Abstract: Description of species of multicellular green algae
Multicellular green algae
Examples of multicellular green algae are Ulotrix and Spirogyra . Kinds genus, aulothrix They live mainly in fresh water, less often in sea and brackish water bodies, as well as in soil. Algae attach to underwater objects, forming bright green bushes up to 10 cm or more in size.
Unbranched ulothrix filaments, consisting of a single row of cylindrical cells with thick cellulose membranes, are attached to the substrate by a colorless conical basal cell, which performs the functions of a rhizoid. The structure of the chromatophore is characteristic, which has the form of a wall plate forming an open belt or ring (cylinder). All cells, except the basal one, are capable of dividing, causing the continuous growth of the thallus.
Asexual reproduction is carried out in two ways: by disintegrating the filament into short sections, each of which develops into a new filament, or by the formation of four-flagellate zoospores in the cells. They leave the mother cell, shed their flagella one after another, attach sideways to the substrate, become covered with a thin cellulose membrane and grow into a new thread.
Reproduction of the filamentous algae ulothrix: red arrows - asexual reproduction, blue arrows - sexual reproduction.
The sexual process is isogamous. After fertilization, the zygote first floats, then settles to the bottom, loses flagella, develops a dense shell and a mucous stalk, with which it attaches to the substrate. This is a resting sporophyte. After a period of rest, reduction division of the nucleus occurs and the zygote germinates as zoospores.
Thus, in the life cycle of ulotrix, there is an alternation of generations, or a change in sexual and asexual forms of development: the filamentous multicellular gametophyte (the generation that forms gametes) is replaced by a unicellular sporophyte - a generation that is represented by a kind of zygote on a stalk and is capable of forming spores.
Spirogyra It is common in stagnant and slowly flowing waters, where it often forms large masses of bright green “mud.” It is a thin thread consisting of long cylindrical cells arranged in one row with a clearly visible cell wall. On the outside, the threads are covered with a mucous sheath.
Spirogyra filamentous algae cell
A characteristic feature of spirogyra is a ribbon-shaped, spirally curved chromatophore located in the wall layer of the cytoplasm. In the center of the cell there is a nucleus enclosed in a cytoplasmic sac and suspended on cytoplasmic cords in a large vacuole.
Asexual reproduction is carried out by breaking the thread into short sections, and there is no sporulation. The sexual process is conjugation. In this case, two threads are usually located parallel to each other and grow together with the help of copulation processes or bridges. Their shells dissolve at the point of contact, and a through channel is formed, through which the compressed contents of the cell of one thread moves into the cell of the other and merges with its protoplast. The zygote formed as a result of fertilization germinates after a period of rest. This is preceded by a reduction division of the nucleus: of the four nuclei formed, three die, and one remains the nucleus of a single seedling emerging through a rupture in the outer layers of the zygote shell.
Spirogyra
(Spirogyra)
Spirogyra(Spirogyra Link.) is a green algae from the conjugate group (see Conjugatae), belongs to the Zygnemeae family. The body of Spirogyra is a non-branching thread, consisting of cylindrical cells. The latter contains a chromatophore characteristic of Spirogyra (see): one or several spirally curled, green ribbons. The chromatophores contain colorless bodies around which starch grains, the so-called pyrenoids, are grouped. The nucleus, very clearly visible under a microscope, suspended on protoplasmic filaments, is located in the middle of the cell. Spirogyra grows by intercalary (uniform) cell division. The sexual process of Spirogyra is copulation or conjugation: cells of 2 adjacent filaments are connected by lateral outgrowths; the shells separating these outgrowths are destroyed and, thus, a copulation channel is obtained, through which the entire contents of one cell (male) passes into another (female) and merges with the contents of the latter; the cell in which the fusion occurred (zygote) becomes rounded, separated from the filament and, covered with a thick membrane, turns into a zygospore. The zygospore overwinters and grows into a young thread in the spring. In the zygote, after the fusion of the contents of the male and female cells, the chromatophore of the first cell dies and only the second remains, the nuclei first merge into one, which is then divided into 4 unequal in size (unequal division of the nucleus); Of these, 2 smaller ones diffuse in the surrounding plasma, and 2 larger ones, merging, form the nucleus of the zygote.
