Protozoan predation of bacteria in soil
Survival of P. fluorescens 10586s FAC510 was studied in liquid culture and soil microcosms in the presence of the ciliate C. steinii and the flagellate Cercomonas sp. Both protozoa caused a reduction in bacterial viable cell concentration, however the ciliate caused a greater decrease in abundance. Starvation of the bacterial prey caused a reduction in grazing rates of both the ciliate and flagellate, indicating the lower nutrional quality of starved as opposed to non-starved cells. Manipulation of the physiological state of prey cells, by starvation, had much the same effect as observed when bacterial prey resources are reduced. Luminescence provided a valuable marker for monitoring P. fluorescens 10586s FAC510 in liquid culture and soil, since detection by luminometry provided a sensitive, rapid, and non-extractive technique for measurement of microbial activity. In the presence of C. steinii, bacterial activity increased, whilst predation by Cercomonas sp. caused a reduction. The different responses of the bacterial inoculum may reflect the divergent balance between bacterial turnover, leading to nutrient regeneration, and grazing strategies, which are selective of the two protozoa. The distribution of bacterial cells in the soil pore network was manipulated by adjustment of the antecedant matric potential prior to inoculation. The ability to predominantly place bacterial and protozan cells in specific pore size classes was confirmed by partial chloroform fumigation and resin impregnated sections of soil.