Population Dynamics of Free-Living Bacteria Related to the Microcystin-Degrading Strain Y2 in Lake Suwa and in Microcystin Amended Enrichments

Abstract

We investigated the population dynamics of the free-living microcystin-degrading isolate Y2 (MCD-isolate) in an eutrophic lake (Lake Suwa, Japan) using fluorescence in situ hybridization (FISH) with a specific probe. Free-living MCD-isolate was successfully visualized by direct viable count method combined with FISH analysis. The highest concentration of MCD-isolate existed in 1999 when high concentrations of dissolved microcystin (Mcyst) and Microcystis populations were present. Mcyst degradation experiments with free-living bacteria collected at different times during a Microcystis bloom in 2000 indicated that three samples of free-living bacteria completely degraded Mcyst-LR and its isomer. The lag time before the degradation was however different for the free-living bacteria. Free-living bacteria present during the mid-bloom of Microcystis completely degraded Mcysts with the shortest lag time. The number of free-living cells of MCD-isolate detected by the FISH method significantly increased when Mcyst was degraded with the fastest degradation rate in the mid-bloom sample. Other bacterial populations collected at mid-and late-bloom increased during the experiments with the exception of the δ-Proteobacteria. However, the community structure was stable. Our findings suggest that MCD-isolate exists with various bacterial consortia in water and degrades Mcysts, the function of which is considered to be induced by exposure to Mcyst.