The described copulation between cells of different threads (dioecious) is called staircase. In the case when a channel is formed between two neighboring cells of the same thread, copulation (monoecious) is called lateral. In most Spirogyra, during the sexual process, the copulation canal is always developed (subgenus Euspirogyra) and both male and female cells are the same, but in some these cells are unequal in size, and the copulation canal is very poorly developed or completely absent, so that the cells merge with each other directly ( subgenus Sirogonium). Due to the size of Spirogyra cells, reaching up to 0.01 mm in some of its species, due to the clarity of their structure, this algae is one of the best studied and serves as a classic object in the study of the anatomy of the cell and nucleus.
Green algae spirogyra
Spirogyra is one of the most common green algae in fresh waters in all parts of the world; it is also found in brackish waters. Its threads are collected in large green clusters that float on the surface of the water or spread along the bottom and are very often found in the mud of standing and flowing waters, in ponds, swamps, ditches, rivers, streams, pools, etc.
Spirogyra under a microscope
In total, up to 70 species of Spirogyra are known, differing from each other in the shape and size of cells and zygospores, as well as the shape and number of chromatophore ribbons found in them, and belonging, as stated above, to 2 divisions - Euspirogyra (the most common: Sp. tenuissima Hass., longata Kg. with one ribbon, Sp. nitida Kg. with very thick cells, etc.) and Sirogonium (Sp. stictica Sm., etc.). For Russia, up to 40 species of Spirogyra are indicated
Ulotrix
Ulotrix(lat. Ulothrix) - a genus of green algae Chlorophyta .
It lives in sea and fresh waters, forming green mud on underwater objects. Filamentous type of thallus differentiation. Chloroplast wall in the form of a belt, closed or open, with several pyrenoids. There is only one core, but without painting it is not visible.
Order Ulotrichales
The ulothrix thallus is built like a single-row unbranched thread. It is composed of cells similar to each other in structure and function (Table 30, 2). Potentially, all cells are capable of dividing and participating in the growth of a plant, just as all cells can form spores and gametes. Only the cell at the base of the filament differs from the rest: with its help, the thallus is attached to the substrate (in attached forms). Ulothrix cells have significant autonomy. This property is associated with the ability for regeneration and vegetative reproduction - individual cells or sections of threads easily break away from the threads and begin to grow independently
The order includes more than 16 genera. Despite the fact that all their representatives are constructed as a simple single-row thread, important differences can be found in their organization, on the basis of which the entire order is divided into three groups. In algae of the first group, the thread is a row of cells loosely arranged in a thick mucous sheath. Such are, for example, algae Geminella genus Geminella. It is interesting that all ulothrixes with a similar structure are planktonic organisms.
The second group includes those filamentous algae that vegetate as single cells or as short chains of 2-4 cells, very loosely connected to each other. Their threads are formed rarely and for a short time. An example of such a structure is genus Stichococcus(Stichococcus, Fig. 216, 2). The algae included in this group lead a terrestrial lifestyle.
The central group of the order is the third group, which includes algae, built as a typical multicellular thread, in which the cells are tightly connected to each other without the help of a mucous sheath. Algae belonging to this group are overwhelmingly attached organisms, at least when young. Their threads are more permanent formations, they no longer fall apart so easily, and they can be distinguished between basal and apical parts. This includes several genera, including the central genus of the order - ulothrix(Ulothrix).
Ulothrix species (more than 25 of them are currently known) live mainly in fresh water bodies and only very few enter brackish and sea waters. These algae can also settle on wet surfaces that are periodically wetted by splashes from the surf or waterfalls.
One of the most widespread and well-studied species is ulothrix girdled(Ulothrix zonata).
The thallus of ulotrix consists of unbranched filaments of indefinite length, which at the beginning of growth are attached to the substrate by a basal cell. The filament cells are cylindrical or slightly barrel-shaped, often short. The cell membranes are usually thin, but often they thicken and can become layered. Ulotrix cells, like the cells of all algae of this order, contain a single wall chloroplast with one or more pyrenoids and one nucleus located along the longitudinal axis of the cell. The chloroplast has the shape of a belt that encircles the entire protoplast or only part of it
Vegetative reproduction of ulotrix is carried out by fragmentation: the threads break up into short segments and each segment develops into a new thread. However, ulothrix does not reproduce in this way as often as other algae of the order that have a loose filament structure.
For asexual reproduction, zoospores are used, which are formed in all cells of the filaments except the basal one. The development of zoospores, like gametes, begins at the top of the filament and gradually invades the underlying cells.
