² H/ ¹ H variation in microbial lipids is controlled by NADPH metabolism

Significance The deuterium/protium ( 2 H/ 1 H) ratio of microbial lipids varies substantially and appears to be correlated with the mode of metabolism in the host organism, with lipids from chemoautotrophs and photoautotrophs 2 H-depleted and heterotrophs in many cases 2 H-enriched. Such patterns suggest that the H-isotope ratios of lipids could be used to infer the metabolism of environmental organisms, with applications ranging from Earth history to global carbon cycling and ecology. Here, we learn to understand this information by using metabolic flux analysis. We show that the full range of lipid 2 H/ 1 H ratios from a diverse set of aerobic heterotrophs can be quantitatively explained by fluxes through various NADP + -reducing reactions due to differences in their kinetic isotope effects.