Algology - Kostikov, I.Yu. - 2009-2013

Chapter 24. Green Algae – Chlorophyta

24.2. The Chlorophytic Lineage

24.2.2. Class Trebouxiophyceae

This Class was established relatively recently, in the 1980s. It was then, based on Electron Cell/15.html">Microscopy data, that the hypothesis was put forward regarding the existence of an independent evolutionary Lineage within the Chlorophyceae class, initially designated as the order Pleurastrales, subsequently as the class Pleurastrophyceae, and finally as the class Trebouxiophyceae. Today, this class comprises about 250 species of coccoid, sarcinoid, filamentous, and heterotrichous Algae that inhabit primarily non-aquatic ecotopes—in soils, aerophytic habitats, as components of Lichens, and more rarely as symbionts of freshwater invertebrates (most commonly Ciliates and Sponges). The class represents a lineage of green algae characterized by cytokinesis mediated by a phycoplast, flagellar basal bodies offset in a counter-clockwise direction with an 11-5 orientation, a cruciate flagellar ROOT system, and monadoid Cells that never bear submicroscopic scales.

Features and CHARACTERISTICS OF THE Class

Biochemical Features

The composition of pigments and assimilation products is identical to that of representatives of Chlorophyceae; thus, no specific biochemical features are unique to this class.

Cytological Features

Cell Coverings. The cells of trebouxiophytes are always covered by a cellulosic-pectic wall, which typically has a three-layered Structure and, in many representatives (such as Chlorella), contains an additional layer of sporopollenin. Monadoid cells, represented solely by zoospores and occasionally by Gametes, are always bounded only by the Plasmalemma; upon coming to rest, they assume a spherical shape and belong to the p-type.

Submicroscopic scales are absent on both the cells and flagella of trebouxiophyte algae.

Nuclear Apparatus and Mitosis. Mitosis within the class is semi-open (in most species) or closed (in prasiolalean algae); the spindle apparatus typically breaks down rapidly in telophase (the exception being prasiolalean algae, in which the telophase spindle is persistent). Centrioles in trebouxiophytes are present only in zoospore-producing species; they are absent in autosporic representatives. When centrioles are present, they do not take direct part in The formation of the mitotic spindle, yet they play a crucial role in constructing the phycoplast.

Cytokinesis proceeds via a Cleavage furrow, which always begins to form from only one side of The Cell. In coccoid and sarcinoid forms, upon completion of cytokinesis, the daughter protoplasts produce their own cell walls while still inside the mother cell, thus making the division correspond to schizogony. In filamentous and heterotrichous species, vegetative division represents true cytotomy (Fig. 24.32).

Fig. 24.32. Cell Division in the sarcinoid alga Friedmannia israeliensis (A) and the heterotrichous alga Leptosira obovata (B): 1, 5-7 - prophase; 2, 8, 9 - metaphase; 3, 10, 11 - telophase; 4, 12 - early interphase; я - nucleus, хл - chloroplast, ц - centrioles (after Lokhorst, Segaar, Star, 1989; Molnar, Stewart, Mattox, 1975).

Flagellar Apparatus. Monadoid cells are represented by zoospores and occasionally by gametes, possessing 2 (rarely 4) isocont and isomorphic flagella. Typically, monadoid cells are dorsiventral and longitudinally flattened.

The transition zone of the flagella contains a stellate structure; spiral ribbons and cylinders are absent in the transition zone. The basal bodies are relatively short, offset in a counter-clockwise direction (11-5 configuration), and overlap each other quite noticeably. The upper cap connecting the basal bodies is transversely striated (Fig. 24.33).

Fig. 24.33. Diagram of The structure of the flagellar root system in trebouxiophyte algae (according to micrographs by Deason, Floyd, 1987).

The Root System is cruciate and generally conforms to the 4-2-4-2 or 5-2-5-2 formula. At the tips of the thick roots, a so-called columnar structure has been detected, which somewhat resembles the multilayered structure of prasinophytes and charophytes. Rhizoplasts have not been reliably identified, although certain representatives exhibit a structure corresponding to a rudimentary transversely striated microfibrillar root.

Types of Morphological Structures

The class encompasses algae representing four types of morphological structures: coccoid, sarcinoid, filamentous, and heterotrichous.

