Phycology - Kostikov I.Yu. - 2009-2013

Chapter 24. Green algae – Chlorophyta

24.2. The Chlorophytan Lineage

24.2.3. Class Ulvophyceae

The Class comprises predominantly marine benthic and periphytic Algae, although freshwater and aerophytic forms are also represented within it. Almost all ulvophycean algae possess multicellular thalli, with filamentous, heterotrichous, and siphonocladous being the predominant structural types. Ulvophyceae represent an evolutionary Lineage of green algae characterized by cytokinesis via a furrowing mechanism, a cruciate flagellar ROOT system, and basal bodies shifted in a counterclockwise direction.

The class includes about 80 genera comprising more than 1000 species.

Features and CHARACTERISTICS OF THE Class

Biochemical Features

A specific feature of many marine siphonocladous representatives is the presence of an additional specific xanthophyll, siphonaxanthin. This xanthophyll is also found in the majority of species belonging to the class Siphonophyceae and in some prasinophyte algae.

Cytological Features

Cell coverings. The Cells are invariably enclosed by a Cellulose-pectin or cellulosic wall. Monad cells of Representatives of the most primitive order, Codiolales, also bear submicroscopic organic scales on their Plasmalemma, which are identical in origin and Morphology to the inner scales of prasinophytes. Monad stages of representatives of other orders are naked and belong to the p-type.

Nuclear apparatus and mitosis features. Mitosis within the class is closed; the spindle is formed with the participation of centrioles and persists for a long time during telophase. Two variants of Chromatin behavior in metaphase are known in ulvophytes: in the first variant, chromatin condenses into true Chromosomes that align at the equatorial plane and subsequently segregate According to the typical eukaryotic pattern; In the second variant, chromatin condenses into a single dense plate perforated by several channels through which bundles of interzonal spindle microtubules pass. A portion of the microtubules attaches to the kinetochores of the plate, driving the subsequent distribution of chromatin between the daughter nuclei (Fig. 24.39).

Fig. 24.39. Diversity of mitosis and cytokinesis in ulvophycean algae. A - codiolalean, acrosiphonalean, and ulvalean algae: chromosomes are visible in metaphase, cytokinesis occurs through the ingrowth of a Cleavage furrow without the participation of a phycoplast or phragmoplast; B - trentepohlialean algae: chromosomes are visible in metaphase, cytokinesis occurs with the participation of a phragmoplast via The formation of a cell plate, and the transverse septum features pores with plasmodesmata; C - cladophoralean algae: chromatin condenses into a dense plate in which individual chromosomes are not discernible, and The Nucleus becomes greatly elongated in anaphase and telophase; upon completion of mitosis, the spindle detaches from the daughter nuclei and is surrounded for some time by remnants of the mother nucleus envelope. 1, 5, 9 - early prophase, 2, 6, 10 - metaphase, 3, 7, 11 - telophase, 4, 8, 12 - early interphase (schematized after Hori, Enomoto, 1978; Sluiman et al., 1980; Chapman, Henk, 1986).

Cytokinesis is accomplished by means of a cleavage furrow or a cell plate. The growth of both the cleavage furrow and The Cell plate is primarily driven by The activity of the Golgi apparatus: Golgi vesicles containing the material for the future cell plate are transported to the division plane, where they either fuse with the nascent cleavage furrow to promote its growth, or coalesce with one another to form a cell plate. During cytokinesis involving a cleavage furrow, Golgi vesicles are transported without the participation of the phycoplast or phragmoplast; The Cell wall ingrows centripetally and lacks pores. During cytokinesis of the cell plate type, the direction of vesicle movement is determined by the phragmoplast, and the transverse septum contains pores with plasmodesmata.

Flagellar apparatus. Monad cells are represented by zoospores and Gametes, possessing 2 or 4 isokont and isomorphic flagella.

The flagella are smooth and typically cylindrical in cross-section, although in trentepohlialean algae each flagellum features a lateral wing-like projection. The transition zone contains a stellate Structure.

The basal bodies of the flagella are elongated, shifted counterclockwise (11-5 configuration), and usually strongly overlapping one another. From a lateral perspective, they are rotated by nearly 180°. The upper connecting fiber linking the basal bodies is smooth (with the exception of cladophoralean algae).

The Root System is cruciate, and the number of microtubules in the roots corresponds to the 3-2-3-2 or 6-4-6-4 formula. The root apices adjacent to the basal bodies are capped with thin terminal caps. In addition, a columnar structure has been discovered in trentepohlialean algae. This structure is similar to that found in trebouxiophytes, but it is associated with the tips of thin roots rather than thick ones (Fig. 24.40).

