Community analysis of aggregated bacteria in southern Lake Baikal

This study focuses on the community structure of aggregated bacteria in Lake Baikal and relationships with free-living bacteria. Fluorescent in situ hybridization (FISH) methods were used in samples of bacteria taken in April, 2001. Bacterial counts of free-living bacteria by DAPI staining ranged from 0.2 × 106 to 3.2 × 106 cells · ml−1, decreasing with depth, whereas aggregated bacterial numbers dramatically increased with depth, ranging from 0.4 to 3.3 × 104 cells ml−1. Ratios of EUB probe binding cells to DAPI counts ranged from 52.3 to 74.1% in free-living bacteria, and from 39.6 to 66.7% in aggregated bacteria, respectively. Community composition of aggregated bacteria was very different from free-living bacteria, especially at the 25 m depth where highest phytoplankton numbers were observed. The vertical profile of aggregated bacteria community was very characteristic. β-Proteobacteria increased with depth down to 100 m. At 250 m, γ-Proteobacteria was 44% of DAPI bound cells, while other groups were less than 1%. We conclude that community structures of free-living and aggregated bacteria were different, and they may sustain the ecosystem in independent ways.