Methods For Extracting And Counting Protozoa

Researchers have favored the culture technique (Singh, 1946), in which small quantities of soil or soil suspensions from dilution series are incubated in small wells, inoculated with a single species of bacteria as a food source. Based on the presence or absence in each well, one can calculate the overall population density ("most probable number"). Adl (2003), Couteaux (1972), and Foissner (1987) favor the direct count approach, in which one examines soil samples, in water, to see the organisms present in the subsample. The advantage of direct counting is that it is possible to observe the organisms immediately present and not rely on the palata-bility of the bacterium as substrate in the series of wells in the culture technique. The disadvantage of the direct count method is that one usually employs only 5-30 mg of soil, so as not to be overwhelmed with total numbers (Foissner, 1987). This discriminates against some of the more rare forms of testaceans or ciliates that may have a significant impact on an ecosystem process, if they happen to be very large.

The culture technique attempts to differentiate between active (trophozoite forms) and inactive (cystic) forms by treatment of replicate samples with 2% hydrochloric acid overnight. The acid kills off the trophozoite forms. After a wash in dilute NaCl, the counting continues. This assumes that all cysts will excyst after this drastic process, an assumption not always met.

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