Fig. 24.40. Flagellar apparatus of ulvophycean algae in lateral and top views: A - Codiolales: monad cells are covered with submicroscopic scales, the upper connecting fiber is smooth; B - Ulvales and Acrosiphonales: the root system is similar to that of ulvales, but submicroscopic scales are absent; C - Cladophorales: the upper connecting fiber is cross-striated; D - Trentepohliales: the number of microtubules in the flagellar roots is doubled, the upper connecting fiber is reduced (after O'Kelly, Floyd, 1984; O'Kelly et al., 1984; Bakker, Lokhorst, 1985; Roberts, 1984).

Thus, by their absolute 11-5 configuration, ulvophytes resemble trebouxiophycean algae. However, the elongation, strong overlap, near-complete Rotation of the basal bodies, presence of a smooth upper connecting fiber, presence of microtubules in multiples of three in the thick roots, and the association of the columnar structure with thin roots clearly distinguish the flagellar stages of ulvophytes from those of trebouxiophytes.

Types of Morphological Structures

Within the class, algae are represented by five types of morphological structures: coccoid, filamentous, heterotrichous, parenchymatous, and siphonocladous.

Coccoid algae are few in number and are represented primarily by endophytic (Chlorocystis) and parasitic (Chlorochytrium) species. In some parasitic representatives, a tendency toward a siphonal Organization is observed.

Filamentous algae are one of the dominant groups within the class. They typically feature unbranched single-row (Ulothrix) thalli, while some forms are characterized by single-layered sheet-like thalli (Monostroma).

Heterotrichous forms are quite numerous and primarily represented by two morphological groups: first, algae with well-developed prostrate and erect filaments (Spongomorpha), the latter sometimes bearing unicellular hairs (setae) with a bulbously expanded base (Acrochaete); and second, forms with a reduced system of prostrate filaments and a well-developed system of erect filaments (Trentepohlia). In some forms, the erect filaments may form parenchymatous discs (Cephaleuros).

Algae with a truly parenchymatous structure are relatively scarce and possess thalli in the form of double-layered sheets (Ulva) or hollow tubes (Enteromorpha).

Siphonocladous forms are quite widespread. Their thalli may appear as unbranched filaments (Rhizoclonium), tufts formed by branched filaments (Cladophora), systems of interconnected vesicles (Valonia), tubes of equal or varying sizes (Siphonocladus), and others.

Reproduction and Life Cycles

Both Selection/8.html">Asexual and sexual reproduction are widely represented within the class. Asexual reproduction occurs via thallus fragmentation, quadriflagellate (rarely biflagellate) zoospores, and aplanospores. Akinetes are also known. The sexual process is predominantly isogamous or heterogamous.

Life cycles are haplodiplophasic, with isomorphic or heteromorphic ALTERNATION OF GENERATIONS. In many representatives, the zygote develops into a specialized unicellular sporophyte known as the "Codiolum" stage. Varieties of parthenogenetic development are also fairly common.

Systematics of the Class

Based on the analysis of various nuclear and chloroplast genes, 5 main groups of species have been established within the class, which can be regarded as taxa of the rank of order (Table 24.5). Interestingly, the molecular (genotypic) distinctness of these groups correlates well with A number of phenotypic features related to ultrastructure (primarily of zoospores and gametes), life cycles, reproductive biology, and, to a certain extent, morphology and ecological characteristics. It is believed that divergence within the class proceeded precisely along these lines.

Table 24.5. Some principal systematic features of orders of Ulvophycean algae

Abbreviations: LC — life cycle, SMS — submicroscopic scales, cocc. — coccoid, fil. — filamentous, h-trich. — heterotrichous, parench. — parenchymatous, siph.-clad. — siphonocladous, IM — isomorphic alternation of generations, HM — heteromorphic alternation of generations.

Codiolales (Ulotrichales). This order unites about 90 species of algae inhabiting predominantly marine environments, although certain representatives are also quite common in fresh continental Water bodies.

Codiolalean algae typically appear as unbranched filaments or profusely branched tufts, and more rarely as solitary uni- or multinucleate cells, or single-layered sheets. Specific features of the order are considered to be, first and foremost, the presence of submicroscopic scales on monad cells and a haplodiplophasic life cycle with a heteromorphic alternation of generations, wherein the sporophyte is represented by the diploid Codiolum stage. Cell walls contain a high amount of pectic substances and usually become slightly mucilaginous.

An example of a coccoid uninucleate alga is the marine genus Chlorocystis. The gametosporophyte of Chlorocystis inhabits the mucous tubes of diatoms or the thalli of macrophytic algae; it possesses spherical uninucleate cells with a parietal, finger-like dissected chloroplast containing a pyrenoid (Fig. 24.41).

