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

Abstract

Abstract. Compound-specific hydrogen isotopes are increasingly used as a powerful proxy for investigating biogeochemical cycle and climate change over the past two 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 a 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 sole organic substrate. Our results show that the abundance-weighted mean fatty acids/water fractionations (εFA/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 εFA/Water of 52 ± 14 ‰ and 44 ± 4 ‰ respectively, and exhibited 2H-depleted using GlcNac (-76 ± 1 ‰) and glucose (-67 ± 35 ‰) as sole carbon sources, relatively small fractionations on acetate (23 ± 3 ‰) and L-alanine (-4 ± 9 ‰). 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 obvious influence on the δ2H values of fatty acids, with strongly 2H-depleted at optimal growth temperature (15 and 20 °C) and relatively small fractionations at non-optimal temperatures (4, 10, and 25 °C). We hypothesized that it is most likely controlled by the temperature effects on the activity of associated enzymes for NADPH production. This study helps understanding the controlling factors of hydrogen isotope fractionation by marine bacteria, lays 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.