The alkane-degrading, sulfate-reducing bacterium Desulfatibacillum aliphaticivorans strain CV2803 T , recently isolated from marine sediments, was investigated for n-alkane metabolism. The total cellular fatty acids of this strain had predominantly odd numbers of carbon atoms (C odd) when the strain was grown on a C-odd alkane (pentadecane) and even numbers of carbon atoms (C even) when it was grown on a C-even alkane (hexadecane). Detailed analyses of those fatty acids by gas chromatography/mass spectrometry allowed us to identify saturated 2-, 4-, 6-, and 8-methyl-and monounsaturated 6-methyl-branched fatty acids, with chain lengths that specifically correlated with those of the alkane. Growth of D. aliphaticivorans on perdeuterated hexadecane demonstrated that those methyl-branched fatty acids were directly derived from the substrate. In addition, cultures on pentadecane and hexadecane produced (1-methyltetradecyl)succinate and (1-methylpentadecyl) succinate, respectively. These results indicate that D. aliphaticivorans strain CV2803T oxidizes n-alkanes into fatty acids anaerobically, via the addition of fumarate at C-2. Based on our observations and on literature data, a pathway for anaerobic n-alkane metabolism by D. aliphaticivorans is proposed. This involves the transformation of the initial alkylsuccinate into a 4-methyl-branched fatty acid which, in addition to catabolic reactions, can alternatively undergo chain elongation and desaturation to form storage fatty acids.The biodegradation of alkanes under aerobic conditions has been studied intensively, and the mechanisms of initial activation have been carefully described. The initial attack requires a mono-or dioxygenase enzyme and the presence of molecular oxygen (18). The demonstration of hydrocarbon biodegradation in the absence of molecular oxygen is more recent. Studies have reported the biodegradation of n-alkanes under nitratereducing (3, 9), sulfate-reducing (1, 4, 23), and methanogenic (29) conditions, but limited information is available so far on the degrading organisms and the mechanisms of degradation (11,24,27). Rabus et al. (17) have demonstrated that the initial oxidation of n-hexane by a denitrifying bacterium, strain HxN1, involves the addition of fumarate at C-2 of the alkane to form a substituted alkylsuccinate. The latter intermediate was shown to be further mineralized via a rearrangement of the carbon skeleton and conversion into specific methyl-branched fatty acids (28). Similarly, a sulfate-reducing enrichment culture, grown on n-dodecane, formed (1-methylundecyl)succinate by the addition of fumarate at the subterminal carbon of the alkane (14). However, neither the complete degradation pathways nor the organisms responsible for the degradation were described. On the other hand, two phylogenetically different sulfate reducers, strains Hxd3 (1, 25) and AK-01 (23), which are able to oxidize alkanes into carbon dioxide under anaerobic conditions, were studied for n-alkane metabolism. These strains utilize alkanes with chain length...
