Vertical diversity of bacteria in an oxygen-stratified humic lake, evaluated using DNA and phospholipid analyses

Microbes play a particularly important role in the food web in lakes with high dissolved organic carbon content. The bacterial community of a polyhumic lake, Mekkojarvi, was studied using DNA techniques and phospholipid fatty acid (PLFA) analysis during the mid-summer period of water column stratification. According to the 16S rRNA gene clone libraries and length heterogeneity analysis (LH-PCR), heterotrophic bacteria dominated only in the oxic epilimmon, in which various Actinobacteria (mostly cluster acI-B) and Betaproteobacteria (especially Polynucleobacter subcluster PnecC) were common. Sequences assigned to heterotrophic, methylotrophic, photoautotrophic, and chemoautotrophic genera were all abundant in the oxic-anoxic boundary layer. Methylobacter and Methylophilus were dominant genera among methylotrophic bacteria. Sequences assigned to the photoautotrophic green Sulfur bacterium Chlorobium sp. dominated in the anoxic water column, in which the microbial PLFA biomass was 6 times higher than in the oxic surface layer. All PLFA-profiles were dominated by 16 monounsaturated fatty acids typical of Gram-negative bacteria, whereas iso- and anteiso-branched PLFAs typical of Actinobacteria were present only in minor proportions. The high biomass of the potentially autotrophic meta- and hypolimnetic bacteria may form an important carbon source for the whole lake after spring and autumn overturns; thus, the role of these bacteria in the seasonal energy mobilization deserves more study in oxygen-stratified humic lakes and ponds.