9. Yellow-green algae. General characteristics. Orders.
Yellow-green algae can have unicellular flagellated, coccoid, filamentous and siphonal forms. Chloroplasts are disc-shaped. Contains chlorophyll a and c pigments, carotenoids. Reserve substances - glucan, fats. They reproduce through cell division or sexually (isogamy, oogamy).
Main representatives: Rhizochloris, Chlorothetium, Tribonema, Botrydium.
10. Brown algae. Structure, reproduction, classes, main representatives, distribution, significance
The department of brown algae (about 1,500 species) includes numerous macroscopic algae, the common external feature of which is the yellowish-brown color of their thalli, due to the presence of yellow and brown pigments. Mostly disc-shaped chloroplasts contain chlorophylls a and c, carotenes and xanthophylls. The pyrenoids are very small. Spare nutrients - laminarin, mannitol (sugar alcohol), and in small quantities - fat. In addition to the usual organelles, the cells contain physodes, structures containing tannins. The cell membranes on the outside are mucilaginous - pectin, the inner layer is cellulose.
The type of structure of the thallus of brown algae is predominantly lamellar, often very complexly organized, and rarely multi-filamentous. The vast majority of representatives of this group are marine forms.
Thallus of brown algae range in size from several tens of micrometers to tens of meters (macrocystis - up to 50 m). In highly organized forms, the thalli are differentiated and resemble flowering plants. Some large representatives have air bubbles. All brown algae grow attached to soil or other algae. For attachment, they form rhizoids or a basal disk. In a cross section of large thalli of brown algae, zones are distinguished, occupied by cells of different structures, specialized to perform individual functions.
The cells are mononuclear, covered with thick membranes with large pores. The shell consists of an inner cellulose and outer layer, the basis of which is proteins combined with alginic acid and its salts. Algulose. Physodes. Chloroplasts are usually small, disc-shaped, less often ribbon-shaped and lamellar. A number of taxa have a pyrenoid in their chloroplasts. In chloroplasts, thylakoids are grouped into lamellae of 3. Pigments: chlorophyll a, c, carotenoids. Reserve nutrients in cells outside chloroplasts are laminarin (a polysaccharide) and mannitol (six-hydroxy alcohol), and lipids in small quantities.
Reproduction. At B.V. All main types of reproduction are found - vegetative, asexual and sexual. Vegetative propagation occurs when branches are accidentally separated from the thallus; these branches do not form organs of attachment and do not form organs of asexual and sexual reproduction. Reproductive organs are formed on them if their development has already begun by the time the thallus is detached from the ground. Asexual reproduction is carried out by zoospores (there are species that reproduce by mono- or tetraspores). The structure of zoospores. Pear-shaped zoospores have two unequal flagella on the side: the anterior one is long, the posterior one is short. The formation of zoospores and gametes in brown algae occurs in containers of two main types: single-locular and multilocular. Meiosis occurs when spores form in single-locular sporangia. The sexual process is isogamous, heterogamous and oogamous. All brown algae, with the exception of Fucus, have alternating generations; in the asexual generation (sporophyte) in zoosporangia (or tetrasporangia), after reduction division, zoospores (or tetraspores) are formed; from them grow haploid sexual plants (gametophytes), dioecious or bisexual. After fertilization, the zygote develops into a new asexual diploid plant (sporophyte) without a dormant period. In Fucus, the entire life of algae occurs in the diploid phase; only sperm and eggs are haploid, before the formation of which reduction division occurs.
Depending on the nature of the alternation of generations and the change of nuclear phases, brown algae are divided into 3 classes.
Class Isogeneratae
In most algae of this class, the sporophyte and gametophyte are the same in shape and size or do not differ very sharply in size. The sexual process is iso-, hetero- or oogamous. Representatives of the genus Ectocarpus are widespread in the seas; There are especially many of them in cold seas. They grow in the littoral and sublittoral zones. They participate in the fouling of ships and buoys. They look like small bushes or tufts, consisting of abundantly branching single-row filaments, often ending in colorless multicellular hairs. The growth of filaments is intercalary. Single-celled zoosporangia form on the sides of the branches. In them, reduction division and a number of divisions of the cell nucleus occur and the formation
many zoospores that emerge from the zoosporangium, after swimming for a short time, germinate into a plant of the same appearance, but haploid. On it, on short lateral branches, multicellular gametangia are formed, in which gametes that are identical in shape and size, but differ in behavior (+ and -) develop. The zygote, without a dormant period, grows into a new diploid thread with a zoosporangium. Ectocarpus is characterized by an isomorphic change of generations. The sexual process is isogamy.
