Impacts Of Protozoa On Ecosystem Function

Several investigators have noted the obvious parallel between the protozoan-microbe interaction in water films in soil and on root surfaces and in open-water aquatic systems (Stout, 1963; Clarholm, 1994; Coleman, 1994). The "microbial loop," defined by Pomeroy (1974) as the rapid recycling of nutrients by protozoan grazers, is a powerful conceptual tool. In a fashion similar to that occurring in aquatic systems, rapidly feeding protozoa may consume one or more standing crops of bacteria in soil every year (Clarholm, 1985; Coleman, 1994). This tendency is particularly marked in the rhizosphere, which provides a ready food source for microbial prey.

The population dynamics of bacteria, naked amoebae, and flagellates were followed in the humus layer of a pine forest in Sweden (Clarholm, 1994). Bacteria and flagellates began increasing in number immediately after a rainfall event and rose to a peak after 2-3 days. Naked amoebae rose more slowly, peaked after 4-5 days, and then tracked the bacterial decrease downward. The flagellates followed a pattern similar to that of the naked amoebae. Further information on protozoan feeding activities and their impacts on other organisms and ecosystem function is given in Darbyshire (1994). Bonkowski et al. (2000) suggest that protozoa, and the bacteria they feed upon in the rhizosphere, produce plant growth-promoting compounds that stimulate plant growth above and beyond the amounts of N mineralized in the rhizosphere. Protozoa and other microfauna are quite sensitive to environmental insults, and changes in the distribution and activities are diagnostic of changes in soil health (Gupta and Yeates, 1997).

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