Population dynamics and activity of bacteria and protozoa in relation to their spatial distribution in soil

Abstract

The effects of protozoan inoculum density and spatial separation of (prey) organisms on the turnover and transport of specific microbial populations in sterilized soil were studied in soil microcosms, applying a combination of techniques. The fate of introduced microbial biomass was followed by monitoring 14C-CO 2 evolution. Dynamics of and genetic transfer between genetically marked bacteria were followed by selective plating using appropriate antibiotics. Three levels of protozoan densities, i.e. no protozoa, protozoa and a 1: 10 diluted protozoan inoculum were established. Two methods of soil inoculation, i.e. joint introduction and separate introduction of protozoa with a donor population and of a 14C-labelled recipient population, respectively, were applied. The activity of protozoa, as judged from increased CO 2 evolution compared to soils not inoculated with protozoa, was larger when 100% of the soil was inoculated with protozoa (joint introduction) compared to inoculation of protozoa into only 33% of the soil which was mixed with 66% of a soil portion not inoculated with protozoa (separate introduction). The activity of protozoa accelerated the turnover of 14C compared to soils without protozoa. The more protozoa added, the faster 14C was respired both in soils with joint, and in soils with separate, introduction. The transfer of genetic material between introduced donor and recipient cells was not influenced by the presence of protozoa but was reduced when separate introduction was applied compared to joint introduction. Transfer of genetic material was only found during the first 2 days of the incubation. Substantial loss of genetic material (plasmid RP4) in the donor population was detected; the presence of protozoa enhanced both donor survival and plasmid maintenance. A stimulating effect of protozoa on cellular metabolism of substrate used by the donor population is hypothesized. These results picture soil as a system in which the distribution of microorganisms in the absence of fluctuations in climatic conditions is not changing in time: protozoa hardly moved through the soil and minimal translocation of bacteria was found.