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Abstract

Dual stable isotope probing has been used to infer rates of microbial biomass production and modes of carbon fixation. In order to validate this approach for assessing archaeal production, the methanogenic archaeonMethanosarcina barkeriwas grown either with H-2, acetate or methanol with D2O and(13)C-dissolved inorganic carbon (DIC). Our results revealed unexpectedly low D incorporation into lipids, with the net fraction of water-derived hydrogen amounting to 0.357 +/- 0.042, 0.226 +/- 0.003 and 0.393 +/- 0.029 for growth on H-2/CO2, acetate and methanol respectively. The variability in net water H assimilation into lipids during the growth ofM.barkerion different substrates is possibly attributed to different Gibbs free energy yields, such that higher energy yield promoted the exchange of hydrogen between medium water and lipids. Because NADPH likely serves as the portal for H transfer, increased NADPH production and/or turnover associated with high energy yield may explain the apparent differences in net water H assimilation into lipids. The variable DIC and water H incorporation intoM.barkerilipids imply systematic, metabolic patterns of isotope incorporation and suggest that the ratio of(13)C-DIC versus D2O assimilation in environmental samples may serve as a proxy for microbial energetics in addition to microbial production and carbon assimilation pathways.