacillariophyceae

Diatoms are very different from other algae. Having studied the pigment plates, as well as the process of photosynthesis occurring in cells, scientists came to the conclusion that these organisms originated from the most ancient flagella. The proof of the theory is their ability to synthesize organic substances using pigments of different colors - green, orange and yellow. Researchers have established family ties of diatoms with brown, golden and yellow-green algae.

Solid silica shells of cells have been deposited on the bottom of the seas and oceans in the form of diatomites and other rocks for hundreds of thousands of years. The most ancient fossil diatoms have been preserved among the Cretaceous deposits of the Mesozoic (the era of middle life). Diatomite and tripoli are mined mainly in quarries and used in construction.

Ecology of diatoms

Diatoms are widely distributed in fresh and salt water bodies around the globe. These organisms live among wet rocks, in the soil, they even mastered hot springs, snow cover and ice of the polar regions. Seas and oceans are inhabited mainly by species belonging to centric diatoms.

Freshwater bodies are typical habitats for pennate diatoms. Among the representatives of this group, there are not only planktonic, but also benthic forms (bottom organisms). There is a great variety of epiphytic diatoms and those that cause "blooming" of water in lakes and reservoirs, their overgrowth.

Meaning of diatoms

In the World Ocean, diatoms make up more than 2/3 of the entire species diversity of algae. They account for 50% of the total biomass of the seas and oceans, on a planetary scale, this figure also looks impressive - 25% of the organic mass.

Diatoms are at the beginning of the food chain in aquatic ecosystems. These photosynthetic cells in plankton serve as food for invertebrates. Fish, crustaceans and other representatives of the marine fauna also feed on diatoms or eat zooplankton. Economic and scientific importance of diatoms:

• are used as indicators or indicators of the state of surface waters;
• form the basis for the formation of sapropel (diatomaceous silt) at the bottom of reservoirs;
• are part of mineral deposits (diatomite and tripoli);
• contribute to the self-purification of water bodies from a number of inorganic contaminants;
• help determine the origin and age of rocks from the shells of fossil forms.

Diatoms in the aquarium

They inhabit glass walls, inhabit the surface of stones and appliances, cover the leaves of underwater multicellular plants. They do not like bright light, so they prefer dark areas of the aquarium. Diatoms must be fought so that the entire artificial biocenosis behind the glass walls is not covered with brown mucus. It is necessary to clean off the film and brown lumps as soon as they begin to appear on stones and appliances. The most difficult thing is to remove plaque from diatoms on the leaves of underwater plants.

Measures taken should be aimed at regulating lighting and water composition. One of the reasons for the "bloom" of the liquid in the aquarium and natural reservoirs is the excess content of silicates. If there are few minerals, then the development of diatoms is slow. The same effect is caused by good aquarium lighting, constant cleaning of glasses and stones from colonies of diatoms.

The Department diatoms ( Bacillariophyta) has more than 20 thousand species. These are photoautotrophic tubulocrysts of microscopic size, with an exclusively cocoid thallus, having covers in the form of a silica shell. shell closely adjacent to plasma membrane and consists of two parts: epithecus and hypotheses. The larger part (epithec) moves with its edges onto the hypotheca like a lid on a box (Figure 3.1). Epithecus consists of a flat or convex sash ( epivalva) with curved edges and a girdle rim ( epicingulum). Hypothesis has similar parts: sash ( hypovalva) with curved edges and a girdle rim ( hypocingulum). The waist bands fit snugly together, making up belt shell. In most diatoms, one or more insert rims, which increase the volume of the cell and promote its growth. The shape of the shell is varied and is characterized, first of all, by the type of valve symmetry. Valves with many axes of symmetry are called radially symmetrical or actinomorphic. Otherwise, it is called zygomorphic. Zygomorphic valves are symmetrical in the longitudinal and transverse directions ( bisymmetric), symmetrical only along one axis ( monosymmetrical), mirror symmetrical or asymmetrical.

The outer and inner patterns of the shell, observed in a light and electron microscope, are called shell structure. It is specific to different taxa and is formed by various structural elements, of which the most important are perforations - a system of holes of various structures located on the valves, through which the protoplast communicates with the external environment. There are small pores ( areola) and large elongated chambers covered with perforated film ( alveoli). In the wings of the shell there may be one or two mucus pores through which mucus is secreted, which serves to attach algae to the substrate and form colonies. Thickenings protruding above the outer or inner surface of the valve are called ribs they provide strength to the shell. In many diatoms, protrusions, bristles, spikes, spines are formed on the outer surface of the shell, which increase its surface and serve to connect cells into a colony. In mobile diatoms, on the valve side of the shell there is the seam in the form of a pair of through slots, as well as nodules- two polar and one central (they are thickenings of the valve walls). Such a structure of the shell, along with a small volume of the protoplast and numerous oil droplets, ensures the soaring of diatoms in the water column. Through the seam, the release and circulation of the cytoplasm occurs, which ensures the reactive movement of the algae.

Cells of representatives of the department have a typical eukaryotic structure. Cytoplasm in them it forms a parietal layer or accumulates at the poles or in the center of the cell, connecting with cytoplasmic bridges. Core lies in the central mass of the cytoplasm or in the parietal layer, closer to the hypotheca (in centric diatoms,), or - in the cytoplasmic bridge in direct contact with the chloroplast, closer to the epithecus (in pennate) (For an explanation of the terms centric and pennate, see later in the text). Mitosis open, but instead of centrioles, the function of organizing the division spindle is performed polar discs. Located at the core golgi complex.

plastids(chloroplasts) secondarily symbiotic, rhodophyte type, four-membrane (two outer membranes are organized in chloroplast endoplasmic reticulum, which goes directly to the kernel shell). Thylakoids, collected in threes and sometimes pierced pyrenoid, available girdle lamella. Set of photosynthetic pigments: chlorophylls a and with, β - and ε -carotenes, xanthophylls (fucoxanthin, diatoxanthin, neoxanthins and diadinoxanthin), which determines the color of the thallus from light yellow, golden to greenish-brown. A cell may have several mitochondria with tubular cristae. Vacuoles - four types: with cell sap, with volutin, with chrysolaminarin and with oils(the last three components are assimilation products diatoms).

vegetative reproduction, most characteristic of diatoms, is carried out cell division in two. Before dividing, oil drops accumulate in the protoplast, it increases in size, pushes the valves apart so that they touch only the edges of the girdle rims. The nucleus divides mitotically, and then the entire protoplast. Each new cell receives one shell flap, which is or becomes an epithecus, and the hypotheca is completed. Multiple vegetative divisions lead to a progressive decrease in the size of cells in the population, since cells that receive the hypotheca of the mother cell (it becomes an epithecus) constantly complete the construction of an even smaller valve (their own hypotheca). Restoration of the original cell sizes characteristic of a given species occurs during the germination of dormant cells, as well as as a result of sexual process, accompanied by education auxospore(growing spores) . The auxospore grows to the maximum possible size for the species, then takes on a typical shape and forms a shell. In a number of species of diatoms, auxospore formation occurs due to autogamy: After meiosis, two nuclei remain viable, which merge inside their cell. Actually asexual reproduction for diatoms is not typical, however, some species are able to form microspores, the nature and ways of formation of which have not yet been studied.

sexual process- iso-, hetero- (aniso-) or oogamy. In the case of oogamy, the only flagellar stage for diatoms is formed - the spermatozoon. It has one flagellum, covered with retronemes, its axoneme lacks central microtubules (formula 9+0, instead of 9+2), the radicular system is reduced, where instead of triplets there are only doublets of microtubules, the basal body is pressed against the nucleus. In some diatoms, the spermatozoon does not have a flagellum and moves with the help of pseudopodia. In the case of iso- and heterogamy, the gametes are gluteless and flow from the shell of one mother cell to another.

The life cycle of all diatoms is diplophase with gametic reduction without generational change.

Under adverse conditions, diatoms go into a dormant state. In this case, the protoplast moves to one of the ends of the cell, loses cell sap and shrinks strongly. The vital activity of these cells resumes when favorable conditions occur. Some planktonic lake species are able to wait out the winter period at the bottom of water bodies in this state. In a number of species, the formation of silica cysts is observed.

Diatoms have a wide distribution and different biotopes: fresh and salty, stagnant and flowing water bodies, wet rocks, soil and even arable land; able to live on snow and ice. The role in nature and practical importance of diatoms is very great: they participate in the creation of organic matter and the absorption of carbon from the World Ocean, are part of the trophic chains of aquatic ecosystems, participate in the silicon cycle and sedimentation, are used in environmental monitoring and archaeological dating of sedimentary rocks.

