Hydrogen isotopic fractionation in lipid biosynthesis by H 2-consuming Desulfobacterium autotrophicum

Abstract

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‰ ⩽ δD ⩽ 993‰). H 2 of constant hydrogen isotope (D/H) ratio was supplied to the cultures. The D/H ratios of water, H 2, and specific fatty acids were measured by isotope-ratio mass spectrometry. The results demonstrate that D. autotrophicum catalyzes hydrogen isotopic exchange between water and H 2, 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 H 2/CO 2 are compared with those from a separate set of cultures grown on D-enriched formate, an alternative electron donor. This comparison rules out H 2 as a significant source of fatty acid hydrogen. Grown on either H 2/CO 2 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 ⩽ α FA–water ⩽ 0.70; the 18:0 fatty acid exhibits a lower fractionation factor of 0.52. The data show that α FA–water generally increases with length of the carbon chain from C 14 to C 17 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 H 2-consuming bacterium ( Sporomusa sp.), but much greater than in most photoautotrophs.