MOLECULAR INSIGHTS INTO THE ANAEROBIC BIODEGRADATION OF HYDROCARBONS AND PETROLEUM

Oxygen is of very limited availability as an electron acceptor for microbial respiration in many petroleum reservoirs. Hence for our understanding of crude oil degradation it was very important that during the past decade the principle capability of anaerobic microorganisms to oxidise hydrocarbons under strictly anoxic conditions has been established. Today we know that all major types of anaerobic respiration (denitrification, sulphate reduction, ferrous iron reduction) and even methanogenesis may be coupled to the specific oxidation of hydrocarbons. Since hydrocarbons belong to the chemically most stable organic compounds it could be expected that their degradation under anoxic conditions requires very specific reactions. There is increasing evidence that activation via radical enzyme reactions initiates hydrocarbon mineralization in different types of bacteria. This contribution will discuss two examples of our studies on anaerobic hydrocarbon degradation in crude oil. The first example are sulphate-reducing bacteria which selectively remove alkylbenzenes from crude oil. This is accompanied by the formation of benzoic acids whose patterns are complementary to the patterns of alkylbenzene depletion. Furthermore, the residual alkylbenzenes are significantly enriched in ¹³C. The second example is a denitrifying bacterium, strain HxN1, which produces (1-methylpentyl)succinate as the initial product of n-hexane degradation. During growth on crude oil various homologous of (1-methylpentyl)succinate are formed indicating that strain HxN1 is able to transform the C₄ to C₈ n-alkanes. It is probable that this mechanism also plays a role in the anaerobic biodegradation of other n-alkanes. From an energetic point of view it must not even be ruled out that methane oxidation during anaerobic methanotrophy proceeds by this type of reaction. Based on our current knowledge we suggest that the activity of anaerobic bacteria in petroleum reservoirs can be traced by i) specific compositional changes of crude oil; ii) transformation products specific for the metabolic pathways of anaerobic bacteria; and iii) the carbon isotopic fractionation of the residual hydrocarbon substrate(s).