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Chlamydomonas

Cladophora

Scenedesmus

Selenastrum

Spirogyra

Ulothrix

Volvox

"In addition to chlorophyll-« (chl.-a) and carotenoids.

Starch is a-l,4-glucan; P-l,2-glucan is called paramylon, and P-l,3-glucan is laminarin. cSome chrysophytes have one or two flagella.

"In addition to chlorophyll-« (chl.-a) and carotenoids.

Starch is a-l,4-glucan; P-l,2-glucan is called paramylon, and P-l,3-glucan is laminarin. cSome chrysophytes have one or two flagella.

to most environmental engineers and scientists, includes very useful (and beautiful!) classic color drawings (by C. M. Palmer) of many important freshwater algae.

Euglenophyta The Euglenophyta are considered to be the first of the algae to evolve, probably from unpigmented flagellates. In fact, they are considered protozoans (Phyto-mastigophora) as well as algae, mainly because they usually have two flagella (one so short it may be internal) and have no cell wall. Most, such as Euglena (Figure 10.38), are able to live chemoorganotrophically in the dark and can also survive in this way if their chloroplasts are removed. Their pigments are similar to those of the green algae and plants, but they are not felt to be closely related to these groups.

Phaeophyta: The Brown Algae The brown algae are multicellular and mainly marine. They include most seaweeds. Giant kelp can exceed 60 m in length. The Sargasso Sea in the North Atlantic Ocean is known for its extensive beds of floating Sargassum. Brown algae are thought to have evolved from early pigmented flagellates and still produce flagellated asexual spores and sexual reproductive cells. Many produce leaflike blades and a holdfast, but not true leaves, stems, or roots.

Pyrrophyta Sem
Figure 10.38 Euglena gracilis. (SEM image courtesy of Brian Leander and Michael A. Farmer, Center for Ultrastructural Research, University of Georgia, Athens, GA.)

Chrysophyta: Chrysophytes and Diatoms The Chrysophyta, named from the Greek chrysos (gold), includes three major classes: yellow-green, golden-brown, and diatoms. They may be either unicellular or colonial. The yellow-green and golden-brown algae (chrysophytes) are mainly found in fresh water, although there are also a few marine species. Diatoms are common in both fresh and salt water, as well as in moist soils. Diatoms are typically single celled, although there also are filamentous species. They do not have flagella, but some have gliding motility.

Diatoms form silica-rich frustules as their cell walls; the two halves fitting together like a petri dish, with one side overlapping the other. These shell-like structures, such as those shown in Figure 10.39, give many diatoms a distinctive, often beautiful appearance, which frequently can be used for classification purposes. Geologic deposits of diatoms are mined to produce the diatomaceous earth used as a common filtration medium. During asexual reproduction, each half of the shell becomes the larger half of one of the two new frustules. Thus, one of the daughters is smaller than the parent. This continues until very small cells are formed, at which point sexual reproduction will occur.

Pyrrophyta: The Dinoflagellates Most dinoflagellates, such as Peridinium (Figure 10.40), are single-celled, mainly marine algae with a cell wall made of tough cellulose-silica plates.

Figure 10.39 SEM images of various diatom frustules. (Courtesy of the University of Iowa Central Microscopy Research Facility.)
Figure 10.40 Peridinium. (SEM image courtesy of C. J. O'Kelly, D. H. Hopcroft, and R. J. Bennett.)

They have two flagella, one wrapped beltlike around the middle in a transverse groove, the other extending backward in a longitudinal groove. This gives them a toplike motion, from which they get their name (from the Greek for "whirl''). Although most are free-living, some are symbionts growing in reef-forming corals, and some are parasitic. The Pyrrophyta are believed to be close relatives of the ciliated protozoans.

Blooms of dinoflagellates in coastal or estuarine waters may sometimes become sufficiently dense to cause red tides (Section 15.2.2). Further, a few dinoflagellate species, such as Gymnodinium, Gonyaulax, and Pfisteria, are able to release a neurotoxin into the water, which may lead to fish kills or to human poisoning due to the consumption of contaminated shellfish (Section 12.3.3). In recent years, Pfisteria has received considerable attention in the Chesapeake Bay, apparently finding a selective advantage by killing fish to secure degradable substrates!

Rhodophyta: The Red Algae The red algae, such as Porphyra, are mostly multicellular and marine, although there are also a few unicellular forms, and some that are freshwater or soil inhabitants. They have pigments similar to those of cyanobacteria, from which their chloroplasts may be derived. They produce no flagellated cells, further suggesting they are not derived from flagellates, as most other algae appear to be. Several red algae produce mucilaginous polymers that are used commercially, including the widely used food additive, carrageenan. Microbiologists also owe them a debt of gratitude, as they are the source of the agar widely used to solidify laboratory media for plating.

Chlorophyta: The Green Algae The green algae are believed to be the group from which higher plants evolved, and in fact they have the same pigments, cell wall composition, and storage products. This is an extremely varied and ecologically widespread group, with habitats that not only include soils and waters (both fresh and salt) but also the surfaces and internal regions of other organisms. Many are unicellular, but there are

Figure 10.41 Scenedesmus.

also colonial (e.g., Scenedesmus Figure 10.41) and multicellular (but undifferentiated) forms, and a few that are coenocytic (filaments in which the cross-walls between cells deteriorate or do not form, so that they become single cells with many nuclei), like fungi.

10.7.3 Slime Molds

The slime molds are aerobic, mainly terrestrial (a few are aquatic) chemoorganotrophs. Although they appear to have no major role in environmental engineering or science, they will be discussed briefly for completeness and because of their interesting life cycle. There are two separate phyla (Table 10.11), both with similarities to some protozoa.

Myxomycota Myxomycetes, or plasmodial slime molds, often form colorful visible growths on decaying wood or vegetation. An example is the genus Physarum, which is bright yellow. The vegetative form is composed of a plasmodium, which is a macroscopic multinucleated mass of cytoplasm with a single plasma membrane enclosing it (hence, their other common name, acellular slime molds). Other genera, such as Echinostelium, however, produce only microscopic plasmodia. The plasmodium crawls over surfaces with amoeboid motion (cytoplasmic streaming), engulfing food particles—mainly bacteria, but also yeasts, spores, and small pieces of organic material—through phagocytosis. Thus, it behaves much like a giant amoeba. However, when there is insufficient food, or the surface dries, the plasmodium forms a stalked sporangium and, after meiosis, releases haploid spores. Under proper conditions a spore germinates to produce a swarm cell (biflagellated) if there is sufficient water for swimming, or a myxamoeba (amoeboid cell) if the surface is only moist enough for crawling. Two such cells fuse to produce a diploid cell, which then undergoes mitosis (without cell division) to produce a new plasmodium.

Acrasiomycota Cellular slime molds, such as Dictyostelium and Polysphondylium, spend most of their active life as free amoeboid cells. Under unfavorable conditions

TABLE 10.11 The Fungi, Oomycetes (Water Molds), and Slime Molds

Group

Phyla

Common Name

Motility

Feeding

Comments

Examples

Fungi

Zyg°-

Lower fungi

None

Absorption

Most

Entomphthora

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