Isotopic fractionation associated with biosynthesis of fatty acids by a marine bacterium under oxic and anoxic conditions

Shewanella putrefaciens (Strain MR-4), a gram negative facultative marine b acterium, was grown to stationary phase under both aerobic and anaerobic co nditions using lactate as the sole carbon source. Aerobically-produced cell s were slightly enriched in C-13 (+ 1.5 parts per thousand) relative to the lactate carbon source, whereas those from anaerobic growth were depleted i n C-13 (-2.2 parts per thousand). The distribution of fatty acids produced under aerobic conditions was similar to that resulting from anaerobic growt h, being dominated by C-16:1 omega 7 and C-16:0 fatty acids with a lesser a mount of the C-18:1 omega 7 component. Low concentrations of saturated even numbered normal fatty acids in the C-14 to C-18 range, and iso-C-15:0 were synthesized under both conditions. Fatty acids from anaerobic cultures (av erage delta(13)C = -37.8 parts per thousand) were considerably depleted in C-13 relative to their aerobically-synthesized counterparts (-28.8 parts pe r thousand). The distinct differences in isotopic composition of both whole cells and individual fatty acid components result from differences in assi milation pathways. Under aerobic conditions, the primary route of assimilat ion involves the pyruvate dehydrogenase enzyme complex which produces acety l-CoA, the precursor to lipid synthesis. In contrast, under anaerobic condi tions formate, and not acetate, is the central intermediate in carbon assim ilation with the precursors to fatty acid synthesis being produced via the serine pathway. Anaerobically-produced bacterial fatty acids were depleted by up to 12 parts per thousand relative to the carbon source. Therefore, de tection of isotopically depleted fatty acids in sediments may be falsely at tributed to a terrestrial origin, when in fact they are the result of bacte rial resynthesis. (C) 1999 Elsevier Science Ltd. All rights reserved.