Fig. 24.41. Selected representatives of Codiolales: A — Life Cycle of Chlorocystis (1 — gametosporophyte; 2 — gametes; 3 — sporophyte, Codiolum stage; 4 — zoospore of the sporophyte; 5 — zoospore of the gametosporophyte); B — Ulothrix (6 — gametosporophyte; 7 — formation of gametes; 8 — successive stages of gamete copulation; 9 — zygote; 10 — sporophyte, Codiolum stage; 11 — formation of zoospores by the sporophyte; 12 — zoospore of the sporophyte; 13 — formation of zoospores by the gametosporophyte; 14 — zoospore of the gametosporophyte); C — Spongomorpha (16 — fragment of the gametophyte thallus; 17 — hook-like lateral branch – cyroid; 18 — cell; 19 — multinucleate sporophyte "Chlorochytrium") (1–5 — after Kornmann, Sahling, 1983; 6–16 — after Moshkova, 1979; 17 — Kostikov, orig., 18 — after Kornmann, 1964).

Asexual reproduction occurs via 8–32 quadriflagellate zoospores, which, upon coming to rest, grow into a new gametosporophyte. Under unfavorable conditions, Sexual reproduction of the heterogamous type is initiated. Both male and female gametes are biflagellate and formed in separate cells, meaning the alga is dioecious. Following copulation, a zygote with a smooth wall is formed, which begins to develop into the diploid sporophyte — the Codiolum stage: a club-shaped outgrowth forms on one side, into which the greater part of the zygote protoplast migrates. The cell grows and increases markedly in size. Several nuclear divisions then take place, the first of which is meiotic. The daughter protoplasts give rise to 32–64 haploid quadriflagellate zoospores, half of which are male and the other half female. Released into the external environment within a mucous vesicle, the zoospores cease movement and grow into morphologically similar female and male gametosporophytes.

Long, unbranched single-row filaments are characteristic of the gametosporophytes of the genus Ulothrix, species of which are widespread in marine waters, though also quite frequently found in freshwater bodies and even in soils. Each cell of the filament (except for the basal cell, by which the alga attaches to the substrate) possesses a single nucleus and a parietal chloroplast shaped as an open ring with one or several pyrenoids.

Vegetative Reproduction occurs through filament fragmentation. During asexual reproduction, quadriflagellate zoospores are formed within any cell and subsequently develop into a new filament.

The sexual process is isogamous. Any cell is capable of producing A large number of biflagellate isogametes, which are released into the external environment and copulate. The zygote retains its flagella for some time and remains motile, after which it comes to rest, develops a thick wall, and grows into a club-shaped unicellular sporophyte (the Codiolum stage). Following meiotic division, the sporophyte produces four quadriflagellate zoospores that grow into new filamentous gametosporophytes. Similar to Chlorocystis, Ulothrix filaments are dioecious.

In the marine genus Spongomorpha, The life cycle involves an alternation of gametophytes and sporophytes. The gametophytes appear as tufts consisting of short prostrate filaments modified into rhizoids, and long branched erect filaments whose lateral branches curve into hooks, particularly numerous near the Base of the thallus. Due to these hook-like branches, the filaments at the lower part of the thallus are heavily intertwined. Spongomorpha thalli grow through the division of the apical Cells of the filaments.

Each cell possesses a parietal perforated chloroplast with numerous pyrenoids. As in the preceding genera, the pyrenoid is bordered by numerous starch granules. A single nucleus is located in the central part of the cell.

Reproduction occurs sexually and via zoospores. In spring or early summer, the gametophyte cells produce a large number of biflagellate isogametes. Following their copulation, a planozygote is formed, which, after a brief motile period, settles onto the thalli of macroscopic algae (predominantly red algae), partially penetrates them, and develops a cell wall. After some time, it begins to grow into a unicellular diploid sporophyte (the Codiolum stage). During this process, the nucleus divides mitotically several times, the cell grows slowly, and acquires a coenocytic structure. In some species of Spongomorpha, this developmental stage was previously described as independent species of the genus Chlorochytrium. Spongomorpha persists as a multinucleate sporophyte throughout the summer, autumn, and part of the winter season. In the second half of winter, the nuclei of the sporophyte undergo Meiosis, producing a large number of quadriflagellate zoospores that subsequently develop into new heterotrichous gametophytes.

Several parasitic and endophytic algae, described as species of the genus Chlorochytrium, have been discovered within the thalli of marine red algae, living and dead cells of duckweed, fish Skin, and salamander egg cases. However, unlike the "Chlorochytrium" stage in Spongomorpha, they do not form multicellular gametophytes and reproduce solely via zoospores. Some of these species, such as Chlorochytrium lemnae, which reproduces via x-type biflagellate zoospores, belong to the Chlorophyceae. Nevertheless, it cannot be ruled out that some chlorochytrium-like species, especially those inhabiting the thalli of red algae, may turn out to be coenocytic representatives of Codiolales that have lost the capacity for sexual reproduction.

