BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, Fungi, Plants

2. Class: Yellow-Green Algae (Xanthophyceae)

Yellow-green Algae encompass all levels of thallus Organization—from amoeboid and monadoid to siphonous. The green METABOLISM/14.html">Chloroplasts turn blue when treated with HCl and contain the xanthophylls heteroxanthin and vaucheriaxanthin instead of fucoxanthin (Table 11.2). The heterokont flagella are laterally attached. Thus, despite their green coloration, yellow-green algae are highly similar to the other classes of Heterokontophyta. This taxon is distinguished from the green Chlorophyta by the absence of chlorophyll b and by the posterior flagellum terminating in a fine Hair (as in brown algae).

In some forms, the Cell wall consists of two overlapping halves. They are largely composed of Cellulose microfibrils and are often impregnated with silicic acid (however, without silicified frustules!). Some species form endogenous cysts with walls impregnated with silicic acid; these cysts are shaped like a box with a lid.

Reproduction. Most yellow-green algae reproduce asexually. Sexual reproduction is known in only one genus (Vaucheria), occurring in a haplontic life cycle (zygotic Meiosis!); this case is discussed below¹.

¹ According to other data, The life cycle of Vaucheria is diplontic (gametic meiosis). — Translator's Note.

During asexual reproduction of Vaucheria, the branch tips swell and are cut off from the rest of the thallus by a septum. The entire multinucleate content of the delimited cell escapes by rupturing its wall, forming a single large, ovoid zoospore (about 1/10 mm in size) (Fig. 11.73, B). Its surface is covered with numerous paired flagella of slightly unequal length that beat synchronously. In the colorless peripheral region of the zoospore, behind each pair of flagella, lie two blepharoplasts and a pear-shaped, pointed Nucleus; the chloroplasts are located closer to the center (Fig. 11.73, C); contractile vacuoles are also present. Morphologically, this Structure corresponds to the aggregate of all zoospores that would form in a single cell, and thus represents a synzoospore.

Sexual reproduction. Oogonia and antheridia (spermogonia) of Vaucheria arise on the thallus filaments as lateral outgrowths (Fig. 11.73, E: o, s). The oogonium primordium (o) initially contains numerous nuclei, all of which, however, except for one—the future egg nucleus—migrate back into the supporting filament along with some of the chloroplasts; only then is a transverse septum formed. The remaining chloroplasts, oil droplets, and The Nucleus move to the lower part of the oogonium, while colorless Cytoplasm accumulates in the beak-like tip, which protrudes as a small sphere when the oogonial wall ruptures. The multinucleate antheridium (s), along with its supporting branch, is curved like a horn. Its apex becomes mucilaginous upon maturity. Numerous tiny spermatozoids escape, enter the opening of the oogonium, and gather in front of the colorless receptive spot of the egg cell. The spermatozoids (Fig. 11.73, F) are heterokont.

After Fertilization of the egg cell by one of the ♂ Gametes, the oil-rich cystozygote becomes surrounded by a multilayered wall, enters a dormant state (hypnozygote), and later, after reduction division, germinates directly into a new haploid filament.

Class="center">Table 11.2. Some chemical characteristics of algal classes

Note. Symbols according to van den Hoek, review of xanthophylls according to Metzner: * — with simple Plastids (arising by primary endocytobiosis); O — with complex plastids and a nucleomorph (arising by secondary endocytobiosis); ∆ — with complex plastids without a nucleomorph (arising by secondary endocytobiosis); + — important pigment or storage polysaccharide; (+) — pigment occurs; (•) — pigment rare or only in minor amounts; pigment or storage polysaccharide absent. In the case of starch: + — outside the chloroplast; © — deposited inside the chloroplast; A — 8-ketocarotene, e.g., fucoxanthin or siphonaxanthin (the latter only in Prasinophyceae and Bryopsidophyceae among green algae);

In the class Xanthophyceae, approximately 400 species from 40 genera have been described, inhabiting freshwater, and to some extent also seas and damp soil.