Cutleria- distributed along the European shores of the Atlantic Ocean and the Mediterranean Sea. It has the appearance of dichotonically branched multilayered ribbons up to 20 cm in length, ending in hairs. Externally, cutleriaceae differ from other algae by the presence of a fringe of delicate hairs at the top or along the edge of the thallus. At the base of the hairs, on some specimens large-celled macrogametangia develop, producing biflagellate macrogametes; on other specimens, small-celled microgametangia develop, yielding microgametes. After fertilization, the zygote develops without a rest period into a new diploid plant - a sporophyte, which has the shape of a multilayer plate or crust, tightly pressed to the substrate. It is so unlike a gametophyte that it has been described under the new generic name Aglaozonia. On its upper side, unicellular zoosporangia develop, in which zoospores are formed after reduction division. Zoospores germinate into a ribbon-shaped bushy gametophyte. The genus Cutlaria is the most famous. She serves as an example of a heteromorphic change of generation; her sexual process is heterogamy.
Dictyota grows mainly in tropical and subtropical seas, and is also found in the Black Sea. It is characterized by a forked-branched thallus with branches located in the same plane. The height of the plant is about 20 cm, the width of the branches is 4-8 mm. Dictyota has an isomorphic alternation of generations. On the sporophyte, large spherical tetrasporangia develop from the surface cells, in which, after reduction division, 4 haploid, immobile, naked tetraspores are formed. From tetraspores, male (with numerous antheridia, producing up to 30,000 sperm) and female (with oogonia, producing one egg) gametophytes develop. Mature eggs fall out of the oogonia. After fertilization, the zygote develops into a new sporophyte. Dictyota is an example of the development of algae with an isomorphic change of generations, in which the sexual process is oogamy.
Class Heterogeneratae
In the development cycle of algae of this class, a macroscopic (in many powerfully developed) sporophyte and a microscopic small gametophyte alternate, which is often called a prothallus by analogy with ferns. The sexual process is isogamous or oogamous. Sporophytes of different genera differ sharply and are the largest lower plants, reaching a length of 60-100 m with significant morphological division and a complex anatomical structure. Sporophytes are divided into leaf blade, petiole and rhizoids or basal disc. At the junction of the plate and petiole there is an intercalary meristem. The sporophyte is perennial, with the exception of the plate, which dies off annually and is replaced by a new one. In the meristoderm of the plate, before its destruction, unicellular zoosporangia are formed. In each zoosporangium, after preliminary reduction division, from 16 to 64 (sometimes 128) zoospores are formed. Billions of zoospores are formed on one algae specimen. After a short period of movement, the zoospores germinate into microscopically small filamentous male and female gametophytes (prothallus). Antheridia produce one sperm, and oogonia produce one egg. After fertilization, a new powerful sporophyte gradually develops from the zygote.
So, heteromorphic alternation of generations is characteristic of heterogenate algae. The sexual process is oogamy.
Laminaria, species of the genus are widespread in the northern seas. The thallus is divided into leaf blade, trunk and rhizoids. It is found vertically from the sea surface to a depth of 200 m.
Macrocystis, growing at a depth of 10-20 m, has a multi-branched trunk up to 50-60 m long. In front of each plate, the branch is usually expanded into a pear-shaped air bubble. Thanks to air bubbles and long branches, the main part of the macrocystis thallus floats at the surface of the sea. It is mined throughout the world and processed into alginates and other chemical products.
Nereocystis It is represented by a long trunk (petiole) up to 15-25 m, which gradually expands upward, carries at the end a bubble with a diameter of 12-20 cm, from which narrow plates extend (up to 9 m).
There are 24-40 such plates in total. Nereocystis grows at depths of up to 20 m, while the upper part of the trunk with a bladder floats near the surface.
The life cycles of the above representatives are similar. Zoosporangia form on leaf blades.
Class Cyclosporeae
This class includes brown algae, which do not have an alternation of generations, but only a change in nuclear phases: the entire algae is diploid, only the gametes are haploid. There is no asexual reproduction. The class contains only one order - Fucus.
Fucuses have leathery, olive- or yellow-brown, dichotonically branched, belt-like thalli 0.5-1 m long, 1-5 cm wide; some have swellings filled with air. Apical growth.
By the time of sexual reproduction, bean-shaped yellowish swellings are formed at the ends of the branches - receptacles, on which the genital organs are located. Gametophytes develop on the receptacles in the form of a layer lining the depressions (conceptacles or scaphidia). Gametangia form in depressions in a layer of cells that arises from a single cell called the conceptaculum initial cell or prospore. The lining layer of the conceptaculum developing from the prospore is the gametophyte. Fucus gametophytes form vertical branched and simple mononuclear multicellular filaments, some of them bear gametangia, and the rest serve as paraphyses.