The systematics of diatoms is based on the structure of the shell, primarily the symmetry of the valves, the presence and structure of the suture. The department is divided into three classes: coscinodiskophic (centric) - Coscinodiscophyceae (Centrophyceae), fragilarioficial (seamless) - Fragilariophyceae, bacilliariophycium (suture) - Bacillariophyceae. The last two classes are traditionally called pennate diatoms.

Class coscinodyscophytic (Coscinodiscophyceae) combines algae with radially symmetrical (actinomorphic) valves, devoid of a seam (Figure 3.2). In most cases, the valves are rounded, so they are often called centric diatoms. The sexual process is oogamy. The class includes 22 orders.

Most widely distributed order Melosirales, whose typical representative is Melosira(Figure 3.3). Melosyra cells are cylindrical, connected in colonies with the help of mucous rollers, spikes or teeth. The shell has high bends of the valves and a complex girdle rim; valves are round, with small areoles.

AT life cycle Melosyra observed oogamous sexual process. Sexual structures differentiate from vegetative cells. the female reproductive cell (corresponding to oogony) by meiosis, followed by degeneration of three nuclei, produces one egg. In the male (corresponding to spermatogonia or antheridium), it first forms a four-flagellated spermatogenic cell, which, after meiosis, buds four single-flagellated spermatozoa. After fertilization, an auxospore is formed from the zygote.

Representatives class fragilariaceae, or seamless, ( Fragillariphyceae) are characterized by zygomorphic shell valves lacking a suture and the absence of flagellar stages in the life cycle. Sexual process iso-, heterogamy, only for species of the genus Rhabdonema an oogamous sexual process is known, but the spermatozoon is flagellaless, moving with the help of pseudopodia.

There are 12 known orders of fragilariefic algae. Typical order – fragilarial ( Fragilariales), with the most common representatives of the genera Fragilaria, Tabellaria,Asterionella(Figure 3.4) .

At kind fragilaria ( Fragilaria) cells often form very long ribbon-like colonies. Shell valves are linear, with transverse strokes. Cells kind asterionella ( Asterionella) are rod-shaped, at one end the valves are connected into star-shaped or spirally curled comb-shaped ribbons. Valves are linear, often with capitate widened ends. Representatives kind tabellaria ( Tabellaria) valves from the side of the girdle have the form of plates connected in zigzag colonies. The valves are linear, widened at the ends and in the middle. In the case of heteropole shells (cell poles of different shapes), as in kind meridion ( Meridion), colonies may take a fan-shaped form.

Similar information.

KINGDOM Heterokontobiontes (Chromista) – Heterokontes

The department is called Diatoms (from the Greek. di- two, volume- incision, dissection), or Bacillaria ( bacillum- wand). Includes unicellular solitary or colonial organisms, almost always microscopic in size; forms visible to the naked eye and reaching 2–3 mm. The presence of a bivalve silica shell is characteristic. About 6-10 thousand species are known.

Cell structure.

Chloroplasts of diatoms of various shapes, usually wall, contain pigments - chlorophylls a and c, carotenes, fucoxanthins.

In centric diatoms, chloroplasts are numerous, small; in pennate diatoms, they are large, often lobed. The color of chloroplasts is brown, yellowish or golden. It is due to the fact that green pigments - chlorophylls - are masked by additional yellow-brown xanthophylls, of which fucoxanthin predominates. There may be one or more pyrenoids, they protrude beyond the chloroplast and are sometimes penetrated by thylakoids.

There are many drops of oil in the cytoplasm. Volutin occurs in the form of large droplets with a characteristic blue sheen.

Mitochondria of diatoms of various shapes (spherical, oval, rod-shaped, filamentous). The Golgi apparatus is located next to the nucleus, it consists of several dictyosomes (up to 20), which contain from 4 to 12 cisterns.

On top of the plasmalemma, diatoms form a special cell cover - a shell. It consists of amorphous silica, similar in composition to opal, therefore diatoms are often called the "precious stones" of the seas ("opal" in Sanskrit means "precious stone"). In addition to silica, the composition of the shell includes an admixture of organic compounds and some metals. Inside and outside it is covered with a thin organic layer consisting of pectin substances. After the death of the alga, the contents of the cell are destroyed and disappear, while the silica backbone of the shell remains unchanged. It doesn't rot.

The shell consists of two halves: the upper larger - epithecus and lower lesser - hypotheses. The epithecus is put on the hypotheca like a lid on a box (Fig. 20). In turn, the epithecus consists of epivalva(upper sash) and epicingulum(girdle rim of the epitheca); the hypothesis is hypovalva(bottom sash) and hypocingulum(girdle rim of the hypotheca). Two search rims, overlapping each other, form a girdle. In diatoms, two projections of the shell are distinguished and used for identification: a view from the valve and from the girdle. The leaf is usually flat. Its curved edge is called the fold of the sash; it can be low or high. In some genera Melozira, Hyalodiscus) shell flaps are closed directly by the edges of the bends of the flaps, and the girdle is formed during cell division.

An inset rim appears between the edges of the fold of the sash and the girdle rim (Fig. 23). There can be many intercalated rims, but the youngest will always be near the bend of the sash, and the oldest near the girdle. The significance of these rims is to ensure the growth of the shell and increase the volume of the cell. In a number of species, thin silicon incomplete partitions grow into the cell cavity from the inner walls of the intercalated rims - septa. They always have one or more holes and partition the cage into semi-isolated chambers, which increases the surface of the cages.

There are two main types of sashes: actinomorphic, through which three or more axes of symmetry can be drawn (such valves are typical for centric diatoms), as well as zygomorphic, through which no more than two axes of symmetry can be drawn (such valves are characteristic of pennate diatoms).

Rice. 23. Diatom Pinnularia(on:): BUT- view from the side of the belt; B– view from the side of the seam; AT- lengthwise cut; G- cross section; D- vegetative reproduction; 1 - epithecus, 2 - hypothecus, 3 - suture, 4 - nodule, 5 - chromatophore, 6 - pyrenoids, 7 - cytoplasm, 8 - nucleus, 9 - vacuole, 10 - valve, 11 - girdle

The shell of diatoms is pierced with perforations, which serve to communicate the protoplast of the cell with the external environment. Perforations on the sash occupy 10–75% of its area. They can fold into rows that are visible as strokes. Strokes can be radial, parallel, convergent. The strength of the shell is given by thickenings protruding above the outer or inner surface of the valve, called ribs. On the surface of the valve, spikes, bristles, protrusions, and spines are often formed, which participate in the formation of colonies.

Some pennate diatoms have a suture system. A slit-like suture consists of a pair of longitudinal slits (seam branches), which are located on the sash. Canal-shaped shovim has the form of a tube located in the thickness of the valve, its comb-shaped thickening - keel , or pterygoid outgrowth surrounding the valve along the edge - wing . The canal-like suture communicates with the external environment with the help of a thin slit, with the internal cavity of the cell - with the help of holes. Diatom sutures provide communication between the protoplast and the external environment, and also take part in movement.

The nucleus is large, contains 1–8 nucleoli; they disappear during mitosis. Centrioles are absent. The centers of organization of microtubules are plates (Fig. 24). They are located at the poles of the spindle. Spindle microtubules are formed outside the nucleus, then pass into the nucleus through the destroyed areas of its shell; the nuclear envelope gradually disappears. Thus, in diatoms, mitosis is open. In the early stages, microtubules travel from pole to pole. Chromosome kinetochores appear to attach to pole microtubules. In anaphase, the chromosomes move towards the poles; in late anaphase, the spindle lengthens. Cytokinesis is carried out due to the formation of a fission furrow by invagination of the cell membrane from the periphery to the center. Cytokinesis is completed with the formation of the membranes of the daughter nuclei. The plane of cell division in diatoms always runs in a plane parallel to the valve.

Rice. Fig. 24. Mitosis in pennate diatoms (according to: C. Hoek van den et al., 1995): A – interphase, almost before prophase; B - prophase; B - metaphase spindle of division; G - anaphase spindle of division; 1 - microtubule center; 2 - core shell; 3 - prophase spindle; 4 - polar plate; 5 - Golgi apparatus; 6 - kinetochore; 7 - chromatid; 8 - chromosomal microtubule; 9 - interpolar microtubule

Motion.

