Top–down control of lower trophic levels, e.g., bacteria, has been suggested to increase along aquatic productivity gradients. The response by the bacterial community may be to become more predation resistant in highly productive environments. To test this hypothesis, samples were taken from 20 aquatic systems along a productivity gradient (dissolved organic carbon from 7 to 71 mg/L), during late summer. The results showed that the biomass of bacteria, phytoplankton, and ciliates increased along the gradient (r 2 = 0.532, 0.426, and 0.758, P < 0.01, respectively). However, the organisms did not increase equally, and the ratio of protozoan to bacterial biomass showed a 100-fold increase along the gradient. Ciliates dominated the protozoan biomass in the more nutrient-rich waters. The edibility of colony-forming bacteria was tested using a ciliate predator, Tetrahymena pyriformis. Bacterial edibility was found to decrease with increases in nutrient richness and ciliate biomass in the aquatic systems (r 2 = 0.358, P < 0.01; r 2 = 0.242, P < 0.05, respectively). Quantile regression analysis indicated that the selection pressures on edible bacteria were increasing along the productivity gradient. Thus, inedible forms of bacteria were selected for in the transition from oligotrophic to eutrophic conditions. Isolated bacteria were distributed among the α-, β-, and γ- Proteobacteria and the Actinobacteria and Firmicutes taxa. We conclude that bacterial predation resistance increases in nutrient-rich waters with high protozoan predation.
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 µg l–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 µg l–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.