In Monostroma, the gametophyte thallus appears as a single-layered blade with undulating margins, growing up to 10-20 cm in length. Each cell contains a single nucleus and a parietal chloroplast with a pyrenoid surrounded by several starch grains. Reproduction occurs via thallus fragmentation, akinetes, or sexual reproduction. Gametes are also capable of parthenogenetic germination, developing into either new gametophytes or haploid sporophytes. Gametophyte thalli develop from zoospores or parthenogenetically germinated gametes: a motile cell (a quadriflagellate zoospore or biflagellate gamete) attaches to a substrate and divides to form initially a two-layered cell cluster. Subsequently, a cavity forms between the First and Second layers, and the upper layer continues to expand, forming a large bladder-like sphere that eventually detaches. This detached portion constitutes the single-layer laminar thallus of the new gametophyte (Fig. 24.42).

During sexual reproduction, any gametophyte cell produces a large number of biflagellate gametes. The thalli in Monostroma are dioecious: some gametophytes produce small male gametes, while others produce female gametes, which are less motile yet noticeably larger than the male ones. The sexual process is heterogamous. Following copulation, the zygote settles on mollusk shells or other calcium-rich substrates and begins to germinate into the Codiolum stage. In this process, a lateral outpocketing "drills" into the substrate and then widens into a club-shaped thickening, into which the protoplast of the zygote migrates. The sporophyte forms numerous lateral outpocketings, resembling marine boring algae of the genus Gomontia. After some time, meiosis occurs, and several quadriflagellate zoospores emerge from the sporophyte cell, which germinate into new gametophytes.

Fig. 24.42. Selected representatives of Codiolales: A - Monostroma grevillei (1 - mature gametophyte thallus; 2, 3 - surface view of cells (2) and transverse section of the thallus (3); 4 - gametes; 5 - young sporophyte, Codiolum stage; 6 - zoospore formation by a mature sporophyte developing within mollusk shells; 7 - sporophyte zoospore; 8-11 - successive Developmental Stages of the attached gametophyte thallus; 12, 13 - young free-swimming thalli); B - Pseudendoclonium basiliense (14 - mature thallus; 15, 16 - zoospore formation; 17 - zoospore; 18-22 - successive stages of zoospore germination into a new thallus); C - Gloeotilopsis sarcinoidea (23 - zoospores, 24 - diversity of thallus forms (1-3, 12, 13 - after Vinogradova, 1979; 4, 6 - after Kornmann, 1962; 14, 22 - after Vischer, 1926; 15-21 - after Ettl, Gartner, 1995; 23, 24 - after Lukesova, 1991).

If gametes germinate parthenogenetically, they still give rise to the Codiolum stage. However, in this case, the sporophyte is haploid, unlike the "Codiolum" formed after sexual reproduction. It is hypothesized that marine boring algae of the genus Gomontia may have evolved from Monostroma-like species that lost the gametophyte stage.

Molecular studies indicate a evolutionary trend within the order toward freshwater and terrestrial habitats, which is associated with the secondary loss of sexual reproduction. The transition to exclusively asexual reproduction occurred both through the loss of gamete formation capabilities and through the loss of gamete copulation ability leading to parthenogenesis. For instance, based on the Analysis of the nuclear Gene encoding 18S rRNA, Codiolales include agamic soil and freshwater algae of the genera Pseudendoclonium and Gloeotilopsis, which exhibit the first and second variants of agamospermy/agamy, respectively.

Pseudendoclonium features thalli shaped as short, branched filaments without a distinct differentiation into prostrate and erect systems. Each cell contains a single nucleus and a parietal, slit-perforated chloroplast with a pyrenoid. Reproduction occurs exclusively via thallus fragmentation and quadriflagellate zoospores. Gloeotilopsis is structurally similar to the previous genus in thallus and cell organization, but reproduces via biflagellate zoospores. According to all currently known molecular phylogenetic reconstructions, species of the latter genus descend from forms with a sexual cycle, quadriflagellate zoospores, and gametes capable of parthenogenetic development into new gametosporophytes.

Acrosiphonales. This order is phylogenetically close to Codiolales, differing from it by its siphonocladous structural type and the absence of submicroscopic scales On the surface of monad cells. Like codiolalean algae, acrosiphonalean algae exhibit a haplodiplontic life cycle with heteromorphic alternation of generations.

Gametosporophyte thalli appear as simple or branched uniseriate filaments. Each cell contains a reticulate chloroplast with numerous pyrenoids surrounded by multiple starch granules, along with several nuclei. Thallus growth occurs via a specialized variant of segregative Cell Division: prior to cytokinesis, nuclei migrate to the future cell plate plane, divide once synchronously, and upon completion of the cross-wall formation, become randomly distributed within the daughter cell protoplasts. Sporophytes are unicellular and, similarly to the previous order, represent the Codiolum stage.