Coccoid algae are the predominant group. Their cells are typically spherical and solitary, generally forming neither colonies nor coenobia. Sarcinoid algae are few in number, typically forming thalli in the shape of packets (Pleurastrosarcina) or tetrads (Friedmannia).

Filamentous algae are fairly numerous, typically forming short filaments that easily break apart into individual cells (e.g., Stichococcus, Diplosphaera), or long filaments with a tendency toward a bi- or multiseriate habit (Prasiola, Schizogonium). Heterotrichous forms possess a well-developed system of branched upright filaments, whereas the prostrate filaments are heavily reduced, typically represented by just a single basal cell (Leptosira, Microthamnion).

Reproduction and Life Cycles

For coccoid representatives, the primary mode of reproduction is asexual via aplanospores and, more rarely, zoospores. Filamentous and heterotrichous forms reproduce predominantly vegetatively—through cell division followed by filament fragmentation—and only under exceptional conditions by means of zoospores and aplanospores. Sexual reproduction, represented by oogamy, occurs in some prasiolalean algae (Prasiola). Algae with a sarcinoidean structure type reproduce both by the fragmentation of multicellular aggregates (packets, tetrads, and complexes thereof) and by zoospores and aplanospores.

The predominant type of life cycle is a simple cyclomorphosis. In species of the genus Prasiola that exhibit a sexual process, The life cycle is haplo-diplophasic, with an isomorphic Morphology/12.html">ALTERNATION OF GENERATIONS and somatic reduction.

System of the Class

Since the class was established relatively recently, and primarily on The basis of cytological and molecular-biological criteria, its taxonomic system is still imperfectly developed. Today, based on the Analysis of the nuclear Gene encoding the 5S and 18S subunits of ribosomal RNA, up to 10 independent evolutionary lineages have been identified within the class, which likely correspond to taxa at the rank of order. However, only four lineages have been granted ordinal status, while two lineages have provisional names that do not meet the requirements of the International Code of Botanical Nomenclature (specifically, lacking Latin diagnoses and types). The remaining lineages are treated as groups of uncertain taxonomic position.

The systematic criteria for delimiting the established orders based on phenotypic characters are, first and foremost, The ability to reproduce via zoospores, the type of morphological structure, the presence of cell or thallus heteropolarity associated with a sessile lifestyle, the type of mitosis, and the presence or absence of a persistent telophase spindle (Table 24.4).

Table 24.4. Main diagnostic characters of the orders of Trebouxiophyceae

Notes: SEM – semi-open mitosis, CM – closed mitosis. Asterisk (*) denotes taxa named without considering the requirements of the ICBN and thus taxonomically invalid.

Chlorellales. This order comprises coccoid, predominantly spherical autosporic algae characterized by semi-open mitosis and a spindle that disintegrates in telophase. Cells of chlorellalean algae are not heteropolar.

Typically, species of Chlorellales are very small (up to 8–10 µm) and reproduce by means of 2–8 autospores released following the rupture of the sporangial wall. Both photoautotrophic (Chlorella) and obligately heterotrophic (Prototheca) forms are known among the representatives of this order.

Mature cells of Chlorella are spherical, containing a single nucleus and a parietal chloroplast with a pyrenoid (Ch. vulgaris) or lacking one (Ch. minutissima) (Fig. 24.33).

Species of the genus Chlorella are a characteristic component of subaerial coenoses. They can be found in soils, on tree bark, and more rarely in aquatic biocenoses, typically those polluted with organic matter. Within the genus, There is a trend toward a symbiotic lifestyle: in particular, some Chlorella species live inside the cells of ciliates and sponges, or within lichen thalli. In many countries, Chlorella species are cultivated industrially as a raw material for vitamin supplements or health foods.

Prototheca is a colorless heterotrophic alga morphologically similar to Chlorella. It reproduces via 2–8 autospores. Prototheca typically parasitizes mammals (including humans), causing characteristic Skin infections known as protothecosis. In a free-living state, it can be found in subaerial coenoses—on tree bark, damp walls, and occasionally in soil, provided the substrate contains sufficient amounts of dissolved organic matter.

