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 predation is one of the main factors responsible for the decline of bacterial populations introduced into soil. Bacteria, however, are not equally susceptible to protozoan predation. We have studied the response of indigenous protozoan populations to the introduction of the poIychlorophenol-degrading bacterium Mycobacterium chlorophenolicum PCP-1 (DSM 43826), and Pseudomonas chlororaphis (ATCC 43928), into soil microcosms. Introduction of P. chlororaphis to the soil resulted in a huge increase in the numbers of heterotrophic flagellates and naked amoebae during the first 8 days of the experiment. Addition of M. chlorophenolicum to soil caused only a slight increase in protozoan numbers, which was similar to the increase caused by addition of water. There was no indication that addition of M. chlorophenolicum to soil resulted in any increase in the number of protozoa able to feed on this bacterium. The number of colony forming units (CFU) decreased rapidly in the treatment amended with P. chlororaphis cells, whereas there was no decrease in CFUs in the M. chlorophenolicum treatment. The only slight increase in protozoan numbers in the M. chlorophenolicum treatment, as well as the apparently low mortality rate of M. chlorophenolicum in the soil microcosms, coincided with significantly lower soil respiration in the soil microcosms amended with M. chlorophenolicum compared to those amended with P. chlororaphis. The results suggest that the indigenous soil protozoa did not graze on M. chlorophenolicum at all, presumably because it is not a suitable food source.