In the marine alga Urospora, gametosporophytes are unbranched and appear as long filaments up to 0.1 mm wide. The filaments attach to the substrate via basal cells modified into rhizoids. Each cell of the filament, with the exception of the basal one, is capable of producing either biflagellate anisogametes or quadriflagellate zoospores (Fig. 24.43).

Fig. 24.43. Selected representatives of Acrosiphonales: A - Urospora: 1-3 - gametosporophyte: basal (1), central vegetative (2), and reproductive (3) PARTS OF THE gametosporophyte; 4 - multinucleate vegetative cell with a reticulate chloroplast containing pyrenoids; 5 - zoospores; 6 - male and female gametes; B - Acrosiphonia: 7 - general appearance of the thallus; 8 - thallus fragment with empty gametangia; 9 - empty gametangia with an aperture for gamete release, gametangium, and vegetative cell; 10 - ULTRASTRUCTURE OF THE pyrenoid; 11, 12 - mitosis and cytokinesis: nuclei positioned in the future cell plate plane (11), upon completion of mitosis and cytokinesis, they synchronously migrate deeper into the cell (12) (1-3, 7-9 - after Vinogradova, 1979; 4-6, 10 - after Lokhorst, Trask, 1981; 11, 12 - after Kornmann, 1965).

The life cycle of Urospora is identical to that of Ulothrix. Urospora gametes are also capable of parthenogenetic germination into a haploid gametosporophyte.

Bushy thalli composed of branched filaments are characteristic of siphonocladalean algae of the genus Acrosiphonia. Cell Structure, cytokinesis, reproduction, life cycle, and habitats in species of this genus are practically identical to those of Urospora.

Ulvales. The order comprises about 200 species of macroscopic algae with heterotrichous or primitive tissue-like organization, inhabiting mainly marine environments. Phylogenetically, the order is close to Codiolales and Acrosiphonales, differing from them by the absence of the Codiolum stage in the life cycle and the presence of isomorphic alternation of generations. Similar to Acrosiphonales, the monad cells of ulvacean algae lack submicroscopic scales.

An example of a heterotrichous ulvacean alga is Acrochaete. Species of this genus are marine periphytic organisms. Gametophyte and sporophyte thalli are morphologically indistinguishable. They are prostrate, consisting of branched uninucleate filaments. In the central part, the filaments partially coalesce to form an irregular pseudoparenchymatous plate. Short rhizoidal filaments extend from the plate, anchoring the alga to the substrate, alongside assimilatory filaments that partially rise above the substrate. Occasionally, assimilatory filaments terminate in unicellular, colorless hairs (setae) bulbously expanded at the base. Each cell (except for setae) contains a single nucleus and a girdle-shaped chloroplast with one or more pyrenoids surrounded by two starch shell Valves. In the cells of assimilatory filaments, the chloroplast is large and occupies most of the protoplast, whereas in rhizoidal cells, the chloroplast is mostly reduced to a small plate (Fig. 24.44).

Fig. 24.44. Heterotrichous representatives of Ulvales: A - Acrochaete: 1 - thallus within the tissue of the brown alga Chorda; 2 - filament fragment with a Hair; 3 - gametangia; 4 - heterogametes and their copulation; 5 - zoosporangium; 6 - zoospore; B - Entocladia: 7 - thallus on The surface of a macroalgal host; 8 - thallus fragment with zoosporangia (after Vinogradova, 1979).

Reproduction occurs via filament fragmentation, zoospores, or sexual reproduction. In the cells of the sporophyte assimilatory filaments, following meiotic reduction division, quadriflagellate zoospores are formed, which emerge into the external environment through a pore in the cell wall and germinate into gametophytes. Gametophytes are monoecious—different cells of the same thallus transform into male or female gametangia. Following the release of gametes into the environment, a heterogamous sexual process takes place, and the zygote germinates directly into a new sporophyte without a resting period.

Species of the genus Entocladia resemble Acrochaete in morphology and life cycle, but lack setae and occur not only in marine waters but also in freshwater habitats, typically as epiphytes on other multicellular green algae (such as Cladophora, Rhizoclonium, Oedogonium).

In the genus Ulva—the green sea lettuce—the thalli are parenchymatous, appearing as a two-layered blade with undulating margins, measuring several centimeters in length. Each cell contains a single nucleus and a parietal plate-like chloroplast with one to several pyrenoids. Gametophyte thalli develop from quadriflagellate zoospores, while sporophyte thalli develop from freshly formed zygotes that retain the gamete flagella (planzygotes): the cell attaches to the substrate, loses its flagella, and initially divides in a transverse plane to form a uniseriate unbranched primary filament. Subsequently, the filament cells divide longitudinally-radially, forming a single-layered tube with a central cavity. The tube walls close together, and the marginal cells continue to divide, resulting in the formation of a two-layered blade with folded margins (Fig. 24.45).

