Vertical distribution of microbial lipids and functional genes in chemically distinct layers of a highly polluted meromictic lake

We examined vertical changes in microbial lipid composition and functional genes in the 4.5 m deep water column of a highly contaminated, meromictic lake in eastern Massachusetts. Lipid based techniques and compound specific isotopic analysis indicate marked differences in community structure between the sulfate and methane laden hypolimnion and the aerated epiliminon. The major intact polar lipids (IPLs) detected throughout the water column include diacyl phosphatidylethanolamine, its methylated derivatives phosphatidylmethylethanolamine and phosphatidyldimethylethanolamine, phosphatidylglycerol, diphosphatidylglycerol, phosphatidylcholine, and the glycolipids monoglycosyldiacylglycerol and glycuronic acid dialkylglycerol. These compounds are attributed mainly to bacteria. The predominance of betaine lipids in the epilimnion is consistent with the dominant role of eukaryotic photoautotrophs in the oxic surface. Bacteriohopanepolyols and slight negative inflections of δ13C values of bacterial phospholipid-bound fatty acids (PLFAs) at the metalimnion indicate a low contribution of methanotrophic bacteria to the pool of metalimnic bacteria. In the hypolimnion, dialkylglycerol-phosphatidylethanolamine, C17-PLFAs and dsrAB gene sequence data suggest the presence of sulfate reducing bacteria (SRB). Five distinct groups of sulfate reducers and methanogens were detected in the hypolimnion. While SRB appear to inhabit the hypolimnion, the methanogens most likely entered the lake through the inflow of methane laden groundwater. We did not detect either of the methyl-coenzyme M reductase genes or characteristic lipids related to known anaerobic methane oxidizing Archaea although the hypolimnion exhibited conditions conducive to anaerobic methanotrophy. The large amount of dissolved organic carbon appears to be the major pool of carbon for the lake's microbial biomass.