Oogonia in Fucus sit directly on the surface of the gametophyte. Antheridia are formed at the ends of branched branches. Developing oogonia (8 eggs) and antheridia (64 sperm) are excreted with mucus
they shoot out from the concentaculum, here the germ cells are released from them. Fertilization occurs in the surrounding water. The fertilized egg grows into a new plant without a dormant period.
Genus fucus (Fucus) distributed in the northern seas, it is the main inhabitant of the coastal zone.
Sargassum - “leaves” are lamellar or subulate. The trunk is short, long branches carrying receptacles extend from its upper part; these branches die off annually. Representatives are distributed mainly in tropical and subtropical seas. Sargassum is used to produce alginates; some species with non-rigid leaves are used as food. In the Sargasso Sea on an area of 4.4 million square meters. km swim two species of sargassum, which reproduce vegetatively and do not have organs of attachment.
Isomorphic or heteromorphic variants of the haploid-diploid life cycle or isomorphic and heteromorphic change of generations. Life cycles of the genera Ectocarpus, Laminaria, Fucus, Conceptacula, Receptacula or Scaphidia.
Distribution and ecology of B.V. Almost exclusively marine plants. They are distributed everywhere from the Arctic to the Antarctic. But the largest species grow mainly in temperate and subpolar latitudes. Representatives of the department grow mainly in shallow water and predominate on rocky coastal substrates in cold regions of the globe. but they are also found at depths of up to 200 m. Species that have air bubbles become free-floating when separated from the ground and sometimes (for example, in the Sargasso Sea) form large accumulations on the surface of the water. The most powerful thickets of B.V. form in waters rich in nutrients. Many large algae are widely used as livestock feed and to fertilize fields (rich in potassium). Some types of kelp are used as food; in addition to food, they also have dietary value, because rich in iodine and useful for atherosclerosis. Previously, iodine was extracted from kelp. Brown algae is used in significant quantities to produce algin, which has great adhesive ability and is used in the production of paper, cardboard, and printing inks. Algae is collected with special harvesting machines, and those thrown ashore are also used.
According to their characteristics (color, structure, methods of reproduction, etc.), algae are divided into several types (divisions): green, blue-green, golden, diatoms, yellow-green, pyrrophytic, euglenic, red and brown algae.
Let's give a brief description of the types.
Green algae - Chlorophyta
Unicellular, colonial, multicellular and noncellular forms. Multicellular forms are represented mainly by filamentous algae. Some are distinguished by a complex internal structure, reminiscent of higher plants in appearance.
Algae are purely green in color, however, in addition to green chlorophyll, the chromatophores contain yellow pigments - carotene and xanthophyll. The cell membrane consists of fiber. Chromatophores with pyrenoids.
Reproduction is carried out by vegetative, asexual and sexual means. Vegetative propagation occurs by dividing the organism into parts. Asexual reproduction is carried out by motile zoospores with flagella of equal size (usually 2-4 of them) or aplaiospores - immobile spores.
With the help of zoospores, green algae not only reproduce, but also disperse. The sexual process of reproduction is varied. Representatives of green algae are Chlamydomonas, Spirogyra, Chlorella, Ulothrpx, Cladophora, Closterium, etc.
Blue-green algae - Cyanophyta
Unicellular, colonial and filamentous forms. Algae have blue-green, yellow-green, olive green and other types of colors. The color is explained by the presence of four pigments in blue-green algae: green chlorophyll, blue phycocyan, red phycoerythryp-pa and yellow carotene. These algae do not have a formed chromatophore and nucleus, flagellar stages and the sexual process are absent. Blue-green algae include: oscillatoria, nostoc, gleotrichia, anabena, etc.
Golden algae - Chrysophyta
Single-celled and colonial forms. They contain chlorophyll and phycochrysipus, which is why the color of representatives of this group of algae is golden or brownish-yellow. The cells are in some cases naked or covered with a poorly differentiated protoplasmic membrane; golden algae are found, the body of which is dressed in a shell or enclosed in a house.
Some forms are mobile and move with the help of flagella, while other forms are immobile in a vegetative state. They reproduce by division or zoospores.
Capable of forming cysts to withstand unfavorable conditions. The sexual process is very rare. Representatives of this type of algae are mallomonas, dinobrion, chrysameba, etc.
