RESUMEN

We have developed a method for determining the potential abundance of free-living protozoa in soil. The method permits enumeration of four major functional groups (flagellates, naked amoebae, testate amoebae, and ciliates) and it overcomes some limitations and problems of the usual 'direct' and 'most probable number' methods. Potential abundance is determined using light microscopy, at specific time intervals, after quantitative re-wetting of air-dried soil with rain water. No exogenous carbon substrates or mineral nutrients are employed, so the protozoan community that develops is a function of the resources and inhibitors present in the original field sample. The method was applied to 100 soil samples (25 plots x 4 seasons) from an upland grassland (Sourhope, Southern Scotland) in the UK. Median abundances for all four functional groups lie close to those derived from the literature on protozoa living in diverse soil types. Flagellates are the most abundant group in soil, followed by the naked amoebae, then the testate amoebae and ciliates. This order is inversely related to typical organism size in each group. Moreover, preliminary evidence indicates that each functional group contains roughly the same number of species. All of these observations would be consistent with soil having fractal structure across the size-scale perceived by protozoa. The method described will be useful for comparing the effects on the soil protozoan community of different soil treatments (e.g. liming and biocides).

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RESUMEN

We have examined and quantified the protozoa living in a productive freshwater pond during a 2-day period in June 1987. Over 90 species were recognised. The planktonic and benthic communities were dominated by ciliates and heterotrophic flagellates although the large amoeba Pelomyxa palustris was abundant (102 ml(-1)) in anaerobic sediments. Picoplankton averaged 1.4 × 10(7) ml(-1), phototrophic nanoplankton 0.8 × 10(5) ml(-1), heterotrophic nanoplankton 0.9 × 10(5) ml(-1) and planktonic ciliates 1.3 × 10(2) ml(-1). Numbers were about two orders of magnitude higher in the sediment. Protozoan biomass ranged from 3% to 61% of the total plankton biomass. Heterotrophic flagellates were the principal grazers of the picoplankton. Planktonic ciliates fed mainly on phototrophic nanoplankton but they probably also ingested heterotrophic flagellates. Benthic ciliates were predominantly bactivorous. Competition between ciliate species was minimised by both spatial and food niche separation. Ten species of planktonic ciliates appeared to contain algal symbionts: one species (Strombidium viride) contained structures resembling sequestered chloroplasts. These findings concerning the diversity and abundance of protozoa in a freshwater pond are consistent with the consensus opinion expressed in the marine literature that protozoa play a fundamental role in microbial food webs within aquatic ecosystems.

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