As discussed earlier, the third domain of life is Eukarya. Early biologists recognized only two kingdoms, Plants and Animals. Fungi and algae (and later bacteria) were considered plants, and protozoans, once they were discovered, were placed with the animals. (Interestingly, recent evidence suggests that fungi may actually be more closely related to animals than to plants!) Later, the eukaryotes were broken into four kingdoms (with the prokaryotes considered a fifth): Animals; Plants; Fungi; and Protista, which included protozoans, algae, and slime molds. However, as with the prokaryotes, genetic testing (in this case, 18S rRNA) has shown that if the differences between plants and animals warrants assignment to separate kingdoms, the protists can be considered to consist of a fairly large number of "kingdoms," including several each of protozoans and algae, two of slime molds, and one that was formerly considered a fungus. Here though, for convenience, we will still use the three groupings within the "protists." Also, although many fungi and algae and the slime molds may be large, multicellular, macroscopic organisms, they will still be discussed below with their microscopic "cousins".

10.7.1 Protozoans

Protozoans are chemoorganotrophic unicellular heterotrophic eukaryotes. They may absorb dissolved nutrients, but most feed mainly by ingestion of small particles (such as bacteria, algae, bits of organic matter, or macromolecules) through one of three methods. In pinocytosis water droplets are drawn into a channel formed by the cell membrane, while in phagocytosis solid particles are engulfed and enclosed by the membrane. In many ciliates, feeding is by a mouth and gullet through which particles are pushed— like swallowing in animals. In each case the ingested particle is enclosed in a membrane-bound food vacuole into which digestive enzymes are secreted. (This is in contrast to the fungi, which excrete enzymes to digest food particles externally.) Since they do not have a cell wall, to maintain osmotic pressure most freshwater protozoans also have a contractile vacuole that is used to expel excess water. Both asexual reproduction (often by fission) and sexual reproduction occur in many species. Most are aerobic, but a few contain a special structure, the hydrogenosome, instead of mitochondria, and are obligate anaerobes. Most protozoans are aquatic or marine, but a large number are parasitic or symbiotic, and others are important members of soil ecosystems. Many free-living types are seen in wastewater treatment systems, where they are thought to aid in purification.

Protozoans are usually motile by one (or more) of four means, at least in one part of their life cycle, and this has led to their being broken into the four major groups described below (Table 10.9). However, it is now recognized that protozoans are a highly diverse group, and include several different phyla and probably even distinct kingdoms. Although there are some colonial forms, no protozoans are multicellular (unlike some algae and most fungi). Still, the single cell may be highly complex, with many specialized organelles, especially among the ciliates. Although some of the simpler flagellates may be only 5 to 10 mm in size, many of the ciliates are 30 to 500 mm, and some Sarcodina exceed 1 mm (although most are much smaller).

Mastigophora (Flagellates) The Mastigophora are motile by means of one or more flagella. This group actually includes several distinct lineages, including the Diplomonads [e.g., Giardia (Figure 10.30), a waterborne human parasite that causes giardiasis; Section 12.2.5], which lack mitochondria and are the most phylogenetically ancient known Eukarya. Another relatively ancient lineage, the Trichomonads, includes the human parasite Trichomonas vaginalis, which can cause vaginal and urinary tract infections (Section 12.6.6). Other flagellates include the Trypanosomes, such as Trypanosoma gambiense, the cause of African sleeping sickness (Section 12.5.2). Bodo is a common free-living flagellate (Figure 10.31) frequently seen in biological wastewater treatment. Other flagellates live in a small individual case called a lorica, often attached to the surface of a rock or another organism. Another group of flagellates, the Euglenophyta, contains chloroplasts and are discussed with the algae. (Often, the flagellates without chlorophyll have been referred to as "Zoomastigophora," to distinguish them from these "Phytomastigophora.") Sponges may have evolved from some of the flagellates. Some flagellates produce resistant cysts, helping them to withstand unfavorable conditions such as drying or exposure to toxic compounds (including disinfection).

