MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 15. MULTICELLULAR PROTISTS: RED, BROWN, AND GREEN ALGAE

Conclusion

This chapter describes three entirely or partially multicellular phyla of protists: red Algae (Rhodophyta), brown algae (Phaeophyta), and green algae (Chlorophyta). Their members are predominantly aquatic, with brown and red algae playing a major role in marine ecosystems, while green algae are frequently abundant in freshwater habitats. However, several genera of green algae inhabit the seas, and some brown and red algae live in fresh Water. Across all these environments, the algae discussed here fulfill ecological roles comparable to those of land plants.

The various algal phyla likely originated through the Symbiosis of heterotrophic, protozoan-like Eukaryotic Cells and photosynthetic Bacteria. Specifically, the METABOLISM/14.html">Chloroplasts of red algae apparently arose from symbiotic cyanobacteria, whereas those of green algae derived from bacteria resembling species of Prochloron. The golden-brown chloroplasts of brown algae, which contain an Abundance of the xanthophyll fucoxanthin and closely related accessory pigments, bear a partial resemblance to the recently discovered bacterium Heliobacterium chlorum. They are also very similar to the chloroplasts of chrysophytes (which in many respects resemble brown algae) and dinoflagellates—a distinct group of protists.

Red algae form a large group, particularly abundant in warm marine waters. Their members are almost invariably attached to a substrate, and some grow at considerable depths (down to 268 m). They contain phycobilins, which impart their characteristic coloration, along with chlorophyll a.

Brown algae comprise the largest and most complexly organized marine protists. In many of their groups, the vegetative body is clearly differentiated into a holdfast, a stipe, and a blade-like lamina. Some possess assimilating conductive Tissues whose structural complexity approaches that of the Vascular Tissues in land plants. The chloroplasts of brown algae contain chlorophylls a and c as well as large amounts of fucoxanthin, which gives them an olive-green or dark brown hue. The sporophyte is typically larger than the gametophyte.

Green algae constitute the largest and most diverse of the three phyla. Land plants likely evolved from these very protists. Their chloroplasts contain chlorophylls a and b as well as carotenoids, and their reserve carbohydrate is starch. In these features, they closely resemble plants. The ancestral forms of green algae were presumably flagellated, scaly unicells. The phylum includes at least five classes, three of which are examined in detail.

Charophyceae and Chlorophyceae inhabit primarily fresh water, whereas Ulvophyceae are predominantly marine. All three classes include unicellular, small-celled, filamentous, and parenchymatous genera, while Chlorophyceae also feature motile and non-motile colonial forms. The motile spores of Charophyceae are asymmetric and bear lateral or subapical flagella oriented at a right angle to The Cell axis, whereas the spores of the other two classes are radially symmetrical with apical flagella.

In Charophyceae and Ulvophyceae, the mitotic spindle persists through telophase, whereas in Chlorophyceae it disintegrates and a phycoplast is formed. The phycoplast is a system of cytokinetic microtubules running parallel to the plane of Cell Division. In Charophyceae, a phragmoplast develops—a system of microtubules oriented at a right angle to the plane of cell division, much like that in plants.

Sexual reproduction in Charophyceae and Chlorophyceae invariably involves The formation of a resting zygospore and zygotic Meiosis, whereas in Ulvophyceae it frequently proceeds with Morphology/12.html">ALTERNATION OF GENERATIONS (meiosis occurring at the spore stage), with resting zygotes being relatively rare.

Class="center">A. Beds of the giant kelp Macrocystis pyrifera off the coast of California. B. Manual harvesting of Nori from submerged ropes in Japan. C. A specially equipped kelp-harvesting vessel in the coastal waters of California. The reciprocating blades at the stern are submerged to a depth of three meters; the vessel moves stern-first through the kelp beds, and the harvested fronds are conveyed via belt systems into an onboard hopper.

Appendix 1. Economic Uses of Seaweeds

People in various Regions of the globe, particularly in the Far East, consume red and brown algae as food. Kelps ("kombu") are regularly eaten as vegetables in China and Japan; although sometimes cultivated, they are harvested primarily from natural populations. The red alga Porphyra ("nori") serves as food for many inhabitants of the North Pacific and has been cultivated in Japan and China for centuries. This industry employs over 30,000 people in Japan alone, with an annual output valued at approximately $20 million. Other red algae are consumed on Pacific islands and along the North Atlantic coast. As a source of CARBOHYDRATES, seaweeds generally lack high nutritional value because humans and most other animals lack the Enzymes required to break down many of The Cell wall components of these protists, such as Cellulose. Nevertheless, seaweeds serve as a source of essential salts, various Vitamins, and Trace Elements, making them a valuable dietary Supplement. Certain green algae, notably Ulva ("sea lettuce"), are also utilized as greens.

Throughout many northern temperate regions, brown algae are harvested for their ash, which is rich in potassium and sodium salts and thus utilized in industrial Applications. Iodine is also extracted from these algae. Frequently, they are applied directly as agricultural fertilizers.

Alginates—a group of substances extracted from kelps, particularly from species of Macrocystis—are widely used as thickening agents or colloid stabilizers in the food, textile, cosmetic, pharmaceutical, and pulp and paper industries, as well as in welding. Along the entire western coast of the United States, Macrocystis beds can yield multiple harvests per year, gathered right at the water's surface. Efforts are currently underway to cultivate giant kelp on a commercial scale.

One of the most widespread applications of algae is The production of Agar from the mucilaginous substance found in the cell walls of certain red algal genera. Agar is utilized in the manufacture of vitamin and drug capsules, dental impressions, cosmetics, and as a culture medium for bacteria and other microorganisms. Furthermore, it is incorporated into baked goods to prevent staling, in instant gelatins and confectionery recipes, and as a temporary protective coating for meat and fish in tropical regions. Agar is produced in numerous countries, most notably in Japan.

Another algal colloid, carrageenan, is often preferred over agar for stabilizing paint emulsions, cosmetics, and dairy products. In the Philippines, the red alga Eucheuma, which serves as a source of this substance, is cultivated on a commercial scale.

Appendix 2. Symbiotic Green Algae

Many algae, including green algae, form symbiotic associations with other organisms. The majority of symbiotic green algae closely resemble Chlorella (see Fig. 15-32). They can be found within many freshwater Protozoans, Sponges, hydras, and certain Flatworms. Many of these algae reproduce via simple cell division inside the host cells, with which they remain in intimate contact. The green alga Tetraselmis convolutae of the class Pleurastrophyceae occurs primarily in the subepidermal Cells of the marine flatworm Convoluta roscoffensis. Within this environment, Tetraselmis lacks a cell wall and a definitive shape. Its Plasma Membrane, whose surface area is vastly expanded through finger-like projections, lies in more or less direct contact with the vacuolar membrane of the host cell. When cultured outside the flatworm, this alga develops a cell wall, four flagella, and an eyespot—features absent during symbiosis.

An example of green algal chloroplast symbiosis is discussed on p. 249.