Algology - Kostikov I.Yu. - 2009-2013

Chapter 24. Green algae – Chlorophyta

24.2. Chlorophytan lineage

24.2.4. Class Siphonophyceae

This Class unites marine Algae exclusively with a siphoneous type of body morphological Structure. Similar to Ulvophyceae, the class represents an evolutionary Lineage of green algae characterized by a cruciate flagellar ROOT system, basal bodies shifted counterclockwise, and the absence of a phycoplast. The oldest fossil remains of siphonophyceans date back to the Precambrian, approximately 1.2 billion years ago.

The class comprises about 200 extant and over 300 extinct species.

Features and CHARACTERISTICS OF THE class

Biochemical features

A characteristic biochemical feature of the class is the presence of specific xanthophylls, siphonaxanthin and siphoxanthin (note: siphonein and siphonaxanthin). Along with starch, the fructose polymer polysaccharide inulin accumulates as a reserve nutrient. The Cellulose of The Cell walls consists of mannan or Xylan rather than glucan, as is the case in other classes of Chlorophyta (though glucans are found in trace amounts in bryopsidalean algae, they do not form cellulose microfibrils in cell walls).

Cytological features

Thalli exhibit a siphonal Organization, except in early developmental stages. The center of the thallus is occupied by a large vacuole containing cell sap, while the periphery consists of a thin layer of Cytoplasm containing METABOLISM/14.html">Chloroplasts, nuclei, and other Organelles (Fig. 24.51).

A specific feature of siphonophyceans is the presence of an ordered cytoplasmic streaming. The cytoplasm moves along specific pathways, carrying chloroplasts and nuclei along with it. This process can be observed under a standard Light Microscope at a velocity of 60–120 µm/min. Movement occurs with the participation of Actin and tubulin. The addition of cytochalasin B, a specific actin inhibitor, to the medium causes the movement to cease. The involvement of microtubules in this process has been proven using immunofluorescence Microscopy. Active cytoplasmic streaming has also been discovered in charophycean algae and higher plants.

Siphonophycean Cells are large (ranging from several millimeters to several centimeters) and are frequently damaged by animals or mechanical forces. However, damage does not lead to the death of the individual, as There is a rapid wound-healing mechanism involving specialized plugs. Within seconds of injury, an actin meshwork forms at the wound site; subsequently, a plug precursor is squeezed out from the vacuole, deposited onto the actin mesh, and polymerized. Following this, cellulosic material is transported to the damaged area and deposited, completing the plug formation.

Cell coverings in many representatives accumulate lime, which is deposited on the cell surface in the form of aragonite. Submicroscopic scales on vegetative and monad cells are absent.

Nuclear apparatus and mitosis features. During their ontogeny, siphonophyceans pass through stages with two morphologically distinct types of nuclei: Primary and secondary. The primary Nucleus is large (100–300 µm in diameter), typically occupies a fixed position within the cell, and develops from the zygote nucleus. External to the nuclear envelope of the primary nucleus lies a thin zone of intermediate cytoplasm devoid of organelles, which is surrounded by a system of small vacuoles forming the so-called lacunar labyrinth. Beyond this lies the main cytoplasm containing chloroplasts, Mitochondria, protein-synthesizing machinery, etc. Communication between the intermediate and main cytoplasm is maintained via connecting cytoplasmic channels. A peripheral fibrillar reticulum is located beneath the nuclear envelope of the primary nucleus (in dasycladals), while dense perinuclear bodies lie above it (in bryopsidals). The nuclear envelope contains numerous pores of typical eukaryotic structure with eight peripheral and one central globule (Fig. 24.51).