Fig. 24.45. Development of the thallus in various ulvacean algae with a parenchymatous morphological body structure: 1 - zoosporangium or gametangium; 2 - zoospore or planzygote; 3 - initial stage of thallus development; 4 - uniseriate filament stage; 5 - multiseriate filament stage; 6 - tubular stage with a cavity; 7 - partial fusion of the tube walls forming a blade; 8 - two-layered blade detaching from the prostrate portion of the thallus.

The life cycle is haplodiplontic, featuring a regular isomorphic alternation of generations and sporic reduction of chromosome number.

In genera closely related to Ulva, such as Enteromorpha, Percusaria, Blidingia, and Ulvaria, the thalli are elongate- laminar, tubular, or appear as multiseriate filaments or single-layered blades (Fig. 24.46). Regarding their life cycle type, cell structure, and reproduction, all these genera are similar to Ulva. The differences lie in the developmental stage at which thallus growth ceases (Fig. 24.45).

Fig. 24.46. Representatives of Ulvales with a parenchymatous morphological structure type. 1 – Percursaria; 2, 3 – Blidingia: thallus surface (2) and transverse section (3); 4–8 – Enteromorpha: tubular (4) and lamellar (5) thalli, transverse sections of the thallus in the bilayered plate zone (6), middle part with a large cavity (7), and basal part (8); 9–15 – Ulva: general appearance (9) and transverse section (10) of the thallus, zoosporangium (11), zoospore (12), gametangium (13), isogamete copulation (14), planzygote (15) (1, 6 – after Vinogradova, 1979; 3–5, 7–10 – after Moshkova, 1979; 11–15 – after Koeman, van den Hoek, 1981).

Specifically, in Enteromorpha, thallus development ceases when the walls of the tube fuse at the apex. As a result, the young alga resembles an intestine (hence its common name), while the mature alga features a single-layered tubular structure with a cavity in its lower part and an elongated bilayered blade in the upper part.

In Blidingia, development terminates at the tubular stage without the walls fusing. Percursaria thalli mature immediately following longitudinal-radial division of the primary filament cells, typically without forming a central cavity; consequently, the alga develops into a multiseriate filament. The genus Ulvaria passes through primary filament and tube stages. Subsequently, the upper portion of the tube ruptures and flattens out, while the cells continue to divide. This leaves the tube structure intact at the base of the thallus, whereas a single-layered blade forms above; if this blade detaches from the lower section, it assumes the morphological appearance of an adult Monostroma.

Representatives of these genera are typical inhabitants of marine coasts; however, certain species of Enteromorpha and Percursaria (P. percursa, E. intestinalis, E. prolifera) are also found in continental freshwater bodies.

Cladophorales (Siphonocladales). This order comprises over 400 species across 32 genera. The vast majority of cladophoralean algae morphologically resemble representatives of Acrosiphonales, yet phylogenetically they constitute a distinct, well-separated group. Cladophorales differ from other orders of Ulvophyceae by a complex set of both biochemical and cytological characteristics.

Cladophoralean algae contain an additional specific xanthophyll, siphonoxanthin, which is also found in Prasinophyceae and Siphonophyceae. Cell walls are composed of parallel cellulose fibrils synthesized by linear terminal enzyme complexes (Fig. 24.47). Aside from Cladophorales, such complexes have been discovered only in certain species of the genus Chlorella. Pectic substances are absent from the cell walls, rendering the cell surface typically rough and layered, which provides a suitable substrate for the attachment of various epiphytic algae and Protozoans.

Fig. 24.47. Cladophoralean algae: A – cell surface view (left) and optical section (right); B – diagram of protoplast structure; C – bilesenticular pyrenoid; D – linear terminal enzyme complex (1 – plasmalemma, 2 – linear array of enzyme subunits, 3 – cellulose microfibril) (A, B – orig.; C – after Hori, Ueda, 1975; D – after Itoh, Brown, 1984).

In monadoid cells, the apical partition connecting the basal bodies is not smooth but transversely striated, resembling that of Trebouxiophyceae. Vegetative cells are multinucleate, possessing a parietal reticulate chloroplast or numerous small disc-shaped METABOLISM/14.html">Chloroplasts interconnected by extremely thin plasmodesma-like strands; small pyrenoids are located within the chloroplasts. Each pyrenoid is traversed in its equatorial plane by thylakoids, dividing it into two hemispheres surrounded by a continuous hemispherical starch plate (Fig. 24.47).

