The effect of location in soil on protozoal grazing of a genetically modified bacterial inoculum

Wright, D.A., Killham, K., Glover, L.A. and Prosser, J.I., 1993. The effect of location in soil on protozoal grazing of a genetically modified bacterial inoculum. In: L. Brussaard and M.J. Kooistra (Editors), Int. Workshop on Methods of Research on Soil Structure/Soil Biota Interrelationships. Geoderma, 56: 633-640. 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 am 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. I N T R O D U C T I O N The introduction of bacterial populations into the soil environment is frequently followed by a decline in viable cell concentration. This decline ceases when a characteristic survival concentration is reached, which appears to be independent of the initial inoculum size (Crozat et al., 1987). The grazing Correspondence to: J.I. Prosser, Department of Molecular and Cell Biology, University of Aberdeen, Marischal College, Aberdeen, AB9 1AS, Scotland, UK. 0016-7061/93/$06.00 © 1993 Elsevier Science Publishers B,V. All rights reserved. 634 D.A. WRIGHT ET AL. activities of indigenous protozoa are considered to be a significant component in the survival and establishment of bacterial inocula (Habte and Alexander, 1978 ). The predatory activities of protozoa are considered to be affected by the matric potential of the soil, since protozoa are dependent upon water for their dispersal and movement through the soil (Sleigh, 1973). The largest protozoan populations are found to exist in saturated soils, and the lowest concentrations in dry soils (Darbyshire, 1976). Postma et al. (1989) observed an increased survival rate of bacterial cells introduced into relatively dry soils in comparison to those inoculated into wetter soils, and suggested that this resuited from reduced protozoan predation. The heterogeneous and discontinuous structure of soil provides a number of distinct or temporally discrete microhabitats, which are influenced by environmental fluctuations (Hattori and Hattori, 1976). Alexander (1981) proposed the importance of such microsites in the survival of bacterial inocula where they exclude the predator and protect the prey. At low matric potentials soil water will be restricted to those pores with small size diameters. The predatory activities of the protozoa under such conditions are thought to be restricted, since they are denied access to their prey due to their larger size. Postma and Van Veen ( 1990 ) described the habitable and protected pore space in the soil matrix by varying the matric potential of the soil with reference to the moisture release characteristic. Heijnen and Van Veen ( 1991 ) found that pores with neck size diameters less than 6/zm positively affected the survival of introduced bacteria, whereas pores with neck size diameters greater than 6/zm had a negative effect. This article describes short-term laboratory experiments performed to investigate the effect of location on protozoal grazing of a genetically modified bacterial inoculum in soil. Bacterial inocula were located in distinct pore size classes with reference to the soil moisture release characteristic. Protozoa were introduced at matric potentials sufficient to ensure their location in larger pores. Introduction of bacteria into pores with small neck size diameters ( < 6 /~m ) was intended to spatially compartmentalise the predator and prey. M A T E R I A L S A N D M E T H O D S Soil type and preparation A sandy loam soil from the Insch series (Grid ref. NJ659223) was used. Soil samples were taken from grass ley sites ensuring minimal pesticide contamination. The soil was sieved to collect the fraction of particle size less than 3 m m and was air dried and stored at 4 ° C. The soil moisture release characteristic was determined by equilibration of initially saturated soil on pressure membrane apparatus. Figure 1 shows the relationship between soil matric EFFECT OF LOCATION IN SOIL ON PROTOZOAL GRAZING 635