Trebouxiales. This order encompasses coccoid and sarcinoidean algae that share systematic features with Chlorellales, yet are capable of reproducing via zoospores. All trebouxiacean algae occur exclusively in subaerial coenoses. The principal genera of the order are Trebouxia, Myrmecia, and Friedmannia (Fig. 24.34).

Fig. 24.34. Some characteristic representatives of Chlorellales (1–3) and Trebouxiales (4–6): 1 – Chlorella vulgaris; 2 – Chlorella minutissima; 3 – Prototheca sp.; 4 – Trebouxia crenulata; 5 – Myrmecia macronucleata; 6 – Friedmannia israeliensis (1–3, 5 – Kostikov, orig.; 4 – Mykhaylyuk, 2000; 6 – after Chantanachat, Bold, 1962).

Trebouxia features spherical cells with a central stellate or axial chloroplast, a pyrenoid, and a single nucleus. Reproduction takes place via naked biflagellate zoospores and aplanospores. The genus comprises about 50 species, which are among the most typical lichen phycobionts.

Myrmecia resembles the preceding genus in morphology and reproduction, but possesses a parietal lobed chloroplast lacking a pyrenoid. Species of this genus are found both as lichen phycobionts and in a free-living state in soils and on rocky substrates within aerophytic habitats.

A distinctive feature of Friedmannia is its ability to form cell tetrads and complexes consisting of tetrads, meaning this alga exhibits a sarcinoidean structure type. The cellular ultrastructure in Friedmannia is identical to that of Myrmecia. Reproduction occurs via naked biflagellate zoospores, aplanospores, or vegetatively through the dissociation of tetrads into individual cells.

Stichococcales. This order includes algae whose cells are capable of dividing in one or two planes to form short filaments. All known Representatives of the order are widespread aerophytic and soil algae.

Algae of the genus Stichococcus form short uniseriate filaments that readily break apart into single uninucleate cells. Each cell contains a parietal disk-shaped chloroplast lacking a pyrenoid; typically, a single oil droplet is present at each cell pole. Reproduction is exclusively vegetative; neither asexual nor sexual reproduction is known. Species of this genus are most frequently found on tree bark and The surface of fruit bodies of old bracket Fungi, quite often causing their "bloom". More rarely, Stichococcus is found in soils and on rocky substrates in aerophytic habitats (Fig. 24.35).

Fig. 24.35. Some characteristic representatives of Stichococcales: 1, 2 – Stichococcus bacillaris; 3, 4 – Diplosphaera chodatii; 5 – Gloeotila spiralis (1, 5 – after Chodat, 1902, 1913; 3 – after Vischer, 1945; 2, 4 – Kostikov, orig.).

Species of the genus Gloeotila, although resembling Stichococcus in having cylindrical cells with a parietal chloroplast lacking a pyrenoid, differ from the latter by forming long filaments that generally do not disintegrate into single cells. Interestingly, molecular data indicate that out of approximately 15 species described in this genus, only those lacking zoospores and inhabiting subaerial ecotopes belong to the Trebouxiophyceae.

Cells of the genus Diplosphaera are capable of dividing in two planes. As a result, not only uniseriate filaments are formed, but also packet-like three-dimensional complexes.

Leptosirales. This order comprises algae with a heterotrichous structural type that reproduce both vegetatively and via p-type zoospores and aplanospores. Like the representatives of the preceding orders, leptosiralean algae inhabit primarily non-aquatic biotopes.

Species of the genus Leptosira possess thalli consisting of short ascending filaments and a well-developed system of prostrate filaments, which frequently form parenchymatous sheets (e.g., L. terrestris, L. obovata). In aquatic cultures, they produce numerous biflagellate zoospores that, upon germination, attach to the surface Water film and grow into short two- or three-celled filaments capable of leading a neustonic lifestyle for an extended period. Further Development of the filaments into multicellular thalli occurs when the cells settle onto a substrate above the water line, thus transitioning to an aerophytic lifestyle.

Several obligately freshwater periphytic species have also been described within the genus Leptosira; however, all these species remain uninvestigated cytologically and molecular-biologically, and their affiliation with the Trebouxiophyceae is not yet definitively proven.

