Hydrogen isotope fractionation during H

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 ( δ D-H 2 O). The δ D 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 δ D were highly correlated between lipids and water ( δ D-lipids = 0.59 × δ D-water – 381‰; R 2 = 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. Received 27 April 2004; accepted 28 July 2004 Corresponding author: D. L. Valentine. Tel.: (805) 893-2973; fax: (805) 893-2314; e-mail: [email protected]