Diatoms - Bacillariophyta
Unicellular and colonial organisms with a silicified shell consisting of two halves called valves. Chromatophore I<ел-того или светло-бурого цвета от наличия в нем, кроме хлорофилла, бурого пигмента диатомина. Размножение осуществляется путем деления клеток на две, у некоторых диатомовых наблюдается образование двужгутиковых зооспор. Известен половой процесс. К диатомовым водорослям относятся пинну л я р ия, навикула, плевросигма, гомфонема, сиредра, мелозира и др.
Yellow-green, or heteroflagellate, algae - Xanthophyta, or Heterocontae
These include unicellular, colonial, filamentous and noncellular forms. These algae contain, in addition to chlorophyll, yellow pigments - xanthophyll and carotene; their color varies from light to dark yellow-green. Reproduction occurs by longitudinal cell division, zoospores (they are characterized by the presence of two flagella of unequal size and unequal structure), and autospores. The sexual process is known. Representative: botridium.
Pyrrophyte algae - Pyrrophyta
Unicellular and colonial forms. In addition to chlorophyll, algae contain the pigment pyrrophyll, which gives the algae a brown and brownish-yellow color. The cells are naked or covered with armored membranes. They reproduce by division, zoosiors, autosiors. They form cysts. Sexual reproduction is rare. Pyrrophyte algae include: peridinium, ceracium, etc.
Euglenophyta - Euglenophyta
Unicellular motile forms with one or two flagella, sometimes without them; the cells are bare, the role of the shell is played by the outer layer of protoplasm, sometimes the cell is located in the house. Most algae are green in color, sometimes light green due to the presence of xaptophylla. Reproduction occurs by longitudinal division, the sexual process is unknown. Representatives of euglena algae are euglena and facus.
Red algae, or purple algae, Rhodophyta
They live mainly in the seas, only a few live in fresh waters. These are multicellular algae, colored red.
(with different shades). The color of algae is associated with the presence in them, in addition to chlorophyll, of other pigments - phycoerythrin and phycocyan.
Asexual reproduction is carried out by aplano-spores. The sexual process is very complex and is characterized by the presence of male organs - antheridia and female organs - oogonia, or carpogones.
The representative is batrachospermum.
Brown algae, Phaeophyta
The name was given due to the yellow-brown color of the thallus, caused by the presence, in addition to green chlorophylls, of a large number of brown carotenoid pigments. Multicellular, predominantly macroscopic algae (the largest organism living in water is the brown algae macrocystis, which reaches a length of 60 m, growing by 45 cm per day).
Reproduction is vegetative, asexual and sexual. Gametes and zoospores bear two flagella on the side, different in length and morphology. Brown algae are widespread in all seas of the planet; they often form underwater forests, reaching their greatest development in the seas of temperate and subpolar latitudes, where they are the main source of organic matter in the coastal zone. In tropical latitudes, the largest accumulation of brown algae is in the Sargasso Sea. Representatives of only a few genera live in desalinated and fresh waters, for example, Pleurocladia, Streblonema, Lithoderma.
Yellow-green algae
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Xanthophyceae P.Allorge ex Fritsch, 1935 |
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Yellow-green algae(lat. Xanthophyceae, or Xanthophyta), or Multiflagellate algae(lat. Heterocontae), or Tribophyceae(lat. Tribophyceae) - a class of lower plants, including algae, the chloroplasts of which are colored yellow-green or yellow. Representatives are unicellular, colonial and multicellular, mainly freshwater organisms. Similar to golden algae, the division of yellow-green algae into classes is based on the variety of morphological organization of the thallus. Class named after type genus Tribonema(from Greek tribe- experienced, skillful, nema- a thread).
Cell structure
Flagella
Monadic representatives (zoospores and gametes) have two flagella of unequal length and morphology: the main flagellum has feathery ciliated hairs, the lateral flagellum is whip-shaped. The exception is synzoospores Vaucheria, in which numerous pairs of smooth flagella of slightly different lengths are located on the surface. Flagella are attached subapically to the cell (in sperm Vaucheria lateral attachment). Mastigonemes are synthesized in cisterns of the endoplasmic reticulum. The short flagellum ends with an acroneme.
The basal bodies of tribophycean flagella have a typical structure, located at right angles to each other. The radicular system is represented by a cross-striated root - rhizoplast and three microtubular roots, each of which consists of 3-4 microtubules.
