Anaerobic bacterial metabolism of hydrocarbons

Many crude oil constituents are biodegradable in the presence of oxygen; however, a substantial anaerobic degradation has never been demonstrated. An unusually low content of n-alkanes in oils of certain deposits is commonly attributed to selective utilization of these hydrocarbons by aerobic microorganisms. On the other hand, oil wells and production fluids were shown to harbour anaerobic sulphate-reducing bacteria, but their actual electron donors and carbon sources were unknown. On the basis of nutritional properties of various bacterial isolates it was assumed that fatty acids and H2 are potential electron donors for sulphate reduction in situ. Here we demonstrate that hydrocarbons in crude oil are used directly by sulphate-reducing bacteria growing under strictly anoxic conditions. A moderately thermophilic pure culture selectively utilizes n-alkanes in oil for sulphate reduction to sulphide. In addition, a mesophilic sulphate-reducing enrichment culture is shown to oxidize alkylbenzenes in oil. Thus, sulphate-reducing bacteria utilizing aliphatic and aromatic hydrocarbons as electron donors may present a significant source of sulphide in oil deposits and oil production plants.