Fig. 24.51. Certain cytological features of siphonophycean algae. A – general view of the cell (right – surface view, left – optical section); B – diagram of interactions between microtubules, actin microfilaments, and main organelles in the peripheral cytoplasmic layer of the cell; C, D – structural diagram of the primary nucleus in dasycladalean (C) and bryopsidalean (D) algae; E – secondary nucleus in telophase; F – chloroplast; G – amyloplast. 1 – Cell wall, 2 – Plasmalemma, 3 – chloroplast, 4 – amyloplast, 5 – secondary nucleus, 6 – cell sap vacuole, 7 – microtubule, 8 – actin microfilament, 9 – nuclear envelope, 10 – lacunar labyrinth in the intermediate zone of the primary nucleus, 11 – peripheral microfibrillar reticulum, 12 – perinuclear bodies, 13 – thylakoids, 14 – thylakoid-organizing body, 15 – starch (A – orig., B, C, D – diagrammatized after Menzel, Schliwa, 1986; Burr, West, 1971; Hori, 1981; Hori, Ueda, 1975).

Secondary nuclei are small (5–15 µm in diameter) and form from the primary nucleus following Meiosis and a series of successive mitoses. Secondary nuclei are not surrounded by intermediate cytoplasm or a lacunar labyrinth; they are typically linked to chloroplasts via microtubules and actin fibrils and circulate throughout the cell along with them during cytoplasmic streaming.

Mitosis in siphonophyceans is generally similar to that of cladophoralean algae: it is closed, the spindle persists in telophase, and nuclei at this phase acquire a characteristic dumbbell shape (Fig. 24.51). However, unlike Cladophorales, centrioles are typically absent. In some bryopsidalean algae, gamete formation involves acentric mitoses, whereas mitoses associated with thallus growth occur with the participation of centrioles. In the latter case, centrioles form de novo near The Nucleus, replicate at the onset of mitosis, migrate to the nuclear poles, where they participate in forming the division spindle, and disappear upon the completion of mitosis.

Photosynthetic apparatus is represented by numerous small, disk-shaped, parietal chloroplasts, predominantly lacking pyrenoids. In halimedalean and dichotomosiphonalean algae, the thallus possesses Two Types of Plastids: pigmented chloroplasts and colorless amyloplasts. Amyloplasts do not directly participate in Photosynthesis but serve to store Reserve Polysaccharides. In many siphonophyceans, chloroplasts and amyloplasts contain specialized thylakoid-organizing bodies (Fig. 24.51).

Monad stages are represented by bi- and quadriflagellate Gametes and stephanokont zoospores (the latter found only in bryopsidalean algae – see below). Basal bodies of gametes are oriented According to the 11-5 pattern, overlap deeply, and diverge at a wide angle (approx. 180^o) in lateral position (Fig. 24.52).

The flagellar root system is cruciate. The root formula generally corresponds to 4-2-4-2, although the number of microtubules in the roots varies both within the same order and even among different gametes of the same individual. For instance, the microtubule count in the flagellar roots of male gametes in Pseudobryopsis is described by the 4-2-4-2 formula, and in female gametes by 4-3-4-3. The upper connecting fiber is smooth; unlike Ulvophyceae, it is not continuous but consists of two halves joined by a striated partition. Terminal caps on basal bodies are present (Bryopsidales, Halimedales) or absent (Dasycladales), and a columnar structure has not been detected.

Fig. 24.52. Flagellar stages and flagellar apparatus of siphonophycean algae. A – typical structural plan of gametes; B – stephanokont zoospore; C, D – flagellar root system of gametes and non-stephanokont zoospores: 1 – thin microtubular root consisting of two microtubules; 2 – thick microtubular root consisting of four microtubules; 3 – smooth upper connecting fiber; 4 – cross-striated partition of connecting fibers; 5 – cross-striated accessory microfibrillar root; E – flagellar root system of stephanokont zoospores: 6 – thin microtubular root consisting of four microtubules; 7 – thick microtubular root consisting of six microtubules; 8 – upper smooth connecting fiber; 9 – flagellar sleeve of the upper ring; 10 – lower connecting ring; 11 – flagellum (A, B – after Sauer, 1977; Vinogradova, 1979; C, D – after Roberts, Stewart, Mattox, 1984; E – after Hori, Kobara, 1982).

Types of body morphological structure

All siphonocladean algae feature a siphonal type of structure. The primary structural element of the thallus is a multinucleate siphon, which resembles a large individual cell not separated from other siphons by cell walls.

