Growth of a ciliate protozoan in model ballotini systems of different particle sizes

The effect of structure (i.e. particle size) on protozoan population development was studied using liquid culture, containing known amounts of the bacterium Erwinia cartovora subsp. carotovora or nutrient growth medium PPY, and the ciliate protozoan Tetrahymena pyriformis. Structure was introduced into each system in the form of different size ranges of ballotini (glass beads), or sand. Even with the smallest particle sizes used, all pore pathways were accessible to protozoa. Incorporation of structure into nutrient solution acted to lower significantly (P < 0.05) protozoan activity in the structured pore network, as compared to the nutrient solution without structure. There were no significant differences (at the 5% level) in the final protozoa population in either substrate system. As particle size decreased, the protozoan population also decreased. Structure was shown to introduce distances between protozoa and bacterial cells thus, in comparison with treatments without structure, protozoan populations were significantly reduced. Further, reducing particle size would increase the time taken to explore the available pore volume, and reduce the amount of food available in each pore. The outcome was that decreasing the particle size reduced the feeding rate and so reduced the rate of population increase. Other possible physical mechanisms which may limit protozoan movement, such as surface area, are examined. © 1994.