Free-living bacilli move along the substrate, as if crawling on the side of the valve. The movement occurs evenly or in jerks in the direction of the longitudinal axis of the cell, moreover, alternately, now in one direction, then in the other, directly opposite to it. Diatoms with a seam are capable of active sliding movement. This movement is carried out at a speed of 0.2–25 µm/s. A number of hypotheses have been put forward regarding the mechanism of their movement. One of the hypotheses connects the movement of diatoms with the release of mucus through the seam, which includes fibrillar polysaccharides. Turning into strands that are thrown forward along the substrate, they provide the movement of diatoms. It is believed that the proteins kinesin and/or dynein are the driving force that leads to the ejection of these strands. Another hypothesis relates the movement of diatoms to the friction of the cytoplasm circulating in the suture. The friction of the flowing cytoplasm against the substrate develops a motor force that moves the cell in the opposite direction to its flow. The third hypothesis explains the movement by alternately taking in water and throwing it out from the opposite end of the cell. The exit of the water flow changes the distribution of hydrostatic pressure in the body of the alga, due to which the latter moves in the direction opposite to the flow.

Reproduction. In diatoms, vegetative and sexual reproduction occurs (isogamy, heterogamy and oogamy).

sexual process in centric and some pennate diatoms, ogamous .

One or two eggs; they are fertilized inside the oogonium or, more rarely, after they enter the water. After fertilization, a diploid zygote is formed, which develops into a growing auxospore. It is dressed in a shell that gradually acquires a structure characteristic of this species, turning into a vegetative cell. In most pennate diatoms, the sexual process is isogamous, but flagella are absent in gametes (Fig. 25, BUT). Before isogamy, meiosis occurs, as a result of which 1–2 haploid gametes are formed. The fusion occurs in such a way that the gamete from one cell crawls into another. Mobile gametes can be considered as male, and remaining in place as female.

vegetative reproduction. More often, the reproduction of diatoms occurs vegetatively, by dividing the cell in two. During division, both valves move apart until the edges touch each other, then the cellular contents are divided into two symmetrical halves in the plane of the girdle. Each of the two newly emerged cells develops the half of the shell (sash) that it lacks, pushing it into the old half that it inherited. In free-living diatoms, young cells quickly separate and disperse, while in colonial ones they remain close to each other. Since, with further repetitions of the division process, new valves are always inserted into the old ones, and therefore there are fewer of them, then after a number of divisions, the generation of diatoms should be significantly reduced. Cells, having reached a minimum of growth, return to their original size in various ways. First, the smaller of the resulting cells may no longer divide. Secondly, in some diatoms, the bands of the shell are more elastic. Thirdly, it is possible to push apart parts of the carapace and equalize the difference in size between the epithecus and the hypotheca. Fourthly, a decrease in the size of cells in diatoms is opposed to an increase in them as a result of the sexual process, through the formation of auxospores.

When the auxospore is formed, the process of copulation occurs, leading to the formation of a zygospore. Two cells approach each other, shed their valves, their protoplasts merge together, are surrounded by a dense cellulose membrane, then increase in volume and turn into an auxospore, giving rise to a new individual, noticeably larger than its parents.

Reduction division in diatoms is observed before the formation of gametes, therefore vegetative individuals are diploid organisms, and the life cycle of diatoms is diplobiont with gametic reduction.

resting stages. Under unfavorable conditions, diatoms can form spores and dormant cells. These structures are rich in reserve substances that will be required during germination. The resting cells are morphologically very close to vegetative cells, while the spore shell becomes thicker, rounded, and its ornamentation changes. Resting cells usually develop in an environment with a low content of dissolved silicon, while spores, on the contrary, require the presence of a sufficient amount of silicon to build their own thick shell. Resting cells are formed more often by freshwater centric and pennate diatoms, while spores are formed by centric marine diatoms. Both resting cells and spores can survive for decades.

Rice. 25. Scheme of the sexual process and the formation of auxospores of pennate diatoms (A) - for example gomphonemes and centric diatoms (B) as an example Melozirs(according to: L. L. Velikanov et al., 1981): 1 - development of spermatozoa; 2 - development of the egg; 3 - fertilization; 4 - formation of auxospores

Systematics

A peculiar movement and a silica shell gave rise to more than once to classify bacilli as animals. It is now generally accepted that diatoms are algae. Most researchers allocate them to a special department. It is believed that about 6–12 thousand species belong to the department Diatoms, but some authors are convinced that the true number of diatom species can reach 1 million. In most systems, diatoms are considered in the rank of the Bacillariophyta department with two classes: Centric - Centrophyceae and Pennate - Pennatophyceae.

Class Pennate diatoms - Pennatophyceae

Pennate diatoms usually include mobile unicellular and colonial representatives, through the valve of which one or two axes of symmetry can be drawn, the valves have a seam. The sexual process is isogamous.

Genus Navicula has single cells, usually mobile, rarely enclosed in gelatinous sheaths, simple or branched, inside which the cells remain mobile (Fig. 26, AT). The valves are longitudinally symmetrical, linear to elliptical in shape. The valve structure consists of transverse striae arranged parallel and radially. Two large lamellar chromatophores are located along the cell and adjacent to its girdle. The species are widespread in fresh and brackish water bodies, less common in the seas. Mostly benthic and rarely planktonic.

Rice. 26. Diatoms (by:): BUT – Pleurosigma; B – Cymbella; AT – Navicula; G – Synedra; D – tabellaria; E – diatom; F – Meridion; W – Cyclotella: 1 - seam, 2 - knot

Genus diatom- cells are connected into ribbon-like and zigzag colonies, sometimes into stellate ones (Fig. 26, E). The carapace from the girdle is linear, with right angles, intercalated rims are sometimes present, septa are absent. The valves are elliptical to linear, the structure of the valves is made of coarse transverse ribs and delicate transverse strokes. Axial field filiform, barely visible. The rhymoportulae are located at the ends of the valve. Chromatophores small, granular, numerous. Freshwater species, predominantly benthic.

Genus tabellaria- cells are rectangular from the girdle, elliptical to elongated linear (Fig. 26, D, rice. 27, G). There are intercalated rims with septa. The seam is missing. Freshwater epiphytes.

Genus Nietzschia- cells from rod-shaped to elliptical, straight, rarely curved, solitary, very rarely connected into filamentous or branched colonies. Valves linear, rarely lanceolate and elliptical, with carinae and transverse streaks. There is a canal-like seam in the keel. One lamellar chromatophore is located along the cell along its diagonal or adjacent to one of the girdle sides. Marine, brackish and freshwater, more often benthic species (Appendix 3B).

Rice. 27. Seamless Diatoms: BUT – Synedra; B – Fragilaria; AT – Asterionella; G – tabellaria: 1 - shell from the sash, 2 - shell from the girdle

Class Centric diatoms - Centrophyceae

Centric diatoms are unicellular and colonial forms, through the valve of which three or more axes of symmetry can be drawn, in which there is no active mobility, there is no seam on the shell, and an oogamous sexual process is observed. Species of centric diatoms are very widely represented in the plankton of the seas and oceans as one of the main producers of organic matter.

Genus coscinodiscus-cells discoid, rarely lenticular, always solitary. The shell is usually rough, often with intercalated rims (Fig. 28, B). Valves are round, flat, convex, sometimes wavy. By

Rice. 28. Centric Diatoms: BUT – Cyclotella; B – coscinodiscus; AT – Melozira; G – Aulacozira; D – Hetoceros from the girdle, part of the chain: 1 – view from the sash, 2 – view from the girdle

the edge of the valve often has a ring of small spines located at a certain angle. Chromatophores are numerous plates located over the entire surface of the cell. The nucleus is central, adjacent to one of the valves, rarely suspended on cytoplasmic strands in the center of the cell. Marine species, predominantly planktonic. Often found in large numbers in desalinated marine areas and in inland brackish seas.

Genus Rhizosoleniya has cylindrical cells. Cells are often strongly elongated in height, in the form of a long stick, straight, rarely slightly curved, solitary, rarely connected into filamentous colonies. The carapace is round to elliptical in cross section, very thin. There are numerous intercalated rims, ring-shaped, trapezoidal, rhombic and scaly. Valves in the form of a cap with an elongated top, the end of which continues in the form of a spike, bristle or outgrowth. Chromatophores numerous, granular or discoid, often accumulate in the center of the cell around the nucleus. Marine, planktonic species (Appendix, 3B).

Genus Hetoceros- cells are low-cylindrical, connected in chains, less often single (Fig. 28, D). The valves are elliptical, flat, convex or concave. One thin bristle extends from the poles of the valve, with the help of which the cells are connected in colonies. In this case, the setae of the valves of adjacent cells in the chain come into contact with each other, bend and cross, quite often fusing at the point of contact. The terminal bristles of the chain usually differ from others in length, thickness, and direction. The openings between adjacent sashes in a chain are called windows, the outlines of which can be very different. The genus includes a large number of exclusively planktonic species that make up the bulk of the coastal phytoplankton of the seas and oceans.

