Coolen, M. J. L., H. M. Talbot, B. A. Abbas, C. Ward, S. Schouten, J. K. Volkman and J. S. Sinninghe Damsté, Sources for sedimentary bacteriohopanepolyols as revealed by 16S rDNA stratigraphy, Environmental Microbiology, 10(7), 1783-1803, (2008)
Bacteriohopanoids are widespread lipid biomarkers in the geosphere and first appeared in the Archean. Many aerobic and anaerobic bacteria are potential sources of these lipids which can make it difficult to use these biomarkers as accurate proxies for climate-induced ecological and environmental changes. Therefore, we applied preserved 16S ribosomal RNA genes (16S rDNA) to identify Holocene biological sources of BHPs in the sulfidic sediments of the permanently stratified post-glacial Ace Lake, Antarctica. A suite of intact bacteriohopanepolyols (BHPs) were identified by HPLC APCI-MS, which revealed a variety of structural forms whose composition differed through the sediment core reflecting changes in bacterial populations induced by dramatically changing salinity. Stable isotopic compositions of the hopanols formed from periodic acid-cleaved BHPs, showed that some were substantially depleted in 13C, indicative of their methanotrophic origin. Using sensitive molecular tools, we found that both Type I and II methanotrophic bacteria (respectively Methylomonas and Methylocystis) were unique to the oldest lacustrine sediments (>9,400 years BP), but quantification of fossil DNA revealed that the Type I methanotrophs, including methanotrophs related to methanotrophic gill symbionts of deep-sea cold-seep mussels, were the main precursors of the 35-amino BHPs amino-pentol, -tetrol, and -triol. After isolation of the lake ~3000 years ago, one Type I methanotroph of the “methanotrophic gill symbionts cluster” remained the most obvious source of aminotetrol and -triol. The cyanobacterial marker 2-methylbacteriohopanetetrol coincided with the unique presence of a Synechococcus phylotype related to pelagic freshwater strains in the oldest sediments deposited under freshwater conditions. This combined application of advanced geochemical and paleogenomical tools has revealed new insights about the paleoenvironmental evolution of Ace Lake.Full article is available here |