The thalli of siphonocladeans generally conform to two morphological plans: in bryopsidalean and halimedalean algae, the body appears as thin or thick filaments or cords, sparingly or richly branched, yet lacking radial Symmetry. In dasycladalean algae, the thalli consist of a central "stem-like" siphon and a system of whorled siphons arranged radially at the apex of the "stem".

A siphon developing from a zygote may retain only a single primary nucleus for a prolonged period.

Reproduction and Life Cycles

Reproduction occurs via bi- or quadriflagellate and stephanokont zoospores, as well as through sexual processes. The sexual process is isogamous or heterogamous, and is oogamous in only a single representative (Dichotomosiphon).

During The formation of reproductive cells, either the entire thallus or only a portion of it may transform into a sporangium or gametangium. The former developmental pathway is termed holocarpic, and the latter eucarpic. In the eucarpic pathway, the siphons where monad Cell Formation begins are separated from the main body of the thallus by plugs. In dasycladaleans, gametes form exclusively within specialized gametic cysts (see below).

Life cycles are haplophasic or haplodiplophasic, either without Morphology/12.html">ALTERNATION OF GENERATIONS or with a heteromorphic alternation. Many representatives exhibit complex cycles featuring digenetic zygote development.

Systematics of the Class

The class is divided into four orders: Bryopsidales, Halimedales, Dichotomosiphonales, and Dasycladales. The first three orders are phylogenetically quite close, whereas the last one represents a distinct, sharply delimited evolutionary branch.

Today, several phenotypic traits have been established that correlate with the division of the class into orders according to molecular phylogenetic reconstructions. The primary ones include the type of thallus symmetry, the presence of stephanokont zoospores, Features of the photosynthetic apparatus (specifically, the presence of amyloplasts), the mode of gamete formation (via holo- or eucarpic types or through gametic cysts), the type of sexual process, and features of primary nucleus organization, notably the presence of perinuclear bodies or a peripheral fibrillar reticulum (Table 24.6).

Table 24.6. Main taxonomic features of the orders of siphonocladean algae

Abbreviations: PFR – perinuclear fibrillar reticulum, PB – perinuclear bodies, ? – not investigated.

Bryopsidales. This order unites algae whose thalli lack radially symmetrical structures and consist of a single giant cell containing numerous (sometimes up to several thousand) nuclei. Cell walls are composed primarily of xylans and Mannans. Bryopsidalean algae are homoplastidic organisms; their photosynthetic apparatus is represented solely by chloroplasts, with amyloplasts being absent. A notable feature of the nuclear apparatus is the presence of a perinuclear fibrillar reticulum in the primary nucleus, which, according to some researchers, consists of A large number of preribosomes. Centrioles are generally absent, except in species of the genus Pseudobryopsis, where centrioles appear during the late interphase in dividing adult nuclei and disappear immediately upon the completion of mitosis.

A characteristic feature of bryopsidaleans is the presence of stephanokont zoospores in their life cycle (Fig. 24.52). These zoospores bear about 30–40 flagella. The basal bodies of each flagellum are connected to a smooth lower spiral protein ribbon, which remains unclosed into a ring and makes 2–4 turns around the anterior end of the cell. Additionally, each basal body is surrounded by a fibrillar collar. These collars attach to the upper smooth ribbon, which originates from the upper fibrillar connecting fiber of the basal bodies. Microtubular roots with four and six microtubules, respectively, attach alternately to the ribbons between the basal bodies.

In bryopsidaleans, gametes are formed via the eucarpic pathway, whereas zoospores are produced both eu- and holocarpically. Bryopsidaleans reproduce through thallus fragmentation, aplanospores, stephanokont zoospores, and sexual reproduction.

Fragmentation occurs through the detachment of a thallus portion, typically under The Influence of various mechanical factors, and is generally observed infrequently. Following such detachment, a cellulosic plug rapidly forms at the rupture site, sealing the wound.