Zoospores are ovoid cells with four flagella at the anterior end. They contain a stigma, several contractile vacuoles and a wall chloroplast. Ulotrix girdled has two types of zoospores - macrozoospores and microzoospores. Large macrozoospores have a broadly ovoid shape, often with a pointed posterior end, and a stigma located at the anterior end (. Microzoospores are distinguished by their smaller sizes, rounded posterior end and the location of the stigma in the middle of the spore. The nature of microzoospores remains not entirely clear. Apparently, they represent is a transitional type between macrozoospores and gametes.
Quite often, zoospores do not leave the sporangia, but secrete a thin membrane and turn into aplanospores. The latter are released as a result of the destruction of the thread, but sometimes they can begin to germinate while in sporangia.
During sexual reproduction, gametes are formed in threads in exactly the same way as zoospores. As a rule, they develop in the same threads as zoospores, or in similar ones. Most often, the transition to sexual reproduction is associated with the end of active growth and the onset of unfavorable conditions. Unlike zoospores, gametes bear two flagella. The sexual process is isogamous. Fusion occurs between gametes of the same or different strands. The zygote remains mobile for a short time, then settles, loses its flagella, becomes covered with a thick membrane and turns into a single-celled sporophyte. It enters a period of rest, during which reserve substances accumulate. The shape of the sporophyte is varied; it is usually spherical with a smooth shell; in some marine species it becomes ovoid and sits on a mucous stalk.
BROWN ALGAE,
Brown algae(Phaeophyta), a type of spore plants, including 240 genera (1500 species), of which 3 are freshwater, the rest are marine. Thallus is olive green to dark brown in color due to the presence of a special brown pigment in the chromatophores 
fucoxanthin (C40H56O6), which masks other pigments (chlorophyll a, chlorophyll c, xanthophyll and beta-carotene). Brown algae vary in shape and size (from microscopic branched filaments to 40-meter plants). In higher brown algae (for example, kelp), tissue differentiation and the appearance of conductive elements are observed. Brown algae are characterized by multicellular hairs with a basal growth zone, which are absent in other algae. Cell membranes contain cellulose and specific substances - algin and fucoidin. Usually each cell has one nucleus. Chromatophores are mostly small and disc-shaped. Some species of brown algae have pyrenoids that are not very similar to the pyrenoids of other algae. In the cell around the core, colorless bubbles containing fucosan, which has many tannin properties, accumulate. As reserve products, mannitol (polyhydric alcohol) and laminarin (polysaccharide), and less often oil, accumulate in the tissues of brown algae. Brown algae reproduce sexually and asexually, rarely vegetatively. Brown algae usually have a sporophyte and a gametophyte; in the higher ones (Laminariaceae, Desmarestiaceae, etc.) they strictly alternate; in Cyclosporans, gametophytes develop on sporophytes; in primitive species (ectocarpaceae, chordariaceae, cutleriaceae, etc.), the gametophyte or sporophyte may drop out of the development cycle or appear once every few generations. Reproductive organs are unilocular or multilocular sporangia. A multilocular sporangium, which more often functions as a gametangium, is formed in the form of a single cell or a series of cells divided by septa into chambers containing one gamete or spore inside. Meiosis usually occurs in unilocular sporangia, in dictyotes - in tetrasporangia. The sexual process is isogamy, heterogamy or oogamy. Pear-shaped spores and gametes usually have an eye and have two flagella on the side, one directed forward, the other backward. brown algae are divided into 3 classes: Aplanosporophyceae (only dictyotes), Phaeosporophyceae (heterogenerate and isogenerate, excluding dictyotes) and Cyclosporophyceae (cyclosporans). brown algae are common in all seas, especially in cold ones, where they form large thickets. They are used to produce alginic acids and their salts - alginates, as well as feed flour and powder used in medicine, containing iodine and other trace elements. Some brown algae are used as food.
Brown algae: 1 - kelp; 2 - dictyota; 3 - ectocarpus; 4 - lessonia; 5 - nereocystis; 6 - alaria; 7 - cystoseira; 8 - elachista bushes on the stem of another algae; 9 - fucus; 10 - dictyosiphon; 11 - sargassum (all except 3 and 8, greatly reduced; 3 - view under a microscope, magnified approximately 40 times).