Ecology and significance

Types of diatoms are widely distributed in nature, they are found in all kinds of biotopes. Diatoms live in marine, brackish and various fresh water bodies: both in stagnant (lakes, ponds, swamps) and in flowing (rivers, streams, irrigation canals). They are common in soil, isolated from air samples, and form rich communities in Arctic and Antarctic ice. Diatoms in aquatic ecosystems dominate other microscopic algae throughout the year. They are abundant both in plankton and in periphyton and benthos. The plankton of the seas and oceans is dominated by centric forms of diatoms, although some pennate forms are also mixed with them. In the plankton of fresh water bodies, on the contrary, pennate diatoms predominate. Benthic (bottom) communities are also distinguished by a large variety and number of diatoms, which usually live at a depth of no more than 50 m. Benthic bacilli crawl along the substrate or attach themselves with the help of mucous legs, tubes, and pads.

Diatoms appear in abundance in stagnant waters (ponds, puddles, ditches), forming yellow-brown mucous membranes at the bottom and near the shore. They are found in mineral springs, in water pipes, even in carafes of water, on the surface of wet rocks, stones, wood, earth (for example, in pots with plants), etc. Some types of epiphytic diatoms attach themselves to sea and freshwater algae, sometimes in such abundance that the algae become simply unrecognizable. After the water bodies dry up, diatoms can be lifted along with the dust by the wind and transferred to large areas. Many species are truly cosmopolitan and are found everywhere. The species composition of bacilli in water bodies is determined by a complex of abiotic factors, of which water salinity is of great importance in the first place. Types of childbirth Rhizosoleniya, Skeletonema, Hetotseros, Biddulphia, Schizonema, Striatella are typical marine. Marine forms are usually larger and have stronger shells. Most members of the genera live in fresh waters. Cymbella, Fragilaria, Navicula, Gomphonema. An equally important factor for the development of diatoms is temperature, the degree of illumination and the quality of light. Diatoms vegetate in the range of 0–70 0 С, but at rest they are able to tolerate lower or higher temperatures.

The communities of fouling and epibionts. Diatoms occupy a dominant position among the epiphytes of higher plants and macroscopic algae in fresh water bodies and seas. Many marine animals can be exposed to diatom fouling, from crustaceans to whales. Such algae are called epizoites. Among the diatoms there are endosymbionts that live in brown algae, foraminifera, etc.

Bacilli are of great importance in nature. Making up a significant mass of phytoplankton, diatoms are start of the food chain. They are eaten by both zooplankton organisms and juveniles and adult fish. Providing about a quarter of the organic matter of the entire planet, they are the most important producers of organic matter in the oceans. Bacilli play a major role in silicon cycling, annually absorbing about 3 billion tons from the World Ocean.

Bilinsky (Bilin in Bohemia) polishing slate, whose deposits, 0.6–5 meters thick, became known earlier than others, all consists of diatom shells, and the main mass is composed of only one species, still living now - Melosira distans. A large deposit of Lüneburg heaths reaches up to 13 m in thickness; it consists mainly of Synedra ulna. The deposit near Berlin reaches 30 m; part of the city itself stands on it. A similar deposit is located near Koenigsberg. All these deposits are of recent origin. But there are also much more ancient, related to the Tertiary era. For example, the city of Richmond in the United States of America is located on such a deposit. Diatoms are also found in amber (tertiary formation). In Russia, shale deposits have been found in the Ulyanovsk and Penza regions and in Siberia.

Diatoms play an important role in sedimentation at the bottom of the ocean. The shells of planktonic diatoms, deposited after the death of algae at the bottom of marine and freshwater basins, formed thick deposits there. diatomite(mountain flour) - a mass of white or light gray, light, porous and hard. Diatomite is 50–80% diatom shells.

The thickness of the diatomite layer in some places reaches several hundred meters. Large deposits of diatomite in Russia are found in Tyumen, the Volga region, the Primorsky Territory and in a number of other places. Only on the territory of the Tyumen region, deposits of diatomites discovered in the last century amount to up to 500 × 10 12 m 3 . It is believed that this discovery can be put on a par with the discovery of oil and gas reserves beyond the Urals, since diatomite is a multi-purpose raw material. It serves as a source of about 100 different products, being used as a material for the production of optical glass fiber, liquid glass, as a filtering agent in various industries, as a polishing and grinding material. It is used and as a building material, as well as for making dynamite.

Mountain flour serves as a good polishing agent (tripoli), it is used to prepare light bricks(Fabroni bricks), and sometimes glass; it goes further to make dynamite, for which it is mixed with nitroglycerin. Mountain flour in need was used for food among the Laplanders, Tunguses and other semi-wild tribes (edible land). Mountain flour was either eaten directly or mixed with ordinary flour and baked bread. The nutritional value of mountain flour, in all likelihood, is due to those residues of organic substances that are still preserved in dead algae.

The shells of diatoms are preserved in the fossil state for a long time, so their used to determine the origin and age of various sedimentary rocks. Diatoms are of particular importance in environmental monitoring, as they serve as good indicators of organic pollution of the aquatic environment.

However, the mass development of some types of diatoms may have negative meaning. A number of diatoms have been found to have toxins. So, representatives of the genera Pseudonitshiya and Nitshia form domoic acid, which causes amnesia poisoning in humans and animals. This acid is soluble in water and insoluble in organic solvents. Domoic acid was first isolated from the macroscopic red alga Chondria (Japanese for "home" - hence the name of the acid). In 1987, more than 100 cases of poisoning people with this toxin after eating mussels were recorded in Canada, four of the victims died. Symptoms of poisoning are, in a mild case, nausea, vomiting, diarrhea; in a severe case, pain sensitivity disappears, hallucinations appear, and short-term memory is lost. Diatoms can clog river mouths and harbors, often cause blooming water and are the cause of the appearance in it unpleasant odors. Clogging the gills of animals, diatoms cause their death.

test questions

1. What are the general characteristics of diatoms

2. The structure of the shell of diatoms.

3. What pigments and types of nutrition are known in bacilli?

4. How do diatoms reproduce? Life cycle of diatoms.

5. Give a description of diatoms of the pennate class.

6. Give a description of diatoms of the centric class.

7. Name typical representatives of pennate and centric diatoms.

8. In what habitats are diatoms found?

9. Importance of diatoms for natural ecosystems.

10. Economic importance of diatoms.

This division includes unicellular and colonial microscopic algae from light yellow to dark brown in color, with an exclusively coccoid body structure. Diatoms are an extensive group of organisms, numbering, according to various authors, from 12 to 25 thousand species.

Diatoms are characterized by a special cell cover - a shell of amorphous silica, resembling opal in composition, with an admixture of metals (aluminum, iron, magnesium) and an organic component.

The shell consists of two halves, which are put on top of each other, like a lid on a box. The larger half is the epithecus, the smaller half is the hypotheca. Each half is formed by a flat or more or less convex sash, which, by its bend, is connected to the girdle ring, or rim. The epithecus covers with its girdle ring the girdle ring of the hypotheca. In many diatoms, intercalated rims of various shapes and in different numbers are formed between the bend of the valve and the girdle ring. Septa develop on the intercalated rims, dividing the cell into chambers.

On the surface of the shell and on the inside there is a thin layer of organic matter. On the shell flap there are dots, strokes, cells, ribs, etc. These are either pores, or chambers that open inward or outward, or alternating thin and thick sections of the shell. Their bottom is usually riddled with numerous holes. The total area of ​​holes in the shell is 75% of the valve area. A number of forms contain various outgrowths of the shell, with the help of which cells are connected in colonies.

In the shell flap of many diatoms there is a suture in the form of two (rarely one) longitudinal, somewhat curved slits (slit-like suture) or in the form of a tubular structure communicating with the external environment with a slit, and with the cell cavity a number of holes separated by silicified partitions (channel-like suture ). Algae with a seam can actively move.

There are a number of hypotheses regarding the mechanism of movement of diatoms. It is assumed that the movement of the cell is due to the movement of the cytoplasm in the gap or channel of the suture or the flow of water in the cavity of the suture. There is evidence that during contraction of fibrils located near the suture, a locomotor substance is released outward, leaving mucus on the substrate.

Cells are mononuclear. The diploid nucleus contains 1–8 nucleoli. The cytoplasm is located in the cell with a thin wall layer, and in some cases it is in the middle of the cell in the form of a bridge in which the nucleus is localized. The rest, most of the cell is filled with vacuole. Chromatophores vary in shape, size, and position. They are usually parietal, in the form of one or two large plates or numerous discs.

