Effects of a Microcystis aeruginosa bloom and bacterivory on bacterial abundance and activity in a eutrophic reservoir

We investigated the changes in bacterioplankton abundance, composition, and fractions of cells incorporating leucine associated with a Microcystis aeruginosa bloom and enhanced protozoan grazing in a eutrophic reservoir. Bacterial community composition and leucine incorporation were studied by means of fluorescence in situ hybridization and microautoradiography. To alter the levels of protozoan grazing and the interactions of bacteria with the M. aeruginosa bloom, a manipulation experiment using size-fractionation and dialysis bag techniques was conducted. Samples collected from the Microcystis-poor dam area and the Microcystis-rich middle part of the reservoir were incubated as <0.8 mu m, <5 mu m, and unfiltered treatments in dialysis bags at their collection sites and, in parallel, one of each of the treatments from one site was transferred to and incubated at the other site. The sample transfer from the dam to the middle part induced a decrease in abundances and doubling times of total bacteria, Betaproteobacteria, and of the R-BT065 cluster (a subgroup of Betaproteobacteria), whereas the transfer to the dam station revealed the opposite trends. In the presence of flagellates and the M. aeruginosa bloom, both abundance and fractions of cells incorporating leucine of Betaproteobacteria and of the R-BT065 cluster transferred to the middle station decreased dramatically, while members of the Sphingobacteria-Flavobacteria lineage became the dominant part of the community. While Gammaproteobacteria accounted for a minor proportion of the total community, their relative proportions and activity increased at the middle station, likely being stimulated by the increased resource availability.