Chloroplast
The chloroplast has a structure typical of ochrophytes. Typically, a cell contains several green or yellow-green disc-shaped plastids. Their color is due to the absence of fucoxanthin, which is responsible for the golden and brown color of other ochrophytes. Among the carotenoids in Tribophyceae there are α- and β-carotenes (predominant), vocheriaxanthin, diatoxanthin, diadinoxanthin, heteroxanthin, lutein, violaxanthin, neoxanthin, etc. Chlorophylls - a And c. In the cells of Tribophyceae, in addition to disc-shaped ones, there are plastids of other forms: lamellar, trough-shaped, ribbon-shaped, cup-shaped, stellate, etc. In a few species, pyrenoids of the semi-constricted type were found. The ocellus consists of a number of lipid globules, located at the anterior end of the body in the chloroplast, oriented towards the basal swelling of the flagellum.
Cell wall
Species with amoeboid, monadic and palmelloid organization lack a cell wall, they are covered only by a cytoplasmic membrane and can easily change shape. Sometimes “naked” cells are found inside houses, the walls of which can be painted brown with manganese and iron salts. The vast majority of tribophyceae have a cell wall that is solid or consisting of two parts. In its composition, studied by Tribonema And Vaucheria, cellulose predominates and contains polysaccharides, consisting mainly of glucose and uronic acids. Young cells have a thin membrane, but with age it thickens. Iron salts can be deposited in it, the compounds of which color it in various shades of brown and red. Most often, silica is present in the cell wall, giving it hardness and shine. It can also be inlaid with lime and be sculptured in various ways (spines, cells, warts, bristles, denticles, etc.). In attached forms, an outgrowth of the shell may form - a leg with an attaching sole.
In filamentous algae with bivalve membranes, when the filaments disintegrate, the cell membranes fall apart into H-shaped fragments, which are tightly connected halves of the membranes of two neighboring cells. As filaments grow, an H-shaped fragment of the cell wall of two adjacent daughter cells is inserted between the two halves of the mother cell wall. As a result, each of the daughter cells is half covered with the old membrane of the mother cell and half with the newly formed membrane.
Other structures
Contractile vacuoles are present in motile representatives. Usually there are 1-2 of them per cell, sometimes more. The Golgi apparatus has a unique structure. Dictyosomes are small, containing 3-7 cisternae.
Reserve nutrients are oils, some have volutin, chrysolamine and leukosin.
Core
There is one nucleus, less often there are many nuclei; in coenotic representatives the cells are always multinucleate. The details of mitosis have been studied in detail only in Vaucheria. Its mitosis is closed, with centrioles located at the poles outside the nucleus. No kinetochores were found. During anaphase, the interpolar microtubules of the spindle greatly elongate, which leads to a significant distance between the daughter nuclei and each other. The nuclear membrane is preserved, so in telophase the daughter nuclei have the shape of a dumbbell. It is believed that such mitosis is not typical for the entire group of Tribophyceae.
Reproduction
Most yellow-greens have vegetative and asexual reproduction. Vegetative propagation is carried out by dividing cells in half, disintegrating colonies and multicellular thalli into parts. During asexual reproduction, amoeboids, zoospores, synzoospores, hemizoospores, hemiautospores, autospores, and aplanospores can be formed. Zoospores are “naked” and usually pear-shaped with two flagella. The sexual process (iso-, hetero- and oogamous) is described in a few representatives.
When unfavorable conditions occur, the formation of cysts is observed. Cysts (statospores) are endogenous, mononuclear, less often multinucleate. Their wall often contains silica and consists of two unequal, or less often, equal parts.
Ecology
Tribophyceae are found on all continents, including Antarctica. They live mainly in fresh waters of temperate latitudes, are also common in soil, and are less common in terrestrial, brackish-water and marine habitats. They inhabit both clean and polluted waters, with varying pH values, but are rarely found in abundance. Tribofyceous algae are significantly more diverse and abundant in soils, where, developing in masses, they can cause a “blooming” of its surface. Aerophytic representatives are found on tree trunks, rocks, and walls of houses, sometimes causing them to turn green. They often live in accumulations of filamentous algae and aquatic higher plants along the banks of rivers, ponds, lakes and reservoirs.
Yellow-green algae are included in various ecological groups - plankton, less often periphyton and benthos. The vast majority of them are free-living forms, but intracellular symbionts - zooxanthellae - are also found in protozoan cells. Seaweed chloroplasts form an interesting intracellular symbiosis V. litorea with clam Elysia chlorotica. For 9 months, this mollusk is capable of photoautotrophic carbon dioxide fixation in culture. This is the longest symbiosis of this type, when the symbiotic plastid is in direct contact with the cytoplasm of the animal. In nature, mollusk larvae feed on threads Vaucheria. As a result of phagocytosis, algae chloroplasts enter the cytoplasm of the mollusk epithelial cells. During this process, the chloroplast envelope becomes three-layered, and the outer membrane of the chloroplast endoplasmic reticulum is lost. This phenomenon can serve as evidence that in the process of evolution, as a result of secondary symbiogenesis due to the loss of membranes, chloroplasts with three membranes could arise.