During reproduction via zoospores and aplanospores in some genera (Derbesia, Bryobesia), portions of the siphons are partitioned by a plug and transform into sporangia, which give rise to numerous stephanokont zoospores. In other genera (Bryopsis), the entire microscopic sporophyte transforms into a sporangium. Thus, bryopsidalean sporophytes can be either holocarpic or eucarpic, whereas gametophytes are invariably eucarpic. Within the order, there is a distinct tendency toward the loss of asexual reproduction capabilities in highly evolved forms (for instance, Codiaceae lack zoospores and reproduce exclusively through sexual means).

The sexual process is predominantly heterogamous. Life cycles in Bryopsidales are haplophasic without alternation of generations or haplodiplophasic with an irregular heteromorphic alternation of generations and a digenetic pathway of zygote development. Sporophytes pass through a microscopic branched filamentous stage known as the "Derbesia" stage.

A typical representative of the order, Bryopsis, is widespread in the seas of tropical, subtropical, and temperate zones (Fig. 24.53). The gametophytes of this alga are differentiated into creeping rhizoidal and ascending assimilatory siphons. The ascending siphons are typically pinnately branched, resembling bird feathers. During gametogenesis, certain lateral siphons are segregated by septa. Female gametangia develop from intensely green-colored siphons, while male gametangia form from lighter, yellowish siphons. Subsequently, the Contents of the gametangia cleave into numerous biflagellate male and female gametes that differ in size. Through an opening in the gametangial wall, the gametes emerge into the external environment, where they undergo pairwise copulation to form a planozygote.

The planozygote settles on a substrate, sheds its flagella, and, without a resting period, begins to grow into a diploid, branched, filamentous sporophyte known as the Derbesia stage. The Development of the Derbesia stage takes 2–4 months. The nucleus increases 20–40 times in diameter, transitioning from the diploid zygotic nuclear stage to the primary nucleus stage. Afterward, the primary nucleus undergoes a series of mitotic divisions, with the daughter nuclei decreasing in diameter to 5–10 µm. The final nuclear division is meiotic. Following meiosis, secondary nuclei formation is completed, and the sporophyte produces unicellular, stephanokont, haploid zoospores via a holo- or eucarpic pathway. These zoospores grow into either new gametophytes composed of pinnate siphons or new sporophytes of the Derbesia stage (Fig. 24.53).

Fig. 24.53. Bryopsidalean algae. 1–11 – Developmental Stages of Bryopsis: 1 – general view of the gametophyte; 2 – apical part of a gametophyte "branch"; 3 – male gametangium; 4 – male gamete; 5 – female gametangium; 6 – female gamete; 7 – general view of the sporophyte (Derbesia stage); 8 – young uninucleate sporophyte with a primary nucleus; 9 – sporophyte with multiple primary nuclei; 10 – sporangium with protoplasts of future zoospores possessing secondary nuclei; 11 – zoospore. 12 – fragment of the gametophyte thallus of Pseudobryopsis with large vegetative siphons bearing smaller generative siphons/gametangia (after Vinogradova, 1977, 1979; Brock, Schnetter, 1997).

Interestingly, under certain conditions, the planozygote can directly develop into a gametophyte without passing through the Derbesia stage. However, the nuclear events underlying this developmental pathway remain uninvestigated.

Thus, Bryopsis exhibits a haplodiplophasic life cycle with irregular heteromorphic alternation of generations and sporomorphic reduction (at least during the typical pathway of zygote development).

A similar Structure and Life cycle are characteristic of the genus Pseudobrypsis, which, unlike the previous genus, exhibits a functional differentiation of lateral siphons into vegetative ones, performing exclusively photosynthetic Functions, and generative ones, from which gametangia develop. The generative siphons are smaller and, even in early developmental stages, become separated from the main thallus body by transverse septa.

The genus Halicystis comprises boring algae that inhabit the calcareous thalli of red algae belonging to the genus Lithothamnion, partially eroding lithothamnion limestone. Their thalli consist of short prostrate, rhizoidal, and vesicular ascending siphons, the latter of which give rise to gametangia. In most species of this genus, The life cycle is haplo-diplophasic, similar to the preceding genera, featuring sporic reduction and a Derbesia stage (Fig. 24.54).

