Hydrogen isotopic fractionation in lipid biosynthesis by H2-consuming Desulfobacterium autotrophicum

We report hydrogen isotopic fractionations between water and fatty acids of the sulfate-reducing bacterium Desulfobacterium autotrophicum. Pure cultures were grown in waters with deuterium (D) contents that were systematically varied near the level of natural abundance ( 37& 6 dD 6 993&). H2 of constant hydrogen isotope (D/H) ratio was supplied to the cultures. The D/H ratios of water, H2, and specific fatty acids were measured by isotope-ratio mass spectrometry. The results demonstrate that D. autotrophicum catalyzes hydrogen isotopic exchange between water and H2, and this reaction is conclusively shown to approach isotopic equilibrium. In addition, variation in the D/H ratio of growth water accounts for all variation in the hydrogen isotopic composition of fatty acids. The D/H ratios of fatty acids from cultures grown on H2/CO2 are compared with those from a separate set of cultures grown on D-enriched formate, an alternative electron donor. This comparison rules out H2 as a significant source of fatty acid hydrogen. Grown on either H2/CO2 or formate, D. autotrophicum produces fatty acids in which all hydrogen originates from water. For specific fatty acids, biosynthetic fractionation factors are mostly in the range 0.60 6 aFA–water 6 0.70; the 18:0 fatty acid exhibits a lower fractionation factor of 0.52. The data show that aFA–water generally increases with length of the carbon chain from C14 to C17 among both saturated and unsaturated fatty acids. These results indicate a net fractionation associated with fatty acid biosynthesis in D. autotrophicum that is slightly smaller than in another H2-consuming bacterium (Sporomusa sp.), but much greater than in most photoautotrophs. 2009 Elsevier Ltd. All rights reserved.