Protozoan Density and the Coexistence of Protozoan Predators and Bacterial Prey

Abstract

Low numbers of Tetrahymena pyriformis provided with °2 x 10 9 Klebsiella pneumoniae or Rhizobium sp. cells/ml reduced the bacterial population only to 10 7 /ml in solution. When 5—7 x 10 8 K. pneumoniae or 1 x 10 8 Rhizobium sp. were preyed upon, 10 6 bacteria/ml survived and coexisted with the ciliate. Low initial densities of the protozoan provided with 3 x 10 6 or 5 x 10 7 K. pneumoniae or 3—4 x 10 7 Rhizobium sp./ml reduced the prey population t 2—3 x 10 5 /ml. However, when the initial number of T. pyriformis was high (2—34 x 10 4 /ml) and the K. pneumonia or Rhizobium sp. density was °10 7 /ml, the animal did not reduce the bacterial population size. The data suggest that the number of K. pneumoniae or Rhizobium sp. cells able to coexist in solution with T. pyriformis was governed by the number of protozoa initially present or developing as a result of predation. By contrast, 10 5 Xanthomonas citri cells/ml coexisted with the ciliate whether this bacterium was provided at 7 x 10 6 or 8 x 10 8 cells/ml. In soil treated with 3.5 x 10 8 and 3.1 x 10 9 Rhizobium sp./g, the density of indigenous protozoa rose to 1—2 x 10 4 and 4—5 x 10 5 /g, respectively, yet the density of rhizobial survivors stabilized at 3—7 x 10 7 /g, a range higher than that observed in solution.