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Large protozoa that have many nuclei, or large amitotic, polyploid nuclei may be best considered acellular rather than unicellular creatures. Nonetheless, some species have been studied extensively by cell biologists as model cells, and ecologists interested in protozoa can glean much useful information from their labors. This section will rely heavily on information from those few taxa. We will first describe some organelles that are important and widely distributed among protozoa and then discuss major groups of protozoa with respect to their more unique organelles. Lastly, we will revert to an overall view in discussing environmental physiology of protozoa. In all sections, we will emphasize those organelles that relate to feeding, locomotion, ecology, and morphology at the light-microscope level.
Protozoa
Nigel Horan, in Handbook of Water and Wastewater Microbiology, 2003
4 PROTOZOAL NUTRITION
Protozoa demonstrate a wide range of feeding strategies of which four types are represented by the protozoa found in wastewater treatment systems. Certain members of the Phytomasti-gophorea are primary producers and capable of photoautotrophic nutrition, in addition to the more usual chemoheterotrophic nutrition.
Heterotrophy among the flagellated protozoa contributes to the process of biochemical oxygen demand (BOD) removal, and uptake of soluble organic material occurs either by diffusion or active transport. Protozoa that obtain their organic material in such a way are known as saprozoic, and are forced to compete with the more efficient heterotrophic bacteria for the available BOD. Amoebae and ciliated protozoa are also capable of forming a food vacuole around a solid food particle (which include bacteria) by a process known as phagocytosis. The organic content of the particle may then be utilized after enzymic digestion within the vacuole, a process which takes from 1 to 24 hours. This is known as holozoic or phagotrophic nutrition, and does not involve direct competition with bacteria, which are incapable of particle ingestion.
The final nutritional mode practised by the protozoa is that of predation. These predators are mainly ciliates, some of which are capable of feeding on algae (and are thus herbivores), as well as other ciliate and flagellate protozoal forms.
All protozoa rely on phagocytosis for their energy and carbon for building cellular material (Fig. 4.2). This involves the enclosure of a solid food particle in a vacuole, which is covered with a membrane and in which digestion occurs. Dissolved nutrients are removed from the vacuole leaving the indigestible remains behind. These are removed from the cell by fusion of the vacuole with the cell surface membrane. The typical lifetime of a food vacuole is around 20 minutes, although this time reduces if the cell is not feeding.
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Fig. 4.2. Degradation of a food particle by phagacytosis. (a) Food particle engulfed by pseudopodia and a vacuole formed; (b) enzymic digestion occurs within the vacuole and digestion products released to the cytoplasm; (c) undigested remains ejected from the body.
In addition to phagocytosis, there are other mechanisms by which a protozoan can obtain energy and cellular building blocks. Some protozoa participate in symbiotic relationships with photosynthetic organisms, whereas others are thought able to take up dissolved nutrients. It is doubtful, however, if this latter mechanism plays any role for the free-living protozoa outside of a laboratory culture.
Although phagocytosis is practised by all the protozoa there are a number of different feeding patterns which are exploited to capture the solids particle and these can be classified into three categories, namely: filter feeders, raptorial feeders and diffusion feeders. Filter feeding involves the creation of a feeding current, which is then passed through a device which acts to filter out the solids particles in the water. In the flagellates this is a collar of straight, rigid tentacles. For the ciliates the water is passed through an arrangement of parallel cilia. The clearance between the tentacles in the collar and the parallel ciliates, dictates the size of particle that is retained. This is typically between 0.3 and 1.5 μm and helps to explain why the presence of a healthy ciliate population in an activated sludge plant generates such a crystal clear effluent with a reduced number of faecal indicator bacteria (Table 4.3).
TABLE 4.3. The effects of ciliated protozoa on the effluent quality from a bench-scale activated sludge plant