Chromatophores are surrounded by a system of membranes. The chromatophore matrix is ​​crossed by three-thylakoid lamellae. There is a girdle lamella. The pyrenoid is central. The chromatophores of diatoms contain pigments: chlorophyll a and with, β- and ε-carotenes and xanthophylls (fucoxanthin, diadinoxanthin, diatoxanthin, etc.). Heterotrophic colorless forms of pigments do not have. Assimilation products are lipids, volutin, chrysolaminarin.

Colonies of diatoms are formed from a single cell as a result of successive divisions. Cells are connected in a colony with the help of mucus, spines, horns and other outgrowths of the shell. Mucous colonies are usually shapeless lumps or films of mucus in which the cells are arranged randomly. The shape of colonies that do not have a common mucus is determined by the way the cells are connected and the outlines of the valves. Often there are colonies in the form of filaments, ribbon-like, fan-shaped, chain-shaped, stellate, bushy.

Diatoms reproduce vegetatively and sexually.

During vegetative division, the contents of the cell increase in size, pushing the epitheca and hypotheca apart in such a way that they touch only the edges of the girdle rims. The nucleus divides mitotically. The protoplast of the cell splits in half parallel to the valves. Each of the formed protoplasts has half of the shell of the old cell, and completes the second, smaller one (hypothesis). After that, the newly formed cells in unicellular forms are separated, but not in colonial ones. Such multiple divisions lead to a gradual decrease in the size of cells in the population.

Restoration of the original size of cells occurs as a result of the sexual process.

In some diatoms (pennates), the sexual process resembles the conjugation of unicellular green algae. Two cells approach each other, covered with mucus. In each cell, the nucleus is reduced by reduction into four, of which three in some species, and two degenerate in others. In the first case, the protoplast of the entire cell becomes a gamete; in the second case, two gametes are formed from it. Thus, two procopulated cells produce one or two zygotes.

In centric diatoms, the sexual process is oogamous. In some cells, after meiotic division of the nucleus, one, less often two, eggs are formed, in others, usually 4 one- or two-flagellated spermatozoa. After the egg is fertilized by the sperm, a zygote is formed, which produces a pectin membrane and turns into an auxospore.

The life cycle of diatoms takes place in the diploid phase with gametic reduction.

Diatoms are not directly related to other divisions of algae. However, the composition of pigments, assimilation products, silicon content in cell membranes and resting spores indicate their distant relationship with golden, yellow-green and brown algae. All of these divisions probably originate from photosynthetic flagellates with a predominance of brown pigments.

Silica shells of diatoms are well preserved as fossils. The most ancient fossil diatoms are known from the Cretaceous deposits of the Mesozoic era. The first marine centric forms were found in the sediments, with a primitive shell, without a girdle. In the Cretaceous layers, pennate layers were also found, initially seamless. At the end of the Paleogene, the first diatoms with a slit-like suture appear on one valve, then on both valves. Later, species with more advanced types of seam appear, including channel forms.

Diatoms are widespread organisms. They are found in reservoirs and streams, in fresh and salt water, on wet rocks, in soil, in hot springs, on snow and ice. In the plankton of the seas and oceans, centric diatoms predominate; in the plankton of fresh water bodies, pennate diatoms predominate. Among the benthic forms, the pennate ones are the most diverse. Epiphytic diatoms, as well as diatoms that develop in eutrophic lakes and reservoirs, are richer in species and quantitative composition.

In the seas and oceans, diatoms account for up to 80% or more of the total species composition of algae, creating half of the entire organic mass of the ocean and almost a quarter of the production of the living matter of the planet.

Diatoms are the main link in the general trophic chain of aquatic biocenoses. Planktonic diatoms serve as food for invertebrates, the latter for fish and other aquatic animals. Many fish feed directly on diatoms. The role of diatoms in the self-purification of water bodies from pollution is great. Many diatoms are used as indicators of polluted water bodies. Dead diatoms form diatomaceous silts and sapropels, as well as rocks - diatomite and tripoli, which are used in various industries and in construction. The shells of fossil diatoms make it possible to determine the origin and age of sedimentary rocks.

The division of diatoms into classes is based on the symmetry of the shell from the side of the valve.

Class Centric diatoms ( Centrophyceae)

Diatoms of this class are represented by unicellular and colonial forms. The carapace valves are usually rounded in outline and radial in structure. Chromatophores are more often in the form of small grains or plates, less often in the form of one or several large plates. Algae do not have active mobility, since there is no seam. The sexual process is oogamous. These are predominantly marine planktonic forms. A very ancient group, whose representatives are known from the early Cretaceous.

Costinodiscal order ( Coscinodiscales). Cells are solitary or connected in filamentous or chain-like colonies. The shape of the shell is lenticular, ellipsoid, spherical, cylindrical. The sashes are round in shape. There are areoles, ribs, tubular outgrowths on the valve.

The genus Cyclotella ( Cyclotella). Cells in the form of a low round box. The marginal zone of the valve has radial streaks or ribs, the middle zone is more or less convex and usually structureless. Chromatophores in the form of small plates are localized in the parietal cytoplasm. Cyclotella cells are sometimes connected by mucus or bristles into straight or curved filaments. Species of the genus Cyclotella are often found in plankton and fouling of stagnant water bodies and streams.

Melozira genus ( Melosira) . Filamentous colonies, usually consisting of cylindrical cells. Cells are connected by marginal spines or mucus. The valves are always round, often covered with radially arranged pores. Chromatophores numerous in the form of discs, often with peripheral lobes. It lives mainly in plankton and benthos of fresh and salt water bodies.

Biddulphic order ( Biddulphiales). Cells are solitary or often connected by marginal spines, setae, or horns into chain-like colonies. The shell is cylindrical or prismatic (ellipsoidal). Valves are round, elliptical, triangular or polygonal in shape, having outgrowths at the poles. The structure of the valves in the form of very small or large areoles. Mostly sea views.

Genus Hetoceros ( Chaetoceros) . Cells are cylindrical, with long hollow setae or spines at the poles of their valves. With the help of bristles, the cells are connected into chains. Setae of neighboring cells cross at the base, forming a filamentous colony. Chromatophores in the form of one or two plates or numerous small grains. Mostly inhabitants of the plankton of the seas and oceans.

Class Cirrus, or Pennate diatoms ( Pennatophyceae)

Unicellular and colonial forms of the cell are linear-elliptical, lanceolate, clavate, rarely rounded on the side of the valve. The structure of the valve with respect to the longitudinal axis is usually pinnate. One or two planes of symmetry can be drawn through the flaps. There are asymmetric sashes. Chromatophores are usually in the form of large plates. Many forms are mobile, have slit-like and canal-like seams. The sexual process resembles conjugation in conjugates. Freshwater and marine algae, living mainly in benthos on various substrates.

Order seamless ( Araphales). Cells are single or connected in colonies of various shapes. Shutters without a seam.

The genus Sinedra ( Synedra). Cells are usually rod-shaped, solitary or in the form of fascicular stellate colonies, free-floating or attached to the substrate. On the valve side of the shell there is a transverse hatching. There are two chromatophores (lamellar) or many (granular). They occur in benthos and fouling, less often in plankton.

Genus Fragilaria ( Fragilaria). The cells are very similar to those of the synedra, but are connected by valves into ribbon-like or zigzag colonies. The carapace is linear from the girdle, from the valve - from narrowly linear to lanceolate. Species of the genus live mainly in fresh water bodies (usually in the littoral), but are often found in the plankton of water bodies, mostly of the eutrophic type.

The genus tabellaria ( Tabellaria). Cells from the side of the girdle often look like plates, unite into zigzag, ribbon-like and star-shaped colonies. Cells with intercalated rims and septa. Chromatophores in the form of small grains. They are found in the littoral and plankton of fresh waters.

Diatom genus ( Diatoma). The colonies are similar to those of Tabellaria, but the cells from the girdle are elongated-rectangular, often with intercalated rims and without septa. Often found in fouling of fresh water bodies.

Genus Asterionella ( Asterionella). Thin rod-shaped cells with slightly expanded ends are connected in the form of stellate colonies. Chromatophores are small, lamellar or granular. It lives in the plankton of the seas and fresh water bodies.

Single-seam order ( Monoraphales ). Cells are usually solitary, rarely collected in ribbon-like colonies. On one leaf there is a slit-like seam, on the other there is no seam. They grow on filaments and other algae, on higher aquatic plants, occasionally found in plankton.