Fig. 24.54. Bryopsidalean algae. A - Halicystis: 1 - adult gametophyte thallus with a rhizoid boring into the calcareous substrate; 2 - gametangium releasing gametes; 3 - portion of the sporophyte thallus (Derbesia stage); 4 - mature zoosporangium; 5 - zoospore. B - Derbesia: 6 - thallus, 7 - zoospore. C - Codium: 8 - general appearance of the thallus; 9 - thallus fragment showing elongated rhizoidal siphons branching into vesicular utricle siphons; 10 - general view of a utricle; 11 - part of a utricle with gametangia; 12 - male and female gametes (after Vinogradova, 1977; Eckhardt, Schnetter, 1984).

In the cosmopolitically distributed marine species Halicystis ovalis, the life cycle includes a dikaryon stage: female and male gametophytes produce anisogametes that copulate upon release from the gametangia. Although the gamete cytoplasms fuse during copulation, karyogamy does not occur, resulting in the formation of a dikaryontic cell. This cell settles on the substrate and develops into a sporophyte known as "Derbesia marina". The thallus of this stage represents a branched system of prostrate and ascending siphons containing numerous small male and female nuclei. Subsequently, short lateral siphons develop on the long ascending siphons and transform into sporangia. During the formation of these sporangia, the "male" and "female" nuclei fuse, producing diploid zygotic nuclei. These undergo immediate meiotic division, and the protoplast of the cell breaks down into a large number of stephanokont haploid zoospores, which subsequently germinate into new male and female gametophytes.

Within the genus Halicystis, There are also species lacking a sporophyte stage, where reproduction occurs exclusively via sexual means, bypassing the Derbesia phase. In these forms, young thalli are diploid and adults are haploid As a result of somatic chromosome reduction. It is believed that such forms originated through The conversion of a digenetic developmental pathway of the zygote into a monogenetic one, leading to the loss of the sporophyte stage.

Species of the genus Derbesia—which gave their name to the sporophytes of Bryopsis and Halicystis—are very common in warm and temperate seas. The thallus of Derbesia consists of short prostrate and ascending filamentous siphons. A sexual process is absent in species of this genus, and reproduction is accomplished solely through stephanokont zoospores or aplanospores. It is likely that various Derbesia species descend from bryopsid- and halicystid-like ancestors whose digenetic cycle was disrupted, leaving only the sporophyte stage with the capacity for self-perpetuation.

An example of an alga with a complexly differentiated thallus is Codium, which inhabits warm seas, typically at depths ranging from 5 to 40 m. In the Black Sea, Codium vermilara is a fairly common species. Its thallus features a central core composed of densely interwoven, elongated rhizoidal siphons. At the periphery, vesicular outgrowths known as utricles branch off from the rhizoidal siphons, forming a cortex-like layer. The system of rhizoidal and utricular siphons forms thick (up to 2 cm in diameter) dichotomously branched cords whose surface appears spongy due to the utricles (Fig. 24.54).

Codium reproduces exclusively via sexual reproduction, specifically heterogamy. Biflagellate female and male gametes are formed within gametangia that are separated from the utricles by septa, making the alga eucarpic. Male gametangia are yellowish, while female ones are dark green. Following copulation, the zygote germinates into a new gametophyte without a resting period. Meiosis in Codium is believed to be gametic, and the life cycle is accordingly diplophasic. Gametes are also capable of parthenogenetic germination.

Halimedales. Based on both phenotypic and molecular characters, the algae of this order are closely related to Bryopsidales: they lack a radially symmetrical structure, gametes are produced within gametangia rather than gametic cysts, the developmental phase during which the thallus possesses only a primary nucleus is short-lived, and the primary nucleus itself lacks perinuclear bodies.

However, Halimedales exhibit several striking specific features. Firstly, their cell walls are composed of xylan and are typically impregnated with lime, which is deposited on the cell surface in the form of aragonite needles. Secondly, halimedalean algae are heteroplastidic, possessing colourless amyloplasts In addition to pigmented chloroplasts; these amyloplasts are incapable of photosynthesis and serve exclusively a storage function. Thirdly, both chloroplasts and amyloplasts contain a unique structure known as the thylakoid-organizing body. Located at one of the poles of the plastid, it consists of a system of three to four small, concentric, closed ring-like thylakoids nested within one another. Fourthly, representatives of this order are holocarpic, meaning that upon gametogenesis, the entire algal thallus transforms into a single giant gametangium.

