BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, fungi, plants

3. Class: Golden Algae (Chrysophyceae)

Most species of this Class are monadoid unicellular forms, some of which form colonies. Less common are representatives with amoeboid (Rhizochrysis), capsal (Chrysocapsa), coccoid (Chrysosphaera), filamentous (Phaeothamnion), and parenchymatous (Thallochrysis) Organization. METABOLISM/14.html">Chloroplasts are most often golden to golden-brown (hence the name 'fucoxanthin'!). The Cell surface in representatives of some genera is covered with characteristic siliceous scales, which are formed inside the cell in vesicles near the chloroplasts and then deposited in finished form on the cell surface; silicified cysts are also found (Fig. 11.74, E). Sexual reproduction (isogamy) has been observed in some species.

This class includes about 1,000 species distributed among approximately 200 genera. They occur mainly in fresh, less frequently in brackish and salt waters.

Freshwater forms prefer unshaded bodies of Water with cool water.

Nutrition is most often photoautotrophic, but can also be heterotrophic and phagotrophic.

1. Order: Chrysomonadales. Unicellular golden-brown forms possess flagella (see Fig. 4; 11.74). In representatives of some genera, endogenous cysts are formed inside the Cells, most often with a silicified wall and a plug (D, E).

In species of the genera Uroglena (Fig. 11.74, F) and Synura, which are common in freshwater plankton, numerous radially arranged cells form spherical colonies; in Synura, the cells are covered with delicate siliceous scales (Fig. 11.74, G).

In Mallomonas, the siliceous scales bear long appendages that facilitate flotation in water. Species of the genus Dinobryon (Fig. 11.74, H), common in fresh water and the sea, form Cellulose loricae around their elongated cells during circular movements; after division, the daughter cells attach to the rim of the mother lorica and form a new lorica, resulting in shrub-like branching colonies.

For copulation, two separate cells, together with their loricae, swim toward each other, fuse, and form a silicified cystozygote.

Species of the genera Ochromonas (Fig. 11.74, A–E) and Monas are externally similar, but in the latter, chloroplasts and the autotrophic lifestyle have been lost.

The Craspedomonadaceae ('choanoflagellates') are also heterotrophic; due to the beating of the flagella, food (detritus, Bacteria) is driven into the protoplasmic collar located at the upper end of the cell.

Only in the seas are naked silicoflagellates found, which are grouped into one.

Fig. 11.74. Chrysophyceae, Chrysomonadales: A–E — Ochromonas (1,000x): A–C — transition from a normal form with two flagella to an amoeboid state with pseudopodia, D — cyst formation in an amoeboid protoplast, E — cyst with an opening and a plug (hatched); F — Uroglena, U. americana (400x); G — Synura, S. glabra, siliceous scales (7,200x); H — Dinobryon, D. sertularia (350x); c — yellow-brown chloroplast; g — flagellum; s — eyespot; v — vacuole

2. Order: Dictyochales. They form an elegant siliceous Skeleton located inside the cell, for example, Distephanus in Fig. 11.75, A. They are known as fossils from the Mid-Cretaceous.

3. Order: Chrysocapsales. In species of this group, the cells are non-motile in the vegetative state, residing in gelatinous coenobia (capsal level of organization, see Box 11.5). The moss-like thalli of Hydrurus inhabit cold mountain streams, attaching to a stony substrate (Fig. 11.75, B).

4. Order: Chrysotrichales. The cells are united into simple or branched filaments, as, for example, in the freshwater alga Phaeothamnion (filamentous level of organization; Fig. 11.75, E). Under certain conditions, the zoospores of these Algae lose their flagella, become surrounded by a thick envelope, and multiply by division ('palmelloid stage'; Fig. 11.75, G). Stichochrysis immobilis has completely lost The ability to form motile flagellated stages, as have many Representatives of the next class.

Fig. 11.75. Chrysophyceae: A — Dictyochales, Distephanus speculum: chloroplasts are mainly in the ectoplasm, outside the siliceous skeleton; flagella are not shown (1,000x); B — D — Chrysocapsales, Hydrurus foetidus: B — young plant (1x), C — zoospore (1,200x), D — terminal part of a branch (450x); E — G — Chrysotrichales, Phaeothamnion borzianum: E — thallus in which zoospore formation occurs (400x), F — zoospore (75x), G — palmella stage (400x)