Genus Kokkoneis ( cocconeis). Cells with elliptical valves. There is a slit-like seam on the lower concave sash. Under the upper, seamless, slightly convex valve, there is a horseshoe-shaped chromatophore plate. Kokkoneis often develops on the threads of cladophora, vosheria and on higher aquatic plants.

Double-seam order ( Diraphales ). Cells are usually solitary, rarely in the form of ribbon-like or bushy colonies. Both sashes have a slit-like seam. The valves are mostly ellipsoidal in outline. The valve structure is represented by strokes, ribs, or areoles in the form of transverse rows.

Genus Pinnularia ( Pinnularia). Valve cells have the shape of an elongated ellipse, usually with a slight expansion in the middle part. Two terminal and one central nodules are located along the middle line of the valve - thickenings on the inner side of the carapace. Between the nodules are usually in the form of a slightly S-shaped slit-like suture, thanks to which the cell can move. Along the edges of the sash there are parallel ribs, which are partitions of narrow transverse chambers on the inside of the sash. Two lamellar chromatophores are located on the girdle sides, so they are visible from the valve as two narrow yellow-brown stripes. The nucleus is located in the center of the cell in the cytoplasmic bridge. On either side of it are large vacuoles with oil drops and volutin grains. Carapace with girdle in the form of an elongated rectangle.

Pinnularia is found at the bottom or in foulings of mostly fresh water bodies with a poor lime content.

Genus Navicula ( Navicula). The largest genus of diatoms. Cells are solitary, rarely form bushy or ribbon-like colonies. The ends of the cells in many are narrowed so that the cell itself has the shape of a boat. Similar types of navicula and pinnularia differ from each other in that the navicula does not have chambers in the valves, but only transverse strokes consisting of dots and dashes.

The navicula is found in the same habitats as the pinnularia, but much more often. Some species of the genus Navicula live in benthos, others in plankton, and others in the soil and on wet rocks. Many of them live in brackish and marine waters.

Genus cymbella ( Cymbella). Cells are solitary, can be attached to the substrate by a mucous stalk. Sometimes the cells are in gelatinous tubes in which they can move easily. Valves semilunar, often asymmetric, one end narrower than the other. The dorsal side is convex, ventral, straight or concave. The suture is usually located closer to the ventral edge. Chromatophore one, located on the girdle side. Very common, mostly freshwater forms. They live at the bottom of reservoirs and in fouling, less often in plankton.

Genus gyrosigma ( Gyrosigma) and the genus Pleurosigma ( Pleurosigma) . The cells from the valve are S-shaped. The seam runs in the middle of the sash. Species of the freshwater genus Gyrosigma differ from species of the genus Pleurosigma living in the seas by hatching of the shell. On the carapace of the gyrosigma, the longitudinal and transverse strokes intersect at a right angle. In the pleurosigma, intersecting strokes are located on the carapace with respect to the longitudinal axis at an angle of 45º.

genus gomphoneme ( gomphonema) . Cells are solitary, attached to the substrate with gelatinous legs or form branched colonies. Valves club-shaped, lanceolate. Their ends are capitate, sometimes pointed. The shell is wedge-shaped from the girdle. The seam runs down the middle of the flaps. Two lamellar chromatophores are located on the girdle sides of the carapace. Freshwater forms live in benthos and fouling.

Tunnel order( Aulonoraphales ). Mostly unicellular mobile forms with a canal-like suture, which is located on the keel or pterygoid outgrowth. Widely distributed in various water bodies.

Genus Nietzschia ( Nitzchia). Cells mostly solitary, rod-shaped, elliptical, S-curved. A keel (narrow protrusion) runs along one edge of each valve, in which a canal-like suture is located. The keel of the hypotheca and the keel of the epithecus are located on opposite sides of the valves. Marine and freshwater forms living in benthos, less often in plankton.

The genus Bacillaria ( Bacillaria). Rod-shaped cells are connected by valves into ribbon-like colonies. Cells can move one relative to the other, changing the shape of the colony. They live in the seas and fresh waters.

Genus Surirella ( Surirella). unicellular forms. Valves are linear, elliptical, ovate, sometimes narrowed in the middle. From the girdle, the shell is rectangular, trapezoidal, sometimes twisted along the longitudinal axis. There are two keels along the edges of each leaf. Therefore, the shell has four keels. In each keel it runs along a canal-like suture. In total, there are four seams in the surriella's shell. Algae is often found in the benthos of the seas and fresh water bodies.

Division of diatoms (Bacillariophyta)

Class Cirrus diatoms - Pennatae

Most aquatic inhabitants of the class lead a bottom lifestyle, some live in plankton.

Pinnularia(Pinnularia) - benthic algae, often entrained in plankton. Let's take from the bottom part of the aquarium, which contains the diatoms collected on the excursion, a drop of liquid and, placing it on a glass slide, cover it with a coverslip. Pinnularia should be found in the specimen in two positions: by the sash facing the observer and from the side of the girdle.

The sizes of pinnularia are very different, as can be seen by comparing several organisms; for observation, you should choose larger ones and study them using a large magnification of the microscope.

From the valve, the pinnularium looks like an elongated ellipse with a slight thickening in the middle part (Fig. 14, 3). Nodules are located along the middle line of the valve: two of them are located at the ends of the cell, and the third is in the center. Between the nodules there is an S-shaped suture - a gap in the valve - through which the protoplast communicates with the external environment. Through the seam, the cytoplasm protrudes outward and, flowing in one direction, causes the jet movement of the body with jerks. The movement of pinnularia is more convenient to observe at low magnification.

Along the edge of the sash, a sculpture in the form of strokes is visible, located with exceptional constancy. The strokes consist of rows of tiny dots containing equally small pores. Pores are also located under these pores in the pectin shell. Through the pores, osmotic and gas exchange with the environment takes place.

The cytoplasm lies along the walls; in the center of the cell there is a cytoplasmic bridge in which the nucleus lies, often visible without color. The cell cavity is occupied by large vacuoles (Fig. 14, 1 and 2).

Pinnularia has two lamellar chromatophores, which are visible from the side of the valve as two narrow brown ribbons along the edge. In diatoms, in addition to the green pigment, the chromatophores contain carotene, xanthophyll of two modifications, and fucoxanthin α and β.

From the girdle, the pinnularia has the shape of a rectangle with slightly rounded corners (Fig. 14, 4). At high magnification in the girdle position, the epithecus and hypotheca are visible. In this position, it is noticeable that the nodules are thickenings in the form of tubercles on the inside of the valves. It can also be seen from the girdle that the sculpture slightly transitions to the side walls. The entire cavity of the cell from the girdle is occupied by a chromatophore, against which droplets of reserve oil are often visible (it has the smell of fish oil). In a drop with a large number of pinnularia, in addition to organisms with brown chromatophores, pinnularia with green chromatophores can be found. These are dying organisms, the brown pigment of which is extracted by water.

Rice. 14. Pinnularia (Pinnularia): 1 - chromatophores in the form of two ribbons are visible in the cell; 2 - a cell from the girdle: a lamellar chromatophore, oil drops are visible, in the center of the cell - a cytoplasmic bridge; 3 - view from the sash: nodules, sculpture, seam are visible; 4 - view from the girdle: on the right - epithecus; on the left - hypothecus; 5 - reproduction by division

Having become acquainted with the structure of the pinnularia, it is necessary to find it in one or another stage of vegetative propagation. In this case, all stages of division can be found - from the initial divergence of the valves of the mother cell to two formed daughter individuals lying in close proximity to each other. When dividing, the chromatophores move to the valves; thus, newly emerged organisms have one chromatophore from the mother cell, each of which then divides longitudinally (Fig. 14, 5).

Each newly emerged cell receives one leaflet from the mother cell, and completes the second one. A hypotheque is always completed. Thus, one of the daughter cells is equal to the parent cell, and the other is smaller than it by the valve thickness. As a result, after a series of successive divisions, a gradual grinding of diatoms occurs. This process does not occur indefinitely, it is prevented by the formation of auxospores - growth spores (see more in the theoretical course).

Navicula(Navicula) is found in a variety of environmental conditions. Some species of navicula live in benthos, others live on the surface of wet rocks, and still others live in the soil. The navicula is similar to the pinnularia, but differs in the pointed ends of the valves, as well as in a different sculpture on them (Fig. 15, 1),

Rice. 15. Diatoms: 1 - navicula (Navicula) from the valve; 2 - pleurosigma (Pleurosigma); 3 - sculpture on the valves of the pleurosigma; 4, 5, 6 - gomphonema (Gomphonema) (4 - from the valve; 5 - from the girdle; 6 - colony; cells sit on mucous legs); 7 - kokkoneis (Cocconeis) - oval little bodies on the shell of the cladophora cell

The genus Navicula is very large, it contains about 1000 species.