Stephanokont zoospores are absent in halimedaleans. Reproduction occurs via thallus fragmentation and sexual means. The sexual process is isogamous or heterogamous. Halimedalean algae inhabit primarily tropical seas.

The thalli of halimedalean algae are macroscopic, complexly segmented, and resemble small herbaceous plants. For instance, Halimeda is differentiated into a system of prostrate rhizomes, rhizoids, and ascending assimilatory "shoots" that, depending on the species, resemble tufts, fans, or articulated whorls. The rhizomes are formed by a filamentous siphon from which densely branched rhizoidal siphons penetrate the substrate. From the rhizome, above each group of rhizoids, a group of assimilatory siphons extends upward to form the "shoots". A "SHOOT" consists of a multiaxial, more or less cylindrical "stalk" and a typically flattened or repeatedly branched apex. Each axis of the "shoot" is a filamentous ascending siphon bearing utricles (Fig. 24.55). The filamentous siphons are densely interwoven and interconnected by numerous anastomoses. The interior of the siphons contains a system of callose beams that functions as an internal Skeleton, while the siphon surface is encrusted with aragonite crystals forming an external skeleton.

Fig. 24.55. Halimedalean algae. A-C - Halimeda: A - general appearance of the thallus; B - diagram of thallus structure (1 - rhizoids, 2 - rhizome, 3 - ascending siphons of the "shoot", 4 - utricles); C - gametangial siphons developing on "shoots" without being separated by septa; D - general appearance of the Udotea thallus; E-G - Caulerpa: E - general appearance of the thallus, F - callose supporting beams (longitudinal and transverse sections through the rhizome); G - male and female gametes (after Vinogradova, 1977; Meinesz, 1980; Fritsch, 1935).

Photosynthesis is carried out by the utricles of the ascending siphons, which contain numerous chloroplasts and amyloplasts. By day, the plastids concentrate in the peripheral region of the utricles; by night, they migrate deep into the thallus and accumulate within the filamentous siphons, which are protected by the external skeleton.

Vegetative Reproduction in Halimeda occurs when rhizomes rupture. The damaged siphons are then sealed with plugs, and the separated thallus segment is capable of attaching to the substrate and growing into a new plant.

During sexual reproduction, the protoplast of the entire plant breaks down into a large number of biflagellate female or male gametes. The gametes are released and undergo copulation. Halimeda is dioecious, with male and female gametes produced on separate plants. The sexual process is heterogamous. The zygote germinates without a resting period into a unicellular spherical plant known as a protosphaera. Within the protosphaera, the nucleus divides mitotically to form numerous primary nuclei. Subsequently, the protosphaera develops multiple protrusions, forms a system of rhizomes and rhizoids, and finally produces ascending "fronds". At this stage, the nuclei are secondary. It is believed that the completion of protosphaera development and the Formation of secondary nuclei result from meiosis, although direct evidence for this is currently lacking.

Species of Halimeda inhabit tropical seas and are significant reef-building algae. Halimeda is particularly abundant in atoll lagoons, where dead algal thalli form a continuous carpet over the bottom, generating a unique aragonite sand characteristic of atolls.

Species of the genus Caulerpa generally share a structure similar to that of Halimeda, but their rhizomes are thick and multiaxial, featuring numerous rhizoidal systems. Ascending "shoots" of various shapes arise from the rhizomes depending on the species. For example, in Caulerpa prolifera, an individual "shoot" takes the form of an elongated lanceolate blade up to 30 cm long. The arrangement of shoots and rhizoids along the rhizome is irregular. Both vegetative and sexual reproduction proceed similarly to Halimeda.

