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 archaeon Methanosarcina barkeri was grown either with H2 , acetate or methanol with D2 O 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 H2 /CO2 , acetate and methanol respectively. The variability in net water H assimilation into lipids during the growth of M. barkeri on 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 into M. barkeri lipids imply systematic, metabolic patterns of isotope incorporation and suggest that the ratio of 13 C-DIC versus D2 O assimilation in environmental samples may serve as a proxy for microbial energetics in addition to microbial production and carbon assimilation pathways.
Highlights
Stable isotopic compositions of microbial lipids provide valuable metabolic and taxonomic information that helps to decipher the role of microorganisms in biogeochemical cycles (Hayes, 1993; Hinrichs et al, 1999; Dawson et al, 2015)
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 dissolved inorganic carbon (DIC) and water H incorporation into M. barkeri lipids imply systematic, metabolic patterns of isotope incorporation and suggest that the ratio of 13C-DIC versus D2O assimilation in environmental samples may serve as a proxy for microbial energetics in addition to microbial production and carbon assimilation pathways
Summary
Stable isotopic compositions of microbial lipids provide valuable metabolic and taxonomic information that helps to decipher the role of microorganisms in biogeochemical cycles (Hayes, 1993; Hinrichs et al, 1999; Dawson et al, 2015). The 13C/12C ratio, expressed as δ13C, of microbial biomass is primarily determined by the carbon sources, fixation pathways and physiological conditions (e.g., Hinrichs et al, 1999; Hayes, 2001; Boschker and Middelburg, 2002; Schouten et al, 2004; Londry et al, 2008; Blaser et al, 2015), whereas the ratio of stable hydrogen isotopes (deuterium/protium ratio; D/H; expressed as δD) is determined by the water and substrate-based sources of hydrogen and the interactions of central metabolic pathways (Valentine, 2009; Zhang et al, 2009a; Wijker et al, 2019). The utilization of organic substrates in SIP experiments distorts the in situ nutrition conditions, for organic matter-lean microbial habitats, and it selects for a subpopulation of the microbial community, which probably characterizes a specific metabolic process rather than the entire community (Kopf et al, 2015)
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