Navicula is a very common diatom found in various samples.

Pleurosigma(Pleurosigma) - S-shaped diatom; on its valves there is a thin rhombic striation. This sculpture is so thin and geometrically correct that the pleurosigma preparation is used to assess the quality of microscope optics (Fig. 15, 2 and 3).

gomphoneme(Gomphonema), like ulotrix, needs a constant supply of oxygen. Therefore, it lives in fast-flowing waters, attaching itself to stones, large green algae (for example, cladophora, etc.), and forms slimy accumulations at the water's edge.

The cells of the colony sit at the ends of long, branched mucous filaments attached to the substrate (Fig. 15, 4, 5, and 6).

Rice. 16. Epiphytic diatoms: 1 — slender sinedra (Synedra gracilis); 2 - colony of Lycmophora (Lycmophora)

We will find the gomphoneme in the position from the sash and from the girdle. From the sash, its shape is guitar-like. A seam runs along the midline; due to its presence, gomphoneme cells, breaking away from the legs, move to active movement.

In the position from the girdle, the gomphoneme has the shape of a trapezoid, the smaller base of which is facing the stem; the corners of the trapezium are rounded. Sculpture on the wings of a gomphoneme in the form of ribs.

Likmophora(Lycmophora) is a colonial epiphytic diatom characterized by a fan-shaped arrangement of cells on a common mucous stalk of the colony (Fig. 16, 2).

Species of the genus Cocconeis also lead an epiphytic lifestyle. Thus, Cocconeus pediculus, which has an ellipsoidal valve, is widespread on the Cladophoran. One of the valves is tightly attached to the shell of the cladophora cell. Developing in masses, it sometimes completely covers the surface of the cladophora (Fig. 15, 7).

Together with kokkoneis, the epiphytic diatom Synedra gracilis, which has an elongated rectangular shape, is very common. Several cells of the sinedra are attached to the common mucus in a bundle and diverge in a fan-like manner (Fig. 16, 1). Other types of sinedra lead a planktonic lifestyle (Fig. 18, 4).

Diatoms of freshwater plankton

Most planktonic diatoms form colonies lying in the water column, which creates significant friction against the water. On the shell of some of them there are various outgrowths that play the role of parachutes. In addition, the shell thickness of many planktonic diatoms is negligible. All these adaptive features, preventing settling to the bottom, allow these organisms to spend their lives in a suspended state.

Rice. 17. Melozira (Melosira): 1 - colony; cells are visible from the girdle; 2 - cage from the sash

Colonies arise as a result of the fact that dividing cells do not disperse, remaining connected by mucus, as a result of which the surface of contact with water increases and the possibility of soaring in it increases. There are several types of structure of colonies: 1) cells lying immersed in the mucus produced by them; 2) ribbon-like and chain-like colonies connected by mucus along valves or corners; 3) colonies attached to slimy stalks, predominantly included in the benthos.

Cirrus diatoms predominate in plankton of fresh waters, representatives of the centric class are predominantly distributed in marine plankton. The plankton of diatoms is not the same at different times of the growing season; thus, asterionella (see below), developing abundantly in the early spring months, predominates in plankton. In summer, it is found in small numbers.

Rice. 18. Planktonic diatoms: 1 - Tabellaria (Tabellaria); in the colony, the cells are visible from the girdle; a separate cell from the sash; 2 - fragilaria (Fragilaria) - a colony and a separate cell from the valve; 3 - asterianella (Asterionella) - stellate colony and a separate cell from the valve; 4, planktonic species of Synedra; needle-shaped cells; on the left - from the sash, on the right - from the girdle

Let's get acquainted with some representatives of colonial diatoms, the most common in freshwater plankton.

tabellaria(Tabellaria). Its cells are rectangular, their corners are slightly rounded. Cells are connected to each other in chains at alternately opposite angles (Fig.

Department of Diatoms - Diafomeophyta

18, 1) and facing the observer with a belt.

Fragilaria(Fragilaria) forms ribbon-like colonies. Its cells have an elongated shape, slightly thickened in the middle part, in the colony they are connected to each other by valves and facing the observer with a belt (Fig. 18.2).

Asterionella(Asterionella) produces star-shaped, graceful colonies. Asterionella cells are rod-shaped with slightly expanded and rounded ends. The cells are connected to each other with the help of mucus by the corners of the narrow sides. Asterionella colonies are fragile and easily break up into groups of cells or into individual individuals (Fig. 18, 3).

The number of cells included in the Asterionella colony is different. There is evidence that in cold weather, when the water density is higher, the number of cells that make up the colony is small. During the warmer months, when water density drops, the number of cells that make up a colony increases.

Of the non-colonial planktonic diatoms, many species of the genus Sinedra have adapted to life in plankton, having developed a needle-shaped body, which increases the surface of contact with water, and hence friction.

Some other planktonic and benthic diatoms are shown in Figure 19.

Department of Diatoms

The Department of Diatoms (diatoms) unites 6 thousand species of unicellular (green pinularia) and colonial organisms living in fresh water bodies, seas and on moist soil. They develop rapidly, forming loose brown accumulations, similar to foam or scale. Their main feature is pectin shells impregnated with silica, which consist of two halves put on like a lid (epithec) on a box (hypothec). Under the shell is the cytoplasm with one nucleus and chloroplasts. The coloration is yellowish-brown with different shades, due to the presence of pigments: chlorophyll, carotenoids and diatomine. The reserve product of diatoms is fatty oil. These algae reproduce vegetatively and sexually. During vegetative propagation, the contents of the cells are divided, and the shell valves diverge. The cells formed as a result of division complete the hypotheca, which leads to the grinding of the algae. Diatoms are diploid organisms. During sexual reproduction (hologamy), their siliceous membranes diverge, and the protoplasts of two cells unite, forming a zygote, or the so-called growth spore - aucospore.

Department of diatoms

The latter is covered with an elastic membrane and grows. When the aucospore acquires an appropriate size, its shell is impregnated with silica, valves are formed, and the alga begins to divide vegetatively. A characteristic representative of the department of diatoms is a unicellular alga pinularia green , which lives in fresh water and on moist soil. Diatoms make up a significant mass of plankton, are found in benthos, and are the main food for many aquatic organisms. After the death of diatoms, their silica shells accumulate on the bottom of reservoirs in large quantities, forming deposits of tripoli and diatoms, which are used as building and polishing material, for the manufacture of insulators, liquid glass, glaze, and in scientific research. From the dead diatoms, powerful deposits of rocks were formed - ditomite and endured. Control fossils are called organisms, the remains of which determine the geological age of rocks. Thus, according to the species composition of diatoms (as well as radiolarians, foraminifers, some mollusks, etc.) of a certain geological rock, one can approximately determine its age.

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Diatoms are unicellular algae with coccoid thallus. An additional pigment is fucoxanthin. Characteristic features: the cells are covered with a silica shell, consisting of two parts: a larger epithecus and a smaller hypotheca. Each part consists of flaps and a belt-shaped rim. Mobile individuals have a seam and nodules on the valves. Flagella (one cirrus) are present only in spermatozoa. Reproduction: vegetative - division into two valves (Two protoplasts arise, each of them eventually leaves one solid valve. The second is completed, after which new organisms of colonial forms of diatoms remain connected, and unicellular ones are separated The result of vegetative propagation is a decrease in cell size from generation to generation); asexual - unknown; sexual - iso- and oogamy (In centric diatoms, an oogamous process takes place, when some individuals give eggs, while others produce flagellar spermatozoa. Fertilization occurs, resulting in a zygote covered with a pectin membrane, then an auxospore is formed). The zygote is dressed in a thin shell and is capable of growth for some time, therefore it is called an auxospore. The life cycle is diploid with gametic reduction. Representatives: pennate group: Navicula, pinnularia, cymbella; centric group: melozira. Ecology: live in plankton and benthos of marine and freshwater reservoirs, in fouling of aquatic plants and other substrates. Significance: the mass of dead diatoms gradually forms the sedimentary rock of diatoms - a multi-purpose raw material. Contribute to the self-purification of water bodies from a number of inorganic contaminants; are used as indicators or indicators of the state of surface waters; help determine the origin and age of rocks from the shells of fossil forms.

24. Class Brown algae (Phaeophyceae). Position in the system, characteristic, reproduction cycle, representatives, ecology, value.