Anaerobic flagellates include symbiotic inhabitants of the hindgut of termites and wood-eating cockroaches. In fact, it is the flagellates (or actually, probably the endosym-biotic bacteria within the flagellates) that produce the enzymes to digest the wood.

TABLE 10.9 The Protozoans

Group Motilitya Feeding1 Comments Examples

TABLE 10.9 The Protozoans

Group Motilitya Feeding1 Comments Examples



























None; flexing


Obligate parasites




a Primary or characteristic method.

a Primary or characteristic method.

If "cured" of its flagellates, the insect starves to death no matter how much wood it eats.

Sarcodina (Amoebas) Sarcodina, such as amoeba (Figure 10.32), typically move by pseudopodia ("false feet") formed by cytoplasmic streaming, and feed by phagocytosis. Some, such as Arcella (Figure 10.33; common in activated sludge), form shells, or tests. A few are pathogens, such as Entamoeba histolytica, the cause of amoebic dysentery (Section 12.2.4). Resistant cyst formation occurs in many species. Foraminiferans are often considered a separate phylum of marine testate organisms; deposits of their shells helped formed the famed white cliffs of Dover in England. Actinopods, which have numerous long projections protruding from their tests, also are now being considered a separate phylum.

Ciliophora (Ciliates) The ciliates appear to be a relatively recent lineage. Unlike other protozoans, they contain two different nuclei; a macronucleus, which carries out most normal nuclear functions, and a micronucleus, which is involved in sexual reproduction. A typical member of this phylum, such as Paramecium (Figure 10.34a), has numerous short hairlike projections, called cilia, which can be moved in a coordinated way for locomotion. In Aspidisca and Euplotes (Figure 10.34b and c) additional cilia fused together form organelles that act like legs for walking over a surface, and perhaps as spines for protection. Food particles are usually ingested by means of a gullet. As opposed to these free-swimming ciliates, some attach to a surface by a stalk; by coordinating the

Figure 10.30 Giardia: sketch and electron micrograph. (SEM courtesy of the US National Park Service.)

movement of its cilia, such a stalked ciliate can create water currents (like small whirlpools) that pull particles into its mouth. Other than the common Vorticella (Figure 10.35), which is solitary, most stalked ciliates (e.g., Epistylis) are colonial, with many organisms on a single branched stalk. One group of ciliates, the suctoreans (e.g., Podophrya, Figure 10.36), are stalked predators, waiting for a free-swimming ciliate to swim by; upon contact, a suctorean holds its prey by means of tentacles, and sucks out its cytoplasm. Some ciliates are anaerobic and are common members of the rumen ecosystem.

Apicomplexa (Sporozoans) The sporozoans are all obligate parasites. They feed by absorption of soluble nutrients from their host. The adult phases lack flagella or cilia but can move by flexing. They usually have a complex life cycle and form sporelike spor-ozoites to infect new hosts. Malaria (Section 12.5.1), one of the most important diseases worldwide, is caused by Plasmodium. Cryptosporidium (Figure 10.37), which has become a major concern in drinking water systems because of its extreme resistance to disinfection

Figure 10.31 Bodo in activated sludge.

by chlorination, is also a member of this group. In 1993, over 400,000 people were sickened in Milwaukee, Wisconsin—the largest known outbreak of any waterborne disease in the United States (Section 12.2.5). Toxoplasmosis (Section 12.3.3), caused by Toxoplasma, is of particular concern for pregnant women who eat undercooked meat or come in contact with cat feces.

10.7.2 Algae

Algae are photosynthetic, oxygenic autotrophs. Most are unicellular, but many are colonial, and some are multicellular. Unlike plants, they do not have fully differentiated roots,

Healthy Activated Sludge Microscope
Figure 10.32 Amoeba in activated sludge.

Figure 10.33 Arcella, a testate amoeba.

Figure 10.33 Arcella, a testate amoeba.

Figure 10.34 Free-swimming ciliates: (a) Paramecium; (b) Aspidisca; (c) Euplotes.

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