A RANDOM ENCOUNTER MODEL FOR THE MICROBIAL PLANKTONIC COMMUNITY

The numeric concentration of procaryotic and eucaryotic cells that form the planktonic microbial community decreases monotonically and rapidly with increasing size. This size distribution has been interpreted to be the result of allometric control of growth and respiration rates, but we propose an alternative "random encounter" hypothesis to explain such a distribution. According to this hypothesis, the size distribution of cells that range from 0.3 to 100-mu-m results from size-selective predation by protozoans. A phagotrophic population will swim randomly through the water encountering both prey and predators. At steady state, such a population must ingest smaller prey at a sufficient rate to balance its rate of loss to larger predators. The size distribution of cells must, therefore, satisfy this condition. In particular, a mathematical statement of the hypothesis is based on the following relationships: volume of a cell varies as the cube of its diameter; clearance rate (L3T-1) by a predator varies as the square of its diameter; and, for a given population, its range of possible prey sizes varies with its prey diameter at the same time that the range of predator sizes able to capture the given population varies with its predator diameter.