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

Abstract

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 strati- fication. According to the 16S rRNA gene clone libraries and length heterogeneity analysis (LH-PCR), heterotrophic bacteria dominated only in the oxic epilimnion, in which various Actinobacteria (mostly cluster acI-B) and Betaproteobacteria (especially Poly- nucleobacter subcluster PnecC) were common. Se- quences 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 bac- terium 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 bio- mass of the potentially autotrophic meta- and hypolim- netic 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.