Geocheminar spring-2015

Microbial sulfate reduction (MSR) is an anaerobic metabolism that oxidizes organic matter and uses sulfate as an electron acceptor. This metabolism is thought to degrade about one third of organic matter in marine sediments. Because MSR fractionates sulfur isotopes, producing sulfide with a smaller 34S/32S relative to that of the initial sulfate, the isotopic composition of sedimentary sulfides and sulfates are used to detect sulfur based metabolisms throughout geologic history. Fractionations larger than 45‰ were also used to constrain the oxygenation of the environment during the Proterozoic. To understand processes that control the magnitude of sulfur isotope fractionation, we study the impact of carbon and phosphate limitation on isotopic and lipid signatures produced by marine sulfate reducing bacteria. Experiments conducted in the absence of oxygen show that microbes can fractionate sulfur isotopes in the 5-65‰ range. Therefore, the type and the availability of organic substrates exert the foremost influence on the magnitude of sulfur isotope fractionation, showing that sulfur isotope fractionations larger than 45‰ are not good indicators of environmental oxygenation. We also report for the first time that, when limited by phosphate, a number of sulfate reducing bacteria produce phosphorus-lacking glycolipids and aminolipids instead of phospholipids. This demonstrates the ability of non-phosphorus lipids to substitute for phospholipids in obligate anaerobes and reduce the requirement of these organisms for phosphorus. Furthermore, the prevalence of aminolipids and glycolipids in modern anoxic water columns and sediments suggests that non-photosynthetic microbes in these environments experience phosphate limitation. The physiological and ecological implications of these widespread lipid substitutions remain to be explored.