Electron-bifurcating transhydrogenase is central to hydrogen isotope fractionation during lipid biosynthesis in sulfate reducing bacteria

Abstract:

A significant range in microbial lipid ²H/¹H ratios is observed in modern marine sediments [Li et al. 2009. GCA]. The magnitude of hydrogen isotope fractionation between microbial lipids and growth water (²ε_lipid-H2O) is hypothesized to relate to the central carbon and energy metabolism [Zhang et al. 2009. PNAS]. These observations have raised the intriguing possibility for culture independent identification of the dominant metabolic pathways operating in environments critical to the geological record. One such metabolism we would like to track for its global significance in sedimentary carbon cycling is bacterial sulfate reduction [Jørgensen. 1982. Nature]. To-date, heterotrophic sulfate reducing bacteria (SRB) have been observed to produce lipids that are depleted in fatty acid H-isotope composition, relative to growth water (²ε_lipid-H2O ~ -125 to -175 ‰), with experiments on different substrates yielding little variability [Campbell et al. 2009. GCA; Osburn. 2013; Dawson et al. 2015. Geobiology]. In stark contrast, aerobic heterotrophs show a wide range in fractionations (²ε_lipid-H2O ~ +300 to -125‰) which seems to scale with the route cellular carbon metabolism [Zhang et al. 2009. PNAS; Heinzelmann et al. 2015. Front Microbio].

Recent work in aerobic methylotrophs [Bradley et al. 2014. AGU] implicates transhydrogenase (TH) activity as a critical control on ²ε_lipid-H2O. This work suggests a specific driving mechanism for this range in fractionation is the ratio of intracellular NADPH/NADH, and more fundamentally, the intracellular redox state. In SRB a key component of energy metabolism is the activity of electron-bifurcating TH [Price et al. 2014. Front Microbio], for which a recent transposon mutant library has generated a number of knockouts in the target gene [Kuehl et al. 2014. mBio] in the model organism Desulfovibrio alaskensis strain G20. In this study we compare growth rates, fatty acid concentrations and ²ε_lipid-H2O from wild type and TH mutants in strain G20. We observed significant growth rate and isotopic phenotypes as a function of substrate when the TH is involved in carbon and energy metabolism. We discuss implications for understanding H-isotope fractionation during microbial fatty acid biosynthesis in sulfate reducers and anaerobes in general in anoxic environments.