Meaning
Tribophycean algae are producers of oxygen and organic substances and are part of trophic chains. They participate in the self-purification of polluted waters and soils, the formation of silts and sapropels, and in the process of accumulation of organic substances in the soil, affecting its fertility. Their economic importance comes down to their use as indicator organisms in determining the state of water pollution; they are part of a complex of microorganisms used for wastewater treatment.
Phylogeny
At the end of the 19th - beginning of the 20th centuries. various genera of Tribophyceae were classified as green algae, which was primarily due to the color and morphological similarity of the thalli. But A. Pascher already included this group in the same evolutionary series with golden algae and diatoms. This point of view was subsequently confirmed in studies at the cytological, biochemical and molecular level. Currently, Tribophyceae are considered as a class within the Ochrophytae division. From tribophyceae, eustigmatophyceae were isolated in the rank of the same class, but, as it turned out, in evolutionary terms they are far from each other. In phylogenetic trees built on the analysis of nucleotide sequences of a number of genes, tribophyceae among ochrophytes are much closer to brown algae than to golden algae, diatoms, sinuraceae and eustigmatophyceae.
Diversity and systematics
About 90 genera and more than 600 species have been described, which are grouped into 6-7 orders (H. Ettl, 1978). The identification of orders is based on the type of differentiation of the thallus and the characteristics of the life cycle. The number of orders depends on the point of view on coenotic tribophycean algae: whether they are classified as one or two orders.
The department includes 2500 species. Representatives of the department are widespread in various habitats, especially in clean fresh water bodies; they are also common in soil. These algae are mainly passive plankters. More often they can be found in accumulations of filamentous algae and among aquatic plants.
These are predominantly microscopic unicellular algae, including colonial, multicellular and noncellular.
The predominant type of thallus structure is coccoid. Monadic, amoeboid, coccoid, palmelloid, filamentous, lamellar and siphonal.
Motile forms and stages have two flagella. Characteristics of flagella.
Outer covers: Some cells are covered only with plasmalemma - these are all amoeboid forms, some are monadic. They form pseudopodia and rhizopodia. Sometimes there are houses inlaid with iron or manganese salts. The vast majority have a dense cell membrane, solid or bicuspid. The cell wall is pectin, sometimes with cellulose and hemicellulose; in the genus Vaucheria it is cellulose. In many representatives, the shell is impregnated with silica or iron salts.
Features of the internal structure: One core, or many cores. Monad forms have 1-2 pulsating vacuoles. Motile and some coccoid forms have a stigma. Chloroplasts come in various shapes. They are surrounded by four membranes. Sometimes chloroplasts contain a pyrenoid. When lamellae form, thylakoids are grouped into groups of 3. Chloroplasts also contain a girdle thylakoid.
Pigments G-Z algae: chlorophylls “a” and “c”, carotenoids. Assimilation products are lipids, chrysolamine and volutin.
Reproduction: vegetative - by longitudinal cell division or disintegration of multicellular organisms into parts, asexual - by biflagellate zoospores, autospores, less often - amoeboids. In the genus Vaucheria, the spores are called synzoospores. The formation of endogenous cysts with a bicuspid membrane containing silica is also known. The sexual process is reliably known only in species of the genus Vaucheria, this is oogamy.
Distributed throughout the globe. They are found mainly in clean freshwater bodies of water, less often in brackish waters and seas. Many representatives are also common in soil. The relatively small division Xanthophyta is distinguished by a wide ecological amplitude.
Representatives: Tribonema, Vaucheria, Botrydium. Morphological and anatomical characteristics of the thallus, features of reproduction.
The most common representatives are:
Botrydium is a terrestrial alga that requires lime in the soil. In summer it can be found on damp soil near the shores of reservoirs, around puddles. It is visible to the naked eye in the form of green shiny bubbles 1-2 mm with a typically siphonal structure.
Vaucheria (Vaucheria) - thallus - sparsely branching threads without partitions, this is one giant multinucleated cell. It is found at the bottom of reservoirs with fast-flowing water, in stagnant reservoirs near the shore, and on heavily moist soil.
Representatives Xanthophyta
with monadic and amoeboid type of thallus structure
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1 - Chlorocardion pleurochloron; 2 - Rhizochloris stigmatica:
A- periplast, b- rhizopodia, V- chloroplast, G- stigma, d- pulsating vacuoles.
