Bacterial diversity and morphology in deep ultraoligotrophic Andean lakes: role of UVR on vertical distribution

We investigated the community composition and the relative size and shape distribution of bacterial assemblages in deep transparent lakes, where bacteria may be affected by harmful ultraviolet radiation (UVR). Summer samples of the euphotic zones of nine ultraoligotrophic lakes located in different drainage basins between 40°27' and 42°49'S in the North Andean Patagonia were analyzed for relative diversity by denaturating gradient gel electrophoresis and the bacterial morphology (free-living cocci and rods vs. filamentous forms) in relation to environmental factors (UVR, photosynthetically active radiation, temperature, nutrients, nanoflagellates, and ciliate abundances). The overall bacterial community composition was similar in all lakes and over depth in each lake. In contrast, the relative proportion of filaments to total bacterial biovolume was higher in the upper layers, which have higher UVR intensities (305-340 nm). Lakes with lower Kd305 (diffuse extinction coefficient, 305 nm) showed a greater proportion of filaments. Filament mean length in the upper layers was also significantly greater than at deeper levels. There was a significant correlation between UVR and filament proportion, whereas neither nanopredator abundances (nanoflagellates and ciliates), nor predation pressure, nor resources (dissolved nutrients) could explain the high proportion of filamentation in the near-surface layers. We propose that UVR plays a decisive role stimulating bacterial filamentation, particularly in lakes with high UVR penetration.