Vertical variation of bulk and metabolically active prokaryotic community in sediment of a hypereutrophic freshwater lake.

This study was conducted to acquire novel insight into differences between bulk (16S rDNA) and metabolically active (16S rRNA) prokaryotic communities in the sediment of a hypereutrophic lake (Japan). In the bulk communities, the class Deltaproteobacteria and the order Methanomicrobiales were dominant among bacteria and methanogens. In the metabolically active communities, the class Alphaproteobacteria and the order Methanomicrobiales and the family Methanosaetaceae were frequently found among bacteria and methanogens. Unlike the bulk communities of prokaryotes, the composition of the metabolically active communities varied remarkably vertically, and their diversities greatly decreased in the lower 20 cm of sediment. The metabolically active prokaryotic community in the sediment core was divided into three sections based on their similarity: 0-6 cm (section 1), 9-18 cm (section 2), and 21-42 cm (section 3). This sectional distribution was consistent with the vertical pattern of the sedimentary stable carbon and nitrogen isotope ratios and oxidation-reduction potential in the porewater. These results suggest that vertical disturbance of the sediment may influence the communities and functions of metabolically active prokaryotes in freshwater lake sediments. Overall, our results indicate that rRNA analysis may be more effective than rDNA analysis for evaluation of relationships between actual microbial processes and material cycling in lake sediments.