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...

Small-subunit (16S) ribosomal DNA clone libraries were constructed using DNA isolated from the anoxic sediments underlying the cyanobacterial mats from two sampling stations of different salinity (Station A, 150-200 per thousand salinity; Station B, 250-320 per thousand salinity) located in the Mediterranean salterns of Salin-de-Giraud (France). Previous studies have shown that the mats at these two sites differ greatly in physicochemical and microbial composition. Sequence analysis of the clone libraries indicated that prokaryotic diversity was high in the sediments from both stations, in both the Bacteria and Archaea domains. Clones related to the alpha- and delta-Proteobacteria (phylum Proteobacteria), the strictly anaerobic fermentative bacteria (phylum Firmicutes), and the Cytophaga-Flavobacterium-Bacteroides (CFB) group (phylum Bacteroidetes) were found in the libraries from both sediments and accounted for the majority of Bacteria domain clones. The results indicated that the populations of delta-Proteobacteria (principally sulfate-reducing bacteria) were significantly different in the two sediments. In addition, several clones from Station A were related either to the gamma-Proteobacteria (phylum Proteobacteria) or to the Spirochaeta, whereas the library from Station B contained several clones related to the uncultured, deep-branching 'KTK group' of Bacteria. Among the Archaea domain clones, all from Station B and the majority from Station A were related to the order Halobacteriales (phylum Euryarchaeota, class Halobacteria). In addition, 12% of the Archaea domain clones from Station A were related to the Methanococci group (phylum Euryarchaeota, class Methanobacteria) and 32% to the phylum Crenarchaeota. This study represents the first molecular analysis of the diversity of halophilic prokaryotes present in these sediments and the results are discussed in the light of our current knowledge of the microbial ecology of these hypersaline ecosystems.

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.

Microbial mats that develop in the gypsum crust of the hypersaline ponds of Salins‐de‐Giraud (Camargue, France) were carefully investigated between 1989 and 1991. During the warm seasons, when these mats were fully developed, analyses of microbial activities and microprofiles of oxygen and sulfide have shown a great activity of the different kinds of bacteria found in the mat below the gypsum crust. Oxygen production could amount to 2 μmol cm−3 h−1 during the maximum daylight whereas the oxidation of sulfide in the light was calculated to be 12.7 μmol cm−3 h−1, i.e. 300 to 180 mmol m−2 day−1 assuming 8–10 hours of constant daylight and a sulfide oxidation zone of 3 mm in thickness. This sulfide oxidation consumes about 65–95% of the diel sulfide production which has been estimated to be 400 to 450 mmol m−2 day−1 originating from sulfate reduction which takes place in the 6 cm depth horizon of sediment plus mat. According to the amounts of sulfate precipitated at the sediment surface in the form of gypsum, sulfate reduction is never limited and was found to be among the highest values reported in the literature (average value of 8200 nmols cm−3 day−1). Completely covered by the gypsum crust, this ecosystem has been found to react as a closed system. Consequently, the sulfide does not escape and accumulate below the crust. It was detected up to the top of the mat after a few hours of darkness. It is reoxidized during the day by the photosynthetic organisms that from the mats. These latter mats were composed of 2 to 3 laminated layers of phototrophic organisms: an upper brown layer of the cyanobacterium Aphanothece, an intermediate green layer of the cyanobacterium Phormidium and an underlying red layer of purple sulfur‐oxidizing bacteria from which two new halophilic species were isolated (Chromatium salexigens and Thiocapsa halophila). It has been found that the accumulated sulfide is oxidized not only by the phototrophic bacteria in the sulfide oxidation zone but also by the oxygen produced by the cyanobacteria which are able to photosynthesize in the presence of sulfide.

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