Predator-induced changes of bacterial size-structure and productivity studied on an experimental microbial community

The grazing impact of 3 different protozoan species on a mixed bacterial co mmunity was studied by means of a simplified and functionally reproducible experimental microbial food web in a 2-stage flow-through system. In the fi rst stage the algae Rhodomonas sp. was grown on an inorganic medium with it s accompanying bacterial community (BC) growing on algal exudates. This mix ture of algae and bacteria was transferred into 4 second stage vessels: (1) a control, and 3 vessels inoculated with (2) a heterotrophic nanoflagellat e, Bodo saltans, (3) a scuticociliate, Cyclidium glaucoma, and (4) a mixotr ophic flagellate, Ochromonas sp. Using image analysis techniques we followe d the changes in bacterial size distributions and bacterial to protozoan to tal biovolume ratios over an experimental period of 15 d. In addition, prod uctivity of the grazed and ungrazed BC was measured using [H-3]thymidine an d [C-14]leucine. As a consequence of total grazing rates and size-selective feeding we observed 3 different responses of the initially identical BC to grazing of the 3 protists. (1) Low grazing by B. saltans caused a slow dec rease of bacterial cell numbers from 14 to 5.9 x 10(6) cells ml(-1), but no significant shift of the mean cell volume (MCV, average 0.107 mu m(3)) and bacterial production. (2) Higher grazing rates of C. glaucoma resulted in the decline of bacterial abundance to 3.3 x 10(6) cells ml(-1) in parallel with a doubling of the MCV to 0.207 mu m(3) and high DNA and protein synthe sis rates. Due to the ciliate's ability to graze also on small prey (<1.5 m u m) an increase in MCV seemed to provide higher grazing resistance or at l east decreased vulnerability. (3) Ochromonas sp. showed the highest grazing rates and reduced bacterial numbers by 20 times within 2 d. Only the small est-obviously more grazing-protected-bacteria (<0.9 mu m) survived, which i ncreased thereafter to almost initial bacterial densities. Although the bac terial MCV dropped to 0.038 mu m(3), [H-3]thymidine uptake rates per cell w ere greatly enhanced and highly variable. Our results reflect the potential of BC responses to different predation regimes and the advantages of pheno typic traits in order to coexist with various grazers. This should be seen in the context of influencing bottom-up effects and the varying potential o f individual bacterial species to change morphology, growth strategies, and activity patterns.