The protoplast of siphonocladalean algae exhibits a complex architecture: chloroplasts are "anchored" near specific Regions of the plasmalemma by microtubule bundles; a single nucleus is situated beneath each isolated chloroplast region (or individual chloroplast). Each nucleus, in turn, is surrounded by radially arranged microtubules, some of which anchor to the outer membrane of the chloroplast.

Mitosis occurs synchronously across different nuclei within a single cell; however, cytokinesis and karyokinesis are uncoordinated, and the nuclei never align in the plane of transverse septum formation. During mitosis, a single dense chromatin plate is observed rather than distinct individual chromosomes.

During the formation of zoospores or gametes, the protoplast of the mother cell simultaneously breaks down into numerous uninucleate daughter protoplasts that will become the future spores or gametes. In this process, the plasmalemmas of the daughter cells are formed from Golgi apparatus vesicles without significant participation of the maternal plasmalemma.

The life cycle is haplodiplontic, with an isomorphic alternation of generations. As in the preceding orders, zoospores are predominantly quadriflagellate, and gametes are biflagellate.

The simplest representatives of the order possess thalli structured as systems of branched vesicles (genus Valonia), whereas more complex forms consist of a central axis and a system of connected lateral segments (Siphonocladus). In some taxa, these lateral segments fuse together to form pseudoparenchymatous structures (Anadyomene) (Fig. 24.48). All these representatives occur in tropical and subtropical marine waters.

Fig. 24.48. Selected representatives of Cladophorales. 1 – Valonia; 2 – Siphonocladus; 3 – Anadyomene; 4–6 – Rhizoclonium: basal part of thallus (4), short lateral branch (5), filament fragments (6); 7–10 – Chaetomorpha: general appearance (7), basal and central parts of filament (8, 9), filament fragment with sporangia (10) (1 – after Vinogradova, 1977; 2 – after Vickers, Shaw, 1908; 3 – after Masiuk, 1993; 4–10 – after Moshkova, 1979).

A significant number of cladophoralean algae feature thalli in the form of unbranched filaments or branched tufts. Representatives of this group are widespread in both marine and continental freshwater habitats.

The genus Rhizoclonium appears as long, unbranched filaments. Each cell contains a small number of nuclei—typically no more than four. Young individuals live attached, anchoring to the substrate via colorless rhizoids. Mature filaments are generally free-floating, forming dense, non-slippery mats where cell widths range from 30 to 50 µm and individual filaments reach lengths of up to several meters. The sporophyte of Rhizoclonium produces biflagellate zoospores, while the gametophyte produces biflagellate anisogametes. The most widespread species in freshwater habitats is R. hieroglyphicum, and in marine environments, R. riparium.

The thalli of Chaetomorpha are likewise unbranched and filamentous. However, the number of nuclei per cell reaches several dozen, and filament widths measure 0.1–0.3 mm. Asexual reproduction occurs via quadriflagellate zoospores and akinetes, while sexual reproduction involves biflagellate isogametes. The most common representative of the genus, Ch. linum, inhabits marine waters and coastal lagoons.

The leading genus of the order is Cladophora. The thalli of Cladophora are tufted, consisting of multinucleate cells that form systems of branched rhizoidal and upright filaments. Cells are typically covered with thick, layered cell walls that serve as attachment sites for numerous epiphytic organisms.

Reproduction occurs via thallus fragmentation, zoospores, akinetes, and an isogamous sexual process.

Meiosis takes place within the apical cells of the sporophyte, resulting in The production of numerous quadriflagellate zoospores. The zoospores escape into the external environment through an opening in the sporangium wall and germinate into gametophytes. The gametophytes of Cladophora are dioecious. Male and female gametes are morphologically indistinguishable, yet they differ physiologically and in the ultrastructure of the cell's apical region. Specifically, female gametes possess a specialized cap beneath the plasmalemma at the apex, which participates in the Formation of the conjugation canal during copulation. Although any vegetative cell in the upright portion of the thallus is capable of transforming into a gametangium, gamete formation typically initiates in the apical cells of the thallus. Gametes are released through an opening in the wall and copulate with gametes of the opposite mating type. Following copulation, the zygote settles onto the substrate and immediately begins germinating into a new sporophyte without a resting period.

Fig. 24.49. Cladophora: thallus fragment (1), zoosporangium and zoospore release (2) (after Vinogradova, 1977).

Gametes of Cladophora (as well as some other cladophoralean algae) are capable of parthenogenesis, developing into either sporophytes or gametophytes. This feature results in a rather diverse array of specific life cycles within the genus, which nevertheless derive from the basic haplodiplontic cycle with sporic meiosis.