It is hypothesized that following appropriate molecular and electron microscopic studies, the group of leptosira-like trebouxiophycean algae will also incorporate many other aerophytic heterotrichous algae (e.g., agents of aerophytic "bloom" on tree bark and rocky substrates belonging to the genera Desmococcus, Apatococcus, Pseudopleurococcus, and others), which are classified under the Chlorophyceae in the classical system, yet are regarded as taxa of uncertain systematic position in the modern system (Fig. 24.36).

Fig. 24.36. Selected representatives of Leptosirales (1-3) and Microthamniales (4-6): 1 - Leptosira obovata; 2 - Apatococcus lobatus; 3 - Desmococcus olivaceus; 4-6 - Microthamnion kuetzingianum (4 - mature thallus, 5 - release of zoospores, 6 - Initial Stages of thallus development) (1, 4-6 - after Moshkova, 1979; 2 - Kostikov, orig.; 3 - after Korshikov, 1953).

Microthamniales. This order includes coccoid and branched filamentous algae capable of developing in both aquatic and terrestrial environments, thus functioning as amphibious organisms.

A specific feature characteristic of the order's representatives is the polarity of their thalli. For instance, in the coccoid alga Fusochloris perforata, which typically inhabits snow, the cells are differentiated into a rounded apical part and an elongated, narrowed basal part.

In the heterotrichous algae of the genus Microthamnion, the thallus consists of a basal cell forming an attachment pad—somewhat resembling the cells of Fusochloris—and a system of branched ascending filaments. Each cell is uninucleate and contains a single parietal chloroplast lacking a pyrenoid (Fig. 24.36).

Both genera reproduce primarily asexually through naked biflagellate zoospores.

Prasiolales. This order unites filamentous algae that exhibit a tendency toward forming multiseriate filaments resulting from cell divisions in both transverse and longitudinal directions. The cells feature a central chloroplast with an inconspicuous pyrenoid. Reproduction occurs via thallus fragmentation or aplanospores. An oogamous type of sexual process has been discovered in certain species. The diagnostic feature that clearly distinguishes prasiolalean algae from other Trebouxiophyceae groups is the presence of closed mitosis and the prolonged persistence of the mitotic spindle in telophase.

Species of the Prasiolales occur in non-aquatic coenoses, typically on substrates rich in ammonium compounds and potassium salts—such as guano in bird colonies, organically polluted soil, etc.

The genus Schizogonium is characterized predominantly by uniseriate filaments; only in certain Regions of the thallus are cells capable of longitudinal division, forming short bi- or triseriate segments (Fig. 24.37).

Fig. 24.37. Characteristic representatives of Prasiolales. 1, 2 - Schizogonium murale; 3 - Prasiola velutina (1, 2 - Masiuk, Kostikov, 1984; 5 - after Lokhorst, Starr, 1988).

Conversely, in species of the genus Prasiola, the majority of cells are capable of division in two planes, resulting in the formation of multiseriate laminate thalli. Sexual reproduction is known in some species.

Thus, in P. stipitata, the vegetative thallus is diploid. In preparation for sexual reproduction, a group of marginal thallus cells undergoes reductional division, followed by a series of mitoses. As a result, a large zone of haploid vegetative cells forms on the diploid thallus, meaning the reductional division is somatic. One fraction of these haploid cells subsequently differentiates into antheridia, which give rise to biflagellate spermatozoids; the other fraction develops into oogonia, within which non-motile spherical egg cells are formed. Upon maturation, in the presence of free moisture, the spermatozoids and egg cells are released into the external environment where copulation takes place (Fig. 24.38).

Fig. 24.38. Life Cycle of Prasiola stipitata. Asp - aplanospore; R! - reductional division; n, 2n - Haploid and Diploid cells, respectively; dk - dikaryotic planzygote; Z - mature zygote (after Friedmann, 1959, 1964, 1969).

The copulation process in Prasiola is unique: a spermatozoid swims up to an egg cell, penetrates it with a single flagellum, and then fuses with the egg cell along one side. Karyogamy does not occur immediately, leaving the zygote binucleate. The zygote retains a single flagellum—specifically the one that did not penetrate the egg cell. For several days, the zygote is capable of slow locomotion utilizing this flagellum. Once movement ceases, the nuclei within the zygote fuse immediately, and without any resting period, it begins to germinate into a new diploid thallus.