Predation pressure on bacteria increases along aquatic productivity gradients

To elucidate how predation pressure on bacteria is affected by increasing productivity in aquatic systems, we ran model simulations of a microbial food web. In addition, we conducted a meta-analysis of data from studies in seas and lakes. The model was run as a flow-through system simulating conditions ranging from oligotrophic to hypertrophic. The organisms included in the model were heterotrophic bacteria, phytoplankton, 3 size classes of protozoa and metazooplankton. Predation pressure was defined as flagellate and ciliate grazing per bacterial biomass. The food-web model simulations showed increasing predation pressure on bacteria with increasing productivity (estimated chlorophyll a concentration 0.2 to 112 mu g 1(-1)). This was explained by an increase in bacterial production and the ratio of protozoan to bacterial biomass with increasing productivity. The results of the model simulation were supported by the meta-analysis, which showed increasing protozoan predation pressure on bacteria with increasing productivity in aquatic systems (chlorophyll a concentration 0.1 to 250 mu g 1(-1)). The ratio of protozoan to bacterial biomass also increased with productivity in the meta-analysis of field data. Our results suggest that protozoa control the bacterial community by predation in high-productivity environments.