Characterization of the depth-related changes in the microbial communities in Lake Hovsgol sediment by 16S rRNA gene-based approaches.

The undisturbed sediment of Lake Hovsgol (Mongolia) is scientifically important because it represents a record of the environmental changes that took place between the Holocene (the present age) and Pleistocene (the last ice age; 12,000 14C years before present day). Here, we investigated how the current microbial communities change as the depth increases by PCR-denaturing gradient gel electrophoresis (DGGE) analysis of the 16S rRNA genes of the microbial communities. The microbial diversity, as estimated by the Shannon index, decreased as the depth increased. In particular, significant changes in archaeal diversity were observed in the middle depth (at 39-42 cm depth of total 60 cm depth) that marks the border between the Holocene and Pleistocene. Phylotype belonging to Beta-and Gamma-Proteobacteria were the predominant bacteria and most of these persisted throughout the depth examined. However, as the depth increased, some bacteria (some genera belonging to Beta-Proteobacteria, Nitrospira, and OP8-9) were not detectable while others (some genera belonging to Alpha-, Beta-, Gamma-Proteobacteria) newly detected by DGGE. Crenarchaea were the predominant archaea and only one phylotype belonging to Euryarchaea was found. Both the archaeal and bacterial profiles revealed by the DGGE band patterns could be grouped into four and three subsets, respectively, subsets that were largely divided by the border between the Holocene and Pleistocene. Thus, the diversity of the current microbial communities in Lake Hovsgol sediments decreases with increasing depth. These changes probably relate to the environmental conditions in the sediments, which were shaped by the paleoclimatic events taking place between the Holocene and Pleistocene.