The most widespread species in temperate and warm seas is Cladophora vagabunda. In freshwater bodies, the most common species is C. glomerata. Morphologically, these two species are indistinguishable; however, the former exhibits a haplodiplontic life cycle with alternation of generations, whereas the latter lacks sexual reproduction, with asexual reproduction occurring via biflagellate zoospores. Special studies have shown that the zoospores of C. glomerata are likely gametes that have lost The ability to copulate while retaining the capacity for parthenogenetic development. Thus, the vegetative thalli of C. glomerata are haploid gametophytes.

Paper is manufactured from Cladophora and Rhizoclonium in some countries, and in the Middle East, locals consume Cladophora as food.

Trentepohliales. This order comprises exclusively terrestrial heterotrichous algae which, much like the cladophoralean algae, possess a whole complex of specific features.

A striking biochemical feature is the ability to accumulate large amounts of hematochrome in vegetative cells, which typically gives the thalli of trentepohlialean algae a bright orange color. This hematochrome is essentially β-carotene dissolved in oil, along with an Abundance of polyhydric alcohols—polyols and alditols. The latter have been found exclusively in representatives of this order. Another characteristic feature is the presence of a sporopollenin layer in the cell wall. It is believed that hematochrome increases cytoplasmic viscosity and protects cells from intense light and excessive moisture loss, while sporopollenin imparts chemical resistance to the cell walls and prevents mechanical damage.

Cytological features are associated with the photosynthetic apparatus, cytokinesis, and the Cytoskeleton. Thus, pyrenoids are absent in Trentepohliales, and starch is synthesized in negligible amounts, being deposited as small granules within the chloroplast stroma.

Cytokinesis proceeds with the participation of a phragmoplast: in telophase, Golgi-derived vesicles containing cellulose microfibrils accumulate between the spindle microtubules in the plane of the future cell plate. These vesicles fuse with one another and with The Plasma Membrane of the nascent cleavage furrow, driving both the inward growth of the furrow and the formation of a cell plate perforated with true pores and plasmodesmata. This type of cytokinesis resembles that of charophytes and zygnematalean algae; however, unlike in the latter two classes, mitosis is closed.

Trentepohlialean algae possess a specialized flagellar apparatus: each flagellum bears a unilateral vane, and each flagellar root contains a doubled number of microtubules, so that the root system formula is not 3-2-3-2 like in other orders of the class, but 6-4-6-4. The tips of the thin microtubular roots feature a columnar structure resembling the multilayered structure found in certain prasinophytes and charophytes. The upper basal body connector is smooth, yet poorly developed compared to other orders and is considered reduced.

The thalli of trentepohlialean algae appear as branched filaments or as unistratose to multistratose parenchymatous plates.

Reproduction occurs via filament fragmentation, akinetes, quadriflagellate zoospores, and sexually. The life cycle is haplodiplontic, with an isomorphic alternation of generations, wherein zoospores are produced by both the gametosporophyte and the sporophyte. The sexual process is isogamous, mediated by biflagellate gametes, which typically are also capable of parthenogenetic development.

All trentepohlialean algae inhabit the aerophytic niche and are either epiphytes developing on tree bark or parasites of various woody plants.

The most widespread genus is Trentepohlia, species of which frequently occur on the bark of deciduous trees, causing red aerophytic "bloom". The thalli of Trentepohlia appear as short, branched, prostrate filaments, with upright filaments being reduced (Fig. 24.50). In many species, the terminal cells are noticeably enlarged and function either as zoosporangia or gametangia. Typically, upon preparing water mounts, one can observe gametes or zoospores being released from the terminal cells within a few minutes.

Fig. 24.50. Representatives of Trentepohliales. A — Trentepohlia: 1 — thalli, 2 — gametangia, 3 — gametes, 4 — zoosporangium, 5 — zoospore; B — Phycopeltis: 6 — thallus, 7 — gametes, 8 — zoosporangia; C — Cephaleuros: 9 — thallus, 10 — single- and double-layered areas of the thallus on the surface of an infected leaf, zoosporangia, and rhizoidal filaments; 11 — zoospore; 12 — vegetative cells (1–8 — after Moshkova, 1979; 9–12 — orig.).

In the genus Phycopeltis, species of which can sometimes be found in greenhouses with tropical plants, the thallus forms a unistratose parenchymatous plate bearing short upright filaments tipped with sporangia.

The epiphytic and endophytic alga Cephaleuros features a thallus shaped as a multistratose plate that gives rise not only to upright filaments with sporangia, but also to rhizoids. Cephaleuros grows on the leaves of tropical and subtropical trees and shrubs; instances are known where species of this genus caused epiphytotics in coffee, citrus, and tea plantations.

By the presence of a phragmoplast and a columnar structure on the flagellar roots, the Trentepohliales resemble primitive members of the Charophyceae; however, molecular phylogenetic reconstructions indicate that this order nevertheless represents an independent, albeit highly specialized, branch of the Ulvophyceae.