Protozoan response to addition of the bacteria Mycobacterium chlorophenolicum and Pseudomonas chlororaphis to soil microcosms

Protozoa are important predators of bacteria in soil and protozoan predatio n is one of the main factors responsible for the decline of bacterial popul ations introduced into soil. Bacteria, however, are not equally susceptible to protozoan predation. We have studied the response of indigenous protozo an populations to the introduction of the poIychlorophenol-degrading bacter ium Mycobacterium chlorophenolicum PCP-1 (DSM 43826), and Pseudomonas chlor oraphis (ATCC 43928), into soil microcosms. Introduction of P. chlororaphis to the soil resulted in a huge increase in the numbers of heterotrophic fl agellates and naked amoebae during the first 8 days of the experiment. Addi tion of M. chlorophenolicum to soil caused only a slight increase in protoz oan numbers, which was similar to the increase caused by addition of water. There was no indication that addition of M. chlorophenolicum to soil resul ted in any increase in the number of protozoa able to feed on this bacteriu m. The number of colony forming units (CFU) decreased rapidly in the treatm ent amended with P. chlororaphis cells, whereas there was no decrease in CF Us in the M. chlorophenolicum treatment. The only slight increase in protoz oan numbers in the M. chlorophenolicum treatment, as well as the apparently low mortality rate of M. chlorophenolicum in the soil microcosms, coincide d with significantly lower soil respiration in the soil microcosms amended with M. chlorophenolicum compared to those amended with P. chlororaphis. Th e results suggest that the indigenous soil protozoa did not graze on M. chl orophenolicum at all, presumably because it is not a suitable food source.