The thermophilic sulfate-reducing archaeon Archaeoglobus fulgidus strain VC-16 (DSM 4304), which is known to oxidize fatty acids and n-alkenes, was shown to oxidize saturated hydrocarbons (n-alkanes in the range C 10 -C 21 ) with thiosulfate or sulfate as a terminal electron acceptor. The amount of n-hexadecane degradation observed was in stoichiometric agreement with the theoretically expected amount of thiosulfate reduction. One of the pathways used by anaerobic microorganisms to activate alkanes is addition to fumarate that involves alkylsuccinate synthase as a key enzyme. A search for genes encoding homologous enzymes in A. fulgidus identified the pflD gene (locus-tag AF1449) that was previously annotated as a pyruvate formate lyase. A phylogenetic analysis revealed that this gene is of bacterial origin and was likely acquired by A. fulgidus from a bacterial donor through a horizontal gene transfer. Based on three-dimensional modeling of the corresponding protein and molecular dynamic simulations, we hypothesize an alkylsuccinate synthase activity for this gene product. The pflD gene expression was upregulated during the growth of A. fulgidus on an n-alkane (C 16 ) compared with growth on a fatty acid. Our results suggest that anaerobic alkane degradation in A. fulgidus may involve the gene pflD in alkane activation through addition to fumarate. These findings highlight the possible importance of hydrocarbon oxidation at high temperatures by A. fulgidus in hydrothermal vents and the deep biosphere.

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...

A novel marine sulfate-reducing bacterium, strain CV2803T, which is able to oxidize aliphatic hydrocarbons, was isolated from a hydrocarbon-polluted marine sediment (Gulf of Fos, France). The cells were rod-shaped and slightly curved, measuring 0·6×2·2–5·5 μm. Strain CV2803T stained Gram-negative and was non-motile and non-spore-forming. Optimum growth occurred in the presence of 24 g NaCl l−1, at pH 7·5 and at a temperature between 28 and 35 °C. Strain CV2803T oxidized alkanes (from C13 to C18) and alkenes (from C7 to C23). The DNA G+C content was 41·4 mol%. Comparative sequence analyses of the 16S rRNA gene and dissimilatory sulfite reductase (dsrAB) gene and those of other sulfate-reducing bacteria, together with its phenotypic properties, indicated that strain CV2803T was a member of a distinct cluster that contained unnamed species. Therefore, strain CV2803T (=DSM 15576T=ATCC BAA-743T) is proposed as the type strain of a novel species in a new genus, Desulfatibacillum aliphaticivorans gen. nov., sp. nov.

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