3 - Stipitococcus vas; 4 – Myxochloris sphagnicola.
Representatives Xanthophyta with coccoid type of thallus structure
Plan:
- Introduction
- 1 Cell structure
- 1.1 Flagella
- 1.2 Chloroplast
- 1.3 Cell wall
- 1.4 Other structures
- 1.5 Core
- 2 Reproduction
- 3 Ecology
- 4 Meaning
- 5 Phylogeny
- 6 Diversity and systematics Notes
Literature
Introduction
Yellow-green algae(lat. Xanthophyceae, or Xanthophyta), or Multiflagellate algae(lat. Heterocontae), or Tribophyceae(lat. Tribophyceae) - a class of lower plants, including algae, the chloroplasts of which are colored yellow-green or yellow. Representatives are unicellular, colonial and multicellular, mainly freshwater organisms. Similar to golden algae, the division of yellow-green algae into classes is based on the diversity of the morphological organization of the thallus. Class named after type genus Tribonema(from Greek tribe- experienced, skillful, nema- a thread).
1. Cell structure
1.1. Flagella
Monad representatives (zoospores and gametes) have two flagella of unequal length and morphology: the main flagellum has feathery ciliated hairs, the lateral flagellum is whip-shaped. The exception is synzoospores Vaucheria, in which numerous pairs of smooth flagella of slightly different lengths are located on the surface. Flagella are attached subapically to the cell (in sperm Vaucheria lateral attachment). Mastigonemes are synthesized in the cisterns of the endoplasmic reticulum. The short flagellum ends with an acroneme.
The basal bodies of tribophycean flagella have a typical structure, located at right angles to each other. The radicular system is represented by a cross-striated root - rhizoplast and three microtubular roots, each of which consists of 3-4 microtubules.
1.2. Chloroplast
The chloroplast has a structure typical of ochrophytes. Typically, a cell contains several green or yellow-green disc-shaped plastids. Their color is due to the absence of fucoxanthin, which is responsible for the golden and brown color of other ochrophytes. Of the carotenoids in Tribophyceae, there are α- and β-carotenes (predominant), voucheriaxanthin, diatoxanthin, diadinoxanthin, heteroxanthin, lutein, violaxanthin, neoxanthin, etc. Chlorophylls - a And c. In the cells of Tribophyceae, in addition to disc-shaped ones, there are plastids of other forms: lamellar, trough-shaped, ribbon-shaped, cup-shaped, stellate, etc. In a few species, pyrenoids of the semi-constricted type were found. The ocellus consists of a number of lipid globules, located at the anterior end of the body in the chloroplast, oriented towards the basal swelling of the flagellum.
1.3. Cell wall
Species with amoeboid, monadic and palmelloid organization lack a cell wall, they are covered only by a cytoplasmic membrane and can easily change shape. Sometimes “naked” cells are located inside houses, the walls of which can be painted brown with manganese and iron salts. The vast majority of tribophyceae have a cell wall that is solid or consisting of two parts. In its composition, studied by Tribonema And Vaucheria, cellulose predominates and contains polysaccharides consisting mainly of glucose and uronic acids. Young cells have a thin membrane, but with age it thickens. Iron salts can be deposited in it, the compounds of which color it in various shades of brown and red. Most often, silica is present in the cell wall, giving it hardness and shine. It can also be inlaid with lime and be sculptured in various ways (spines, cells, warts, bristles, denticles, etc.). In attached forms, an outgrowth of the shell may form - a leg with an attaching sole.
In filamentous algae with bivalve membranes, when the filaments disintegrate, the cell membranes fall apart into H-shaped fragments, which are tightly connected halves of the membranes of two neighboring cells. As filaments grow, an H-shaped fragment of the cell wall of two adjacent daughter cells is inserted between the two halves of the mother cell wall. As a result, each of the daughter cells is half covered with the old membrane of the mother cell and half with the newly formed membrane.
1.4. Other structures
Contractile vacuoles are present in motile representatives. Usually there are 1-2 of them per cell, sometimes more. The Golgi apparatus has a unique structure. Dictyosomes are small, containing 3-7 cisternae.
Reserve nutrients are oils, some have volutin, chrysolamine and leukosin.
1.5. Core
There is one nucleus, less often there are many nuclei; in coenotic representatives the cells are always multinucleate. The details of mitosis have been studied in detail only in Vaucheria. Its mitosis is closed, with centrioles located at the poles outside the nucleus. No kinetochores were found. During anaphase, the interpolar microtubules of the spindle greatly elongate, which leads to a significant distance between the daughter nuclei and each other. The nuclear membrane is preserved, so in telophase the daughter nuclei have the shape of a dumbbell. It is believed that such mitosis is not typical for the entire group of Tribophyceae.
