Impact of metabolism and temperature on H-2 / H-1 fractionation in lipids of the marine bacterium Shewanella piezotolerans WP3

Compound-specific hydrogen isotopes have increasingly been used as a powerful proxy for investigating biogeochemical cycles and climate change over the past 2 decades. Understanding the hydrogen isotope in extant organisms is fundamental for us to interpret such isotope signals preserved in natural environmental samples. Here, we studied the controls on hydrogen isotope fractionation between fatty acids and growth water by an Fe-reducing heterotrophic marine bacterium Shewanella piezotolerans WP3 growing on different organic substrates, including N-acetyl-D-glucosamine (GlcNac), glucose, acetate, pyruvate, L-alanine, and L-glutamate. Meanwhile, we also evaluated the impact of growth temperature on the hydrogen isotope composition of fatty acids using GlcNac as the sole organic substrate. Our results show that the abundance-weighted mean fatty-acid / water fractionations (eFA/water) display considerable variations for cultures grown on different substrates. Specifically, WP3 yielded the most 2H-enriched fatty acids growing on L-glutamate and pyruvate with an eFA/water of 52 +/- 14 parts per thousand and 44 +/- 4 parts per thousand, respectively, and exhibited 2H depletion using GlcNac (-76 +/- 1 parts per thousand) and glucose (-67 +/- 35 parts per thousand) as sole carbon sources and relatively small fractionations on acetate (23 +/- 3 parts per thousand) and L-alanine (-4 +/- 9 parts per thousand). Combined with metabolic model analysis, our results indicate that the central metabolic pathways exert a fundamental effect on the hydrogen isotope composition of fatty acids in heterotrophs. Temperature also has an obvious influence on the 82H values of fatty acids, with strong 2H depletion at an optimal growth temperature (-23 +/- 2 parts per thousand and -23 parts per thousand growing at 15 and 20 degrees C, respectively) and relatively small fractionations at non-optimal temperatures (4 +/- 5 parts per thousand, -4 +/- 12 parts per thousand, and 15 +/- 41 parts per thousand at 4, 10, and 25 degrees C, respectively). We hypothesized that this may be associated with temperature-induced enzyme activity for nicotinamide adenine dinucleotide phosphate (NADPH) production. This study helps understand the controlling factors of hydrogen isotope fractionation by marine bacteria, laying the foundation for further interpreting the hydrogen isotope signatures of lipids as an important proxy to decode the biogeochemical cycles and ecological changes in marine sediments.