THE EFFECT OF LOCATION IN SOIL ON PROTOZOAL GRAZING OF A GENETICALLY MODIFIED BACTERIAL INOCULUM

Short-term laboratory experiments were performed to investigate the effect of location on protozoan grazing of a genetically modified bacterial inoculum in soil. Pseudomonas fluorescens (strain 10586, containing chromosomally borne genes encoding bioluminescence and antibiotic resistances) was introduced into varying pore size classes by adjustment of the soil matric potential with reference to the moisture release characteristic. The soil ciliate protozoan Colpoda steinii was subsequently introduced to the soil at conditions close to field capacity to ensure initial location in larger pores.

When the Ps. fluorescens was predominantly located in small pores (less than 6 mum pore neck diameter), the decline in viable cell concentration was less than that when located in larger pores. This suggests that the bacterial inocula introduced into soil may be protected by spatial compartmentalisation. This protection may be from protozoan grazing, in which case the predator activity of the introduced Colpoda inoculum was not significant in comparison to that of the indigenous protozoa. Further work is therefore required to determine the mechanism of protection but the findings demonstrate that the antecedent matric potential and pore size characteristics will be critical in determining the survival characteristics of microbial inocula in soil.