1 Bridging the Gap Between Basic Research and the Application of New Technology: Anaerobic Hydrocarbon Metabolism, Bioremediation and Future Energy Supplies Joseph M. Suflita, Todd Townsend, Lisa Gieg, Irene Davidova, Mark Nanny, and Roger C. Prince † Institute for Energy and the Environment and Department of Botany and Microbiology The University of Oklahoma; Norman, Oklahoma 73019-0245 and † ExxonMobil Research and Engineering Co. Annnandale, NJ 08801 email: JSuflita@ou.edu Keywords: Anaerobic biodegradation, hydrocarbons, sulfate reduction, methanogenesis, CO 2 emissions Introduction The worldwide demand for oil is expected to grow unabated for the foreseeable future. It is estimated that global oil consumption will increase annually by an average of more than 4 x 10 7 barrels per day to eventually reach 4.3 x 10 10 barrels per year by 2020 [1]. This reliance on oil as the dominant form of energy has many ecological consequences. Most obvious is by the release of hydrocarbons into the environment. Fortunately less than 0.02% of U.S. annual consumption enters the ultimate environmental repository – the world oceans, with approximately an equal amount coming from natural seeps. Nevertheless, localized spills and discharges can have a substantial impact on marine, freshwater, and soil environments. Understanding the processes that control the fate of oil in the environment is paramount for determining the associated environmental risks and for designing appropriate remedial measures. It is well recognized that the susceptibility of hydrocarbons to microbial biodegradation governs the persistence of these chemicals in the environment. The metabolic diversity and patterns of hydrocarbon biodegradation exhibited by aerobic microorganisms have been well studied [2]. This fundamental knowledge base helps underpin biotechnological advances for aerobic biodegradation, biotransformation and biocatalysis processes as summarized in a report of a recent meeting [3]. However, a comparable knowledge base on the transformation of hydrocarbons by anaerobic microorganisms does not exist. This is because historically anaerobic hydrocarbon biodegradation was considered slow, metabolically limited and ecologically insignificant. This view has been completely altered in recent years. An appreciation for the metabolism of hydrocarbons coupled with the electron acceptors other than oxygen has grown substantially in the last two decades. There is no doubt that anaerobes are capable of catalyzing remarkable reactions that have far reaching biotechnological implications. A detailed review of the mechanisms of anaerobic hydrocarbon decay by microorganisms is beyond the scope of this communication. Rather, our objective will be to offer an overview of some of the more generalizing metabolic features pertinent to the destruction of important classes of hydrocarbons. We will also attempt to illustrate how such bioconversions can be of practical significance for selected environmental problems that impact the energy industry. Lastly, we will speculate on the potential role of such processes on the reduction of greenhouse gas emissions and future energy supplies. Patterns of anaerobic hydrocarbon biodegradation Crude oils are enormously complex mixtures containing tens of thousands of individual components [4,5]. Even condensates and refined products include a dizzying array of constituent hydrocarbons. Once released in the environment, the relative concentrations of these chemicals change over time reflecting individual susceptibilities to the various fate processes such as sorption, volatilization, dispersion and biodegradation. One way of assessing the susceptibility of such complex mixtures to anaerobic biodegradation is to consider individual chemical classes of hydrocarbons and to determine how metabolism varies with structural complexity. For instance, the anaerobic biodegradation of the BTEX hydrocarbons (benzene, toluene, ethylbenzene and the xylene isomers) has been most extensively studied because of the relatively high water solubility of these compounds and their toxicological impact. There is no doubt that each of these hydrocarbons can be mineralized under a variety of anaerobic conditions. While generalizations are often difficult to make, toluene is the most readily metabolized hydrocarbon of this group while benzene is far more recalcitrant. Not only are the BTEX compounds metabolized, evidence from a variety of laboratories clearly show that other classes of chemicals including the n-alkanes, branched alkanes, olefins, alicyclic and polynuclear aromatic hydrocarbons (PAHs) are amenable to biodegradation under anaerobic conditions [for reviews see 6-8; 9-11]. The quantitatively most important group of hydrocarbons, the n-alkanes, are rather labile under a variety of electron accepting conditions. As the degree of structural complexity increases by methylation, biodegradation of the branched alkanes tends to slow, but is far from precluded. Even the highly branched pristine and phytane, long considered to be suitable biomarker chemicals in crude oils because of their relative persistence, can no longer be considered as such since their anaerobic biodegradation has been documented [12,13]. Similarly, the cyclic alkanes are readily metabolized as are most of the mono- and dimethylated and even ethylated derivatives. However, more complicated substitution patterns leads to a more limited attack on structural isomers and generally increased resistance to anaerobic biodegradation. The latter