These protozoa feed on bacteria and this capacity of protozoa plays an important role in the cycling of carbon and other nutrients. In addition, the population density of protozoa serves as an indirect measure of the bacterial growth rate.
The dependence of protozoa on the bacterial flora is also reflected on their distribution within the groundwater ecosystem. As in the case of bacteria, the protozoan number declines with the depth in horizons beneath the soil surface.
Protozoa have been found in least numbers in clay regions. In regions like coarse textured sands and gravels, where bacteria are seen in abundance, protozoan number is also high.