Aquatic protists modulate the microbial activity associated with mineral surfaces and leaf litter

Aquatic heterotrophic protists structure biofilm morphology and stimulate organic matter processing, but knowledge about their effects on the activity of surface-associated communities is still missing. Microcosm experiments revealed that the community respiration of young biofilms (7 d old) at mineral surfaces was not affected by co-cultivation with the raptorial feeder Chilodonella uncinata or the suspension feeder Tetrahymena pyriformis. However, grazing by both ciliates reduced the bacterial abundance and probably enhanced nutrient availability by recycling. Our data indicated an increased individual bacterial activity under grazing pressure, resulting in no net effect on the community respiration. In a second experiment, the respiration of leaf-associated microbial communities composed of the fungus Heliscus lugdunensis and a multispecies bacterial assemblage was significantly enhanced in the presence of T. pyriformis after 7 d of incubation. The stimulation was observed under both normoxic (turbulent) and hypoxic (turbulent and stagnant) conditions. After longer incubation, presumably matching an advanced phase of leaf degradation, T. pyriformis did not affect community respiration exposed to hypoxic stagnant conditions. In contrast to former studies, no impact of protists on leaf mass loss was observed. By stimulating leaf-associated community respiration, protists seem to affect processes involved in the initial phase of leaf processing.