Hydrogen isotope fractionation during H_2/CO_2 acetogenesis: hydrogen utilization efficiency and the origin of lipid-bound hydrogen

Abstract

Hydrogen metabolism was studied in the anaerobic bacterium, Sporomusa sp. strain DMG 58, by measuring natural abundance levels of deuterium in H_2, H_(2)O, and individual fatty acids during acetogenic growth on H_2/CO_2. Four cultures were grown, each in medium with a distinct hydrogen-isotopic composition (δD-H_(2)O). The δD value of H_2 was quantified in the residual gas exiting the growth chambers and found to decrease concurrently with net H_2 consumption, indicating rapid isotope exchange between H_2 and H_(2)O. An isotopic mass balance was used to constrain the efficiency with which H_2 was activated by the cell and the reducing equivalents catabolized, which we term the H_2 utilization efficiency. Results indicate that H_2 utilization efficiency in these cultures is less than 20% during the growth phase, and less than 2% after the growth phase. The gross rate of cellular H_2 activation was similar in the growth phase and afterward. Biomass harvested at the end of each experiment was used to analyse the D/H of individual membrane lipids. Values of δD were highly correlated between lipids and water (δD-lipids = 0.59 × δD-water – 381‰; R2 = 0.995), indicating the source of lipid hydrogen is in isotopic equilibrium with water. Results are consistent with two possibilities: (i) water is the sole source of hydrogen to lipids, and the fractionation during biosynthesis is significantly larger than previously observed (α = 0.59), or (ii) hydrogen from H_2 is incorporated into lipids, but only after reaching isotopic equilibrium with H_(2)O. Fatty acids were strongly depleted in deuterium relative to all other organisms studied thus far, and such large depletions may prove useful as biomarkers for studying H_2 cycling in anoxic environments as well as in the geological record.