In the genus Udotea, thalli appear as fan-shaped blades anchored in the sediment by a system of rhizoidal siphons, entirely lacking rhizomes. A distinctive feature of Udotea's life cycle is that secondary nuclei are formed immediately after the very First Division of the primary nucleus, thereby bypassing the multinucleate primary-nucleus siphon stage entirely.

Dichotomosiphonales. This order includes a single genus with a single species, Dichotomosiphon tuberosum. Unlike other members of the class, this alga inhabits continental freshwater bodies. It possesses very simple thalli formed by dichotomously branched tubular siphons anchored in the substrate by short rhizoids, and features an oogamous Sexual process in which the zygote germinates only after a resting period (Fig. 24.56). Nevertheless, much like Representatives of the preceding orders, Dichotomosiphon exhibits cytoplasmic streaming, basal bodies oriented according to the 11-5 type and rotated by 180^o, and a smooth, discontinuous upper connecting fiber of the basal bodies (consisting of four segments rather than two, unlike other siphonocladalean algae). The number of rootlets (R-roots) is 4 or 5.

Fig. 24.56. Dichotomosiphon: 1 - general appearance of the thallus; 2 - fragment of the apical region of the thallus; 3 - apical siphon with oogonia and antheridia; 4 - mature oogonia and antheridium; 5 - zygote; 6 - thallus fragment with propagules; 7 - apical part of a spermatozoid (1-6 - after Sauer, 1977; Starmach, 1972; 7 - after Moestrup, 1975).

In A number of features, this alga resembles the halimedaleans: its photosynthetic apparatus is heteroplastidic, comprising both chloroplasts and amyloplasts; structures resembling thylakoid-organizing bodies have been identified within the plastids; and stephanokont stages are absent. Reserve polysaccharides are deposited not only within the chloroplast stroma but also directly in the cytoplasm (a phenomenon also observed in dasycladalean algae).

The alga reproduces via gemmae (brood buds) and sexually. Antheridia and oogonia develop at the apices of lateral thallus branches. Numerous biflagellate spermatozoa are formed within the antheridia, whereas a single egg cell is produced in the oogonium. The spermatozoa are released into the surrounding medium, penetrate the oogonium through a small papilla-like neck, and fertilize the egg cell. The zygote becomes enclosed in a three-layered wall and enters a resting stage, upon the completion of which it germinates into a new siphonal dichotomously branched thallus.

Dasycladales. This order represents an independent, sharply delineated evolutionary lineage of siphonal algae characterized by a complex of specific features. The thalli are radially symmetrical, consisting of an axis bearing a system of apical whorled siphons. The cell walls are composed primarily of mannan and are impregnated with carbonates, most frequently in the form of aragonite. The photosynthetic apparatus is homoplastidic, represented solely by chloroplasts lacking pyrenoids and thylakoid-organizing bodies. Reserve polysaccharides, similarly to Dichotomosiphon, are deposited both within the chloroplast stroma and directly in the cytoplasm. A specific feature of the flagellar apparatus is the absence of terminal caps on the basal bodies. The primary nucleus differs from the nuclei of other siphonocladalean algae by the presence of perinuclear bodies.

As in other siphonocladaleans, active cytoplasmic streaming occurs within the cells of dasycladads. However, its specific feature is the presence of two distinct streams: a relatively slow surface stream with a velocity of about 50-100 µm/min, and a rapid deep stream with a velocity ranging from 200-600 µm/min. Cytoplasmic movement in the deep stream involves not only actin, but also tubulin.

During their ontogeny, dasycladals sequentially pass through a two- or three-year unicellular plant stage with a primary nucleus, followed by a short-lived stage of a truly siphonal plant possessing multiple secondary nuclei. The sexual process is isogamous, with gametes forming not in gametangia, but within specialized gametic cysts.

The most well-known representative of the order is the genus Acetabularia. The thalli of this alga are perennial. During the first two to three years, they consist of a central axis 2-3 cm in length, rhizoids, and one to three rings of unfused whorls located at the apex of the central axis. A single large primary nucleus, approximately 100 µm in diameter, is situated at the Base of the central axis. At the end of the growing season, the apical region of the thallus dies off, and only the rhizoidal part and a small basal section of the central axis containing the primary nucleus remain viable (Fig. 24.57).