Pigment: chlorophyll A and C; additional pigments: xanthophylls: fucoxanthin, etc.; carotenes: B-carotene. Type of thallus differentiation: filamentous, multifilamentous, pseudotissue, real tissue. In the class, only multicellular representatives, most are macrophytes; Spare substances: chrysolaminarin, mannitol. They are deposited outside the chloroplasts, in the cytoplasm. The cell wall has two layers: the primary one is cellulose (algulose) and insoluble alginates (salts of alginic acids), the second one is a mucilaginous layer of soluble alginates and fucoidans. Burning stages: zoospores, gametes; usually there are two flagella (pinnate and smooth) attached laterally. Life cycle - haplo-diploid with sparious reduction with isomorphic or heteromorphic change of generations; diploid with gametic reduction. Reproduction: vegetative - by groups of cells on rhizoids (Vegetative reproduction occurs when branches are accidentally separated from the thallus, these branches do not form attachment organs and do not form organs of asexual and sexual reproduction); asexual - by zoospores or aplanospores (Spores of asexual reproduction are formed as a result of meiosis and subsequent mitoses in single-celled sporangia); sexual: iso-, hetero, and oogamy (Gametes are formed in multi-nested gametangia. In each nest of such a gametangium, one gamete is formed). Representative: ectocarpus, dictoid, kelp, fucus, sargassum. Ecology: marine (more often northern seas), mostly benthic, deep-water; there are non-attached species (plankton). Value: food, medicinal (iodine-containing, fucoidan), additives in the food industry (pectin-alginates for juice clarification), etc.

25. Division Green algae (Chlorophyta). Characteristic. Principles of classification, reproduction cycles (volvox, ulotrix, spirogyra). Main classes and representatives, ecology.

Pigments: chlorophylls a and b; additional - carotenoids: alpha and beta-carotenoid, xanthophylls: lutein, neoxanthin, etc. chloroplasts of pure green (herbaceous) color, different shapes, surrounded by 2 membranes, thylakoids are collected in grana. Type of thallus differentiation: all types (except omeboid), a significant part are unicellular, colonial and filamentous thalli. Only in this division are known species with a siphon-clad thallus. Reserve product: starch, deposited in the chloroplast. Cell wall: predominantly cellulose, rigid. Flagellar stages: vegetative cells (monadic unicellular tholli), zoospores, gomets; in type - 2 identical smooth flagella. Reproduction: vegetative (fragmentation, division of unicellular individuals in half), asexual (by zoospores, less often by aplanospores), sexual. Types of the sexual process: holo-, iso-, hetero-, oogamy. Life cycles: haploid with zygotic reduction prevails, less often diploid or with a change of generations. Representatives: 1) Class: Actually green algae, Representative: chlamydomonas, volvox, chlorococcus; 2) Class: Ulva, Representatives: Ulothrik, Ulva, Caulerpa; 3) Class: Trentopoly, Representative: Trentopoly. Ecology: ubiquitous, all ecological groups. Most are aquatic species, mostly freshwater, rarely marine. There are terrestrial, snow and ice algae, as well as lichen symbionts. Meaning: producers of organic matter; food, feed, medicinal.

26. Class Charophyceae. Characteristics, position in the system, evolutionary connections.

Characeae are highly organized macrophyte algae with a complex thallus and multicellular reproductive organs. The cell wall is two-layered, the inner one is cellulose, the outer one is callose, in the latter lime is deposited. Tall is a complicated variant of the heterotrichous type: jointed with coiled branching, consists of axes of unlimited and limited growth, core filaments, and is attached by resoids. Apical growth. The apical cell is alternately separated by a septal cell-segment, each of which is divided into a biconcave cell (transformed into a node and then gives lateral branches), and biconvex (transformed in the internode). Reproduction: vegetative with the help of nodules in the lower nodes of the thallus and on rhizoids, fragmentation of thalli and the formation of "offspring" from the nodes with further rooting; asexual reproduction is absent; sexual - oogamy, reproductive organs are complex multicellular. Ecology: they live in freshwater reservoirs with a high content of calcium. Evolutionarily, characeae were considered as a dead-end branch of evolution, which separated early from the common trunk of green plants. They have the most complex organs of sexual reproduction among algae with green pigments.

27. Evolutionary connections of algae.

1 line: Chlorophyll a, phycobilins. Absence of mobile stages with flagella, thylakoids are solitary, located in the cytoplasm or chloroplasts - Department of the prokaryotic cyanophyte. Additional pigments are phycobilins and carotenoids.

Lecture 3. Department of Diatoms. General characteristics. Classification

There is murein in the cell wall. Zap product cyanophycin starch. The size is vegetative and asexual (by spores and hormogonia) unicellular colonial filamentous tolls. Often surrounded by mucus - Department of Rhodophyte (cr water) in the cell wall of cellulose, pectins. Chloroplasts in the form of grains or plates with 2 shells: red starch. Variation of vegetative, asexual (mono and tetraspores) sexual atypical oogamy with a sample of carposporophyte. F cycle is often complex of three generations. Macrophytes with pseudotissue, filamentous and multifilamentous thalli predominate, a dead-end branch of evolution 2 line : chlorophylls a and b. mobile stages with two identical smooth flagella; Additional pigments are carotene and castnophylls. Grass-colored chloroplasts, with 2 membranes. Cellulose in the cell wall. Zap thing - starch. Reproduction by all types. G cycles: haploid, diploid, with a change of generations. all types of thallus except amoeboid. - Charophyte department. Pigments, chloroplasts. The zap product is similar to the green ones. In the cell wall of the cell wall of the charic complements caso3. Differences from green in the details of cell division, the structure of the flagella. Thallus trichalum. W cycle haploid. - higher plants 3 line: chlorophylls a and c. motile stages with two different flagella pinnate and smooth. Thylakoids are collected by 2-3 around the chloroplast - Department of Ochrophyte additional pigment fucosanthin or wocherioxanthin. Cellulose in the cell wall. The stock of things is chrysolaminarin. Reproduces all types. F cycles haploid diploid with cm generations, sometimes without sexual times.

Alom all types. They gave rise to chlorine-free branches of pseudofungi - the department of oomycot. pseudo-fungi

Diatoms are a division of unicellular algae, a distinctive feature of the cells of which is the presence of a strong silicon shell. About 300 genera and more than 12 thousand species are known, which are distributed throughout the globe - in water bodies, both salty and fresh, on the surface of moist soil, stones. Diatoms are the predominant component of plankton, benthos at a depth of no more than 100 meters. The habitats of benthic diatoms are always associated with some kind of substrate. They move along it or are attached through tubules, legs . According to the way of feeding, the vast majority of diatoms are phototrophs, sometimes there are mixotrophic forms, symbiotrophs and heterotrophs. Structure diatoms has characteristic features. These are unicellular single or colonial forms. Each cell is covered with a strong silica shell, which is a replacement for the typical cellulose shell of algae. According to the shape of the shell, all diatoms are divided into two broad groups: with a bilaterally symmetrical and with a radially symmetrical shell. In the shell, two halves are distinguished: the upper and lower. Both valves penetrate multiple pores through which the exchange of substances between the algae organism and the surrounding space takes place. The cells of the colonies are not interconnected, but are connected by the surface of the shell valves. Mandatory elements of diatom cells are chloroplasts. Their color may be different depending on the combination and quantitative ratio of pigments. Carotene is predominant, xanthophyll and diatomine are also present. Spare nutrients in diatom cells are lipids in the form of many drops of oils. One large nucleus can contain up to eight nucleoli. Under the microscope, mitochondria, the Golgi complex, and a large vacuole with cell sap are also distinguishable in the cell. reproduction most often asexual, vegetative, occurs by cell division in two during mitosis. Two daughter cells receive one shell of the mother's shell each, and the second is formed independently. The sexual process in diatoms is possible according to the type of isogamy, oogamy, heterogamy, hologamy. Representatives of the class Diatoms (Bacillariophyceae) are single-celled, colonial or filamentous organisms and are widely distributed in fresh and marine waters. Some of them live in the bottom layers, others in suspension ( plankton). Diatoms are divided into 11 orders and 2 groups: pennate and centric. In the former, the cells have bilateral symmetry, while the latter have a radial structure. To the group pennate belong pinnularia,Navicula,Tabellaria, Asterionella. To the group centric are unicellular cyclotella and colonial melozira (Melosira), consisting of a large number of barrel-shaped cells. Meaning diatoms in nature is extremely large. They, being a valuable and most numerous group in the composition of plankton, produce up to 25% of the organic matter of the Earth. Their role is the main source of food and the initial link in the food chains of aquatic animals. At the bottom of the seas, these algae currently form diatom silts. Due to the fact that diatom shells have a regular, clear organization, they are used to test the resolution of optical microscopes.