MORPHOLOGICAL AND COMPOSITIONAL SHIFTS IN AN EXPERIMENTAL BACTERIAL COMMUNITY INFLUENCED BY PROTISTS WITH CONTRASTING FEEDING MODES

In a two-stage continuous-flow system, we studied the impacts of diffe rent protozoan feeding modes on the morphology and taxonomic structure of mixed bacterial consortia, which were utilizing organic carbon rel eased by a pure culture of a Rhodomonas sp. grown on inorganic medium in the first stage of the system. Two of three second stages operated in parallel were inoculated by a bacterivorous flagellate, Bodo saltan s, and an algivorous ciliate, Urotricha furcata, respectively. The thi rd vessel served as a control. In two experiments, where algal and bac terial populations grew at rates and densities typical for eutrophic w aters, we compared community changes of bacteria, algae, and protozoa under quasi-steady-state conditions and during the transient stage aft er the protozoan inoculation. In situ hybridization with fluorescent o ligonucleotide probes and cultivation-based approaches were used to te ntatively analyze the bacterial community composition, Initially the c ell size distribution and community structure of all cultivation vesse ls showed similar patterns, with a dominance of 1- to 2.5-mu m-long ro ds from the beta subdivision of the phylum Proteobacteria (beta-Proteo bacteria). Inoculation ,vith the ciliate increased bacterial growth in this substrate-controlled variant, seemingly via a recycling of nutri ents and substrate released by grazing on algae, but without any detec table effect on the composition of bacterial assemblage. In contrast, an inoculation with the bacterivore, B. saltans, resulted in a decreas ed proportion of the beta-Proteobacteria. One part of the assemblage ( <4% of total bacterial numbers), moreover, produced large grazing-resi stant threadlike cells, As B. saltans ingested only cells of <3 mu m, this strategy yielded a refuge for similar to 70% of total bacterial b iomass from being grazed. Another consequence of the heavy predation i n this variant was a shift to the numerical dominance of the alpha-Pro teobacteria. The enhanced physiological status of the heavily grazed-u pon segment of bacterial community resulted in a much higher proportio n of CFU (mean, 88% of total bacterial counts) than with other variant s, where CFU accounted for similar to 30%, However, significant cultiv ation-dependent shifts of the bacterial community were observed toward gamma-Proteobacteria and members of the Cytophaga/Flavobacterium grou p, which demonstrated the rather poor agreement between cultivation-ba sed approaches and oligonucleotide probing.