Hydrogen-Isotopic Systematics of Lipid Biosynthesis in Hydrogen-Consuming Anaerobes and Aerobes

In anoxic sediments, molecular hydrogen (H₂) is a key intermediate in the transfer of electrons between H₂-producing (e.g., fermentative) bacteria and H₂-consuming microbes, including sulfate-reducing bacteria (SRB). H₂ is a potential source of lipid-bound hydrogen for SRB, as are water and organic matter. Relative to these other potential sources, H₂ typically is markedly depleted in deuterium. If hydrogen from strongly D-depleted H₂ is incorporated into SRB lipids, the isotopic signal could be preserved over geologic time in biomarker compounds in the sediments. The accumulation of characteristically D-depleted SRB biomarkers may thus provide a quantitative measure of sulfate reduction (and hence of carbon remineralization by SRB) in the ancient environment. Ongoing experiments are designed to quantify the relative contributions of H₂, water, and organic matter to lipid-bound hydrogen in SRB, as well as to determine the associated hydrogen-isotopic fractionations. Desulfobacterium autotrophicum, a facultative autotroph, is grown in pure culture under various isotopically defined conditions. Water in the media and key metabolites are monitored for D/H. The produced biomass is harvested, and D/H ratios of individual lipid compounds are measured. Isotopic mass-balance calculations based on these data will allow us to determine 1) hydrogen-isotopic compositions of SRB lipids, 2) effects of growth conditions on D/H ratios, and 3) the biochemical sources for lipid-bound hydrogen. Similar experiments are underway to identify and quantify the controls on stable hydrogen-isotopic fractionation during lipid biosynthesis in syntrophic cocultures and in pure cultures of H₂-consuming, aerobic (i.e., knallgas) bacteria. Taken together, these experiments will provide a first test of our hypothesis that D/H ratios in lipids can be used to quantify carbon remineralization by SRB in modern, and potentially ancient, sediments.