Fig. 24.57. Successive Stages of the Acetabularia life cycle: 1, 2 - sterile thalli of the First and Second years with the primary nucleus in the rhizoidal region; 3 - fertile thallus of the third year with segments forming the cap; 4 - transverse section through the cap (2, 4 - after Vinogradova, 1977; 1, 3, 5-12 - after "Entwicklung von Acetabularia (Dasycladales)", IWF, 1978).

In the third or fourth year of life, the apical whorls fuse together to form the "cap" characteristic of Acetabularia. At this time, the primary nucleus decreases in diameter by a factor of 2-3 and undergoes a reductional division (somatic reduction), followed by a series of mitoses that give rise to approximately 20,000 small haploid secondary nuclei. These nuclei are carried by cytoplasmic streaming into the apical region of the thallus, accumulating in the "cap," whereupon the alga acquires a truly siphonal structure. Within the rays of the cap, the protoplast of the alga initially constricts and subsequently breaks down into uninucleate segments. The latter become encased in thick cellulosic walls and transform into gametic cysts with a large opening sealed by a specialized plug¹.

The cysts remain within the "cap" for a period ranging from several weeks to several months. During this time, the nuclei divide slowly, and the cysts become multinucleate. Subsequently, the cap disintegrates, the cysts are released, and a large number of uninucleate biflagellate isogametes are formed within them. The cysts open, and the gametes emerge and copulate. Without a resting period, the zygote immediately begins to germinate into a new plant possessing a primary nucleus, rhizoids, a central axis, and a ring of unfused apical whorls. Thus, Acetabularia exhibits a diplohaplophasic life cycle without alternation of generations, accompanied by somatic reduction.

The genus Dasycladus is characterized by thalli in which the central axis is covered with numerous closely spaced whorl rings. Each branch of the whorl is dichotomously or trichotomously branched three times (Fig. 24.58).

Fig. 24.58. Dasycladalean algae. A - Dasycladus: 1 - general appearance of the thallus; 2 - transverse section fragment of the thallus; 3 - fertile spherical segment among sterile segments. B - scheme of vegetative Hybridization in 7 - a nucleated stalk section of A. mediterranea (1, white) was grafted onto a rhizoidal system section containing the nucleus of A. wettsteinii (3, dotted). The transplanted plant (2) developed the "cap" of A. wettsteinii (A - after Vinogradova, 1977; B - after Masiuk, 1993).

All dasycladalean algae are marine, inhabiting primarily tropical and subtropical waters. For instance, species of Acetabularia occur in the warm seas of the Northern Hemisphere (specifically, A. mediterranea is a common species in the Mediterranean Sea). Other dasycladals are predominantly inhabitants of tropical seas (e.g., Dasycladus grows in the Caribbean Sea). Due to the high calcium carbonate content in their cell walls, these algae are well preserved in the fossil record, which has made it possible to trace the evolutionary pathways of this order.

The earliest remains of Dasycladales are known from the Upper Precambrian (Proterozoic, approximately 1 billion years ago). Morphological evolution within this order is well documented by fossil records: it has been established that the most primitive dasycladalean algae possessed a thallus lacking any assimilation shoots; subsequently, species appeared with assimilation branches arranged on the central axis first spirally, then oppositely, and finally in whorls along the entire length of the central axis. In the youngest forms, the whorls assumed an exclusively apical position.

Fossil dasycladals have played a significant role in carbonate rock formation. For example, certain limestone massifs in the Alps are formed precisely by the remains of algal thalli belonging to this order. Owing to the presence of a giant primary nucleus, dasycladalean algae have become a popular model Organism for cytological and physiological research. In particular, it was using Acetabularia that Hämmerling conducted his classic experiments investigating nuclear function and vegetative hybridization.

¹ Until recently, it was believed that meiosis takes place precisely at this stage, prior to gametogenesis. However, modern electron microscopic studies have demonstrated that the reductional division occurs much earlier—during the first division of the primary nucleus.