2. Reproduction
Most yellow-greens have known vegetative and asexual reproduction. Vegetative propagation is carried out by dividing cells in half, disintegrating colonies and multicellular thalli into parts. During asexual reproduction, amoeboids, zoospores, synzoospores, hemizoospores, hemiautospores, autospores, and aplanospores can be formed. Zoospores are “naked” and usually pear-shaped with two flagella. The sexual process (iso-, hetero- and oogamous) is described in a few representatives.
When unfavorable conditions occur, the formation of cysts is observed. Cysts (statospores) are endogenous, mononuclear, less often multinucleate. Their wall often contains silica and consists of two unequal, or less often, equal parts.
3. Ecology
Tribophyceae are found on all continents, including Antarctica. They live mainly in fresh waters of temperate latitudes, are also common in soil, and are less common in terrestrial, brackish-water and marine habitats. They inhabit both clean and polluted waters, with varying pH values, but are rarely found in abundance. Tribofyceous algae are significantly more diverse and abundant in soils, where, developing in masses, they can cause a “blooming” of the soil surface. Aerophytic representatives are found on tree trunks, rocks, and walls of houses, sometimes causing them to turn green. They often live in accumulations of filamentous algae and aquatic higher plants along the banks of rivers, ponds, lakes and reservoirs.
Yellow-green algae are included in various ecological groups - plankton, less often periphyton and benthos. The vast majority of them are free-living forms, but intracellular symbionts - zooxanthellae - are also found in protozoan cells. Seaweed chloroplasts form an interesting intracellular symbiosis V. litorea with clam Elysia chlorotica. For 9 months, this mollusk is capable of photoautotrophic carbon dioxide fixation in culture. This is the longest symbiosis of this type, when the symbiotic plastid is in direct contact with the cytoplasm of the animal. In nature, mollusk larvae feed on threads Vaucheria. As a result of phagocytosis, algae chloroplasts enter the cytoplasm of the mollusk epithelial cells. During this process, the chloroplast envelope becomes three-layered, and the outer membrane of the chloroplast endoplasmic reticulum is lost. This phenomenon can serve as evidence that in the process of evolution, as a result of secondary symbiogenesis due to the loss of membranes, chloroplasts with three membranes could arise.
4. Meaning
Tribophycean algae are producers of oxygen and organic substances and are part of trophic chains. They participate in the self-purification of polluted waters and soils, the formation of silts and sapropels, and the process of accumulation of organic substances in the soil, affecting its fertility. Their economic importance comes down to their use as indicator organisms in determining the state of water pollution; they are part of a complex of microorganisms used for wastewater treatment.
5. Phylogeny
At the end of the 19th - beginning of the 20th centuries. various genera of Tribophyceae were classified as green algae, which was primarily due to the color and morphological similarity of the thalli. But A. Pascher already included this group in the same evolutionary series with golden algae and diatoms. This point of view was subsequently confirmed by studies at the cytological, biochemical and molecular level. Currently, Tribophyceae are considered as a class within the Ochrophytae division. From tribophyceae, eustigmatophyceae were isolated in the rank of the same class, but, as it turned out, in evolutionary terms they are far from each other. In phylogenetic trees built on the analysis of nucleotide sequences of a number of genes, tribophyceae among ochrophytes are much closer to brown algae than to golden algae, diatoms, sinuraceae and eustigmatophyceae.
6. Diversity and systematics
About 90 genera and more than 600 species have been described, which are grouped into 6-7 orders (H. Ettl, 1978). The identification of orders is based on the type of differentiation of the thallus and the characteristics of the life cycle. The number of orders depends on the point of view on coenotic tribophycean algae: whether they are classified as one or two orders.
Notes
- According to EOL. See card.
Literature
- Plant life. In 6 volumes / Ch. ed. Al. A. Fedorov. - M.: Education, 1977. - T. 3. Seaweed. Lichens. Ed. M. M. Gollerbach. - 487 p. - 300,000 copies.
- "Course of lower plants", ed. M. V. Gorlenko - M.: “Higher School”, 1981
- V. A. Kryzhanovsky, G. L. Bilich “Biology. Full course. In 3 vols. Volume 2. Botany." - M.: LLC Publishing House "ONICS 21st Century", 2002
- “Small workshop on lower plants” - M.: “Higher School”, 1976
- N. P. Gorbunova “Algology”, M.: “Higher School”, 1991