GROWTH, NATURAL RELATIONSHIPS, CELLULAR FATTY-ACIDS AND METABOLIC ADAPTATION OF SULFATE-REDUCING BACTERIA THAT UTILIZE LONG-CHAIN ALKANES UNDER ANOXIC CONDITIONS

Natural relationships, improvement of anaerobic growth on hydrocarbons , and properties that may provide clues to an understanding of oxygen- independent alkane metabolism were studied with two mesophilic sulfate -reducing bacteria, strains Hxd3 and Pnd3. Strain Hxd3 had been former ly isolated from an oil tank; strain Pnd3 was isolated from marine sed iment. Strains Hxd3 and Pnd3 grew under strictly anoxic conditions on n-alkanes in the range of C-12-C-20 and C-14-C-17, respectively, reduc ing sulfate to sulfide. Both strains shared 90% 16 S rRNA sequence sim ilarity and clustered with classified species of completely oxidizing, sulfate-reducing bac bacteria within the delta-subclass of Proteobact eria. Anaerobic growth on alkanes was stimulated by alpha-cyclodextrin , which served as a non-degradable carrier for the hydrophobic substra te. Cells of strain Hxd3 grown on hydrocarbons and alpha-cyclodextrin were used to study the composition of cellular fatty acids and in vivo activities. When strain Hxd3 was grown on hexadecane (C16H34), cellul ar fatty acids with C-odd chains were dominant. Vice versa, cultures g rown on heptadecane (C17H36) contained mainly fatty acids with C-even chains. In contrast, during growth on 1-alkenes or fatty acids, a C-ev en substrate yielded C-even fatty acids, and a C-odd substrate yielded C-odd fatty acids. These results suggest that anaerobic degradation o f alkanes by strain Hxd3 does not occur via a desaturation to the corr esponding 1-alkenes, a hypothetical reaction formerly discussed in the literature. Rather an alteration of the carbon chain by a C-odd carbo n unit is likely to occur during activation; one hypothetical reaction is a terminal addition of a C-1, unit. In contrast, fatty acid analys es of strain Pnd3 after growth on alkanes did not indicate an alterati on of the carbon chain by a C-odd carbon unit, suggesting that the ini tial reaction differed from that in strain Hxd3. When hexadecane-grown cells of strain Hxd3 were resuspended in medium with 1-hexadecene, an adaptation period of 2 days was observed. Also this result is not in favor of an anaerobic alkane degradation via the corresponding l-alken e.