Monadoid forms are grouped into the order Chloramoebales (1st order—e.g., Ankylonoton, Fig. 11.72, D). The capsoid level of organization is represented in the order Heterogloeales (2nd order); the marine genus Heterocapsa belongs to it. The order Mischococcales (3rd order) groups floating or sessile forms with a rigid cell wall. In the branched, terrestrial species of Capitulariella (Fig. 11.73, C), the sporocysts detach as a whole and only then release zoospores (analogous to Trentepohlia, see Fig. 11.93, C, D). Tribonematales (4th order) are represented in freshwater and on damp soil by frequently occurring species of the genus Tribonema (Fig. 11.72, E), the walls of whose unbranched filaments are composed of H-shaped pieces in longitudinal section.

B — allenic carotene, e.g., vaucheriaxanthin and neoxanthin (the latter in Euglenophyta, Chlorophyta, Eustigmatophyta, and in some Heterokontophyta, Rhodophyta); C — acetylenic carotenoids; D — carotenoid esters, i.e., xanthophylls bearing fatty acid residues on one or both hydroxyl groups. The table does not include 4-keto-carotenes, e.g., echinenone in Euglenophyta + and Chlorophyta +, and in Heterokontophyta (+). Cyanelles — photosynthetic Organelles with a peptidoglycan wall. Rhodoplasts — plastids of Rhodophyta and photosynthetic organelles derived from them. Chloroplasts — plastids of Chlorophyta and photosynthetic organelles derived from them.

To the Heterosiphonales (5th order) belongs the vesicular alga Botrydium (Fig. 11.73, G), which grows on damp mud and whose cell, about 2 mm in size, is anchored in the mud by rhizoids. The vesicle contains numerous nuclei and discoid chloroplasts in its parietal cytoplasm, with a wall consisting of a pectin-like fraction and cellulose. When Botrydium is flooded with Water, numerous heterokont zoospores are formed, which escape after the vesicle wall ruptures and grow into a new vesicle on a suitable substrate. Upon drying out, A large number of multinucleate cysts are formed from protoplasts that migrate into the rhizoid.

Fig. 11.73. Xanthophyceae, Heterosiphonales; A — F — Vaucheria (A — C — V. repens, D, E — V. sessilis, F — V. synandra): A — sporangium primordium (150x), B — synzoospore escaping from the sporangium (150x), C — margin of the synzoospore (500x), D — plant with rhizoid and gametangia developing from synzoospore B (70x), E — portion of a filament with gametangia (150x), F — spermatozoid (700x); G, H — Botrydium, B. granulatum: G — entire plant (30x), H — zoospore (1000x); c — chloroplasts; o — oogonium; s — antheridium (spermogonium); z — synzoospore primordium

The widespread species of the genus Vaucheria also have a siphonous structure. They inhabit freshwater or damp soil, anchoring to the substrate by a tuft of rhizoids, and possess a branched thallus lacking transverse septa, which forms a system of filaments (Fig. 11.73, D) with numerous nuclei and plastids. The Cell wall contains a pectin-like substance and cellulose.

The walls of some species are incrusted with lime and can form calcareous tufa.

Closely related to the yellow-green algae is a small group (often treated as an independent division, Eustigmatophyta), which differs from Xanthophyceae in several ultrastructural features: chloroplasts lack a peripheral girdle lamella; pyrenoids are present only in the chloroplasts of vegetative Cells; and the eyespot is located at the anterior end of the cell, outside the chloroplast¹. Heterokont flagellated cells may occur in the Life Cycle of capsoid and coccoid organisms. In Chlorobotrys, several cells form a colony within a mucilaginous envelope; some species of this genus are widespread in bog pools.

¹ This group, most often treated as a class, differs from Xanthophyceae in that most representatives actually possess only one forward-directed, tinsel flagellum; instead of a second flagellum, only a basal body is present. In only two representatives does the posterior smooth flagellum emerge externally. Furthermore, the predominant pigment is violaxanthin rather than diatoxanthin. — Translator's note.

The classes (3–5) discussed below are characterized by fucoxanthin as an accessory pigment in chloroplasts that range in color from yellow to brown.

Classes 3 and 4, discussed below, predominantly comprise unicellular or colonial algae and, less frequently, forms that produce unbranched filaments. Parenchymatous thalli, which occur only as exceptions, are microscopic in size. Morphology/12.html">ALTERNATION OF GENERATIONS is unknown.