METHANOGENIC BIODEGRADATION OF N-ALKANES AND N-ALKYLATED CYCLOHEXANES AND BENZENES IN THE OIL SPILL LONG-TERM STUDY SITE AT BEMIDJI, MN

The 1979 oil spill at Bemidji, MN has been a USGS study site for over 20 years under the leadership and with the strong support of Mary Jo Baedecker. Research here has resulted in significant fundamental advances in the understanding of redox reactions and biodegradation of hydrocarbons. Because of its well-documented methanogenic conditions, the Bemidji spill site provides an ideal laboratory in which to study methanogenic biodegradation.

In general, more is known about anaerobic biodegradation of aromatic hydrocarbons (e.g., BTEX compounds) than aliphatic. However, with the recognition that aliphatic compounds such as n-alkanes are also degraded under anaerobic conditions, the mechanism of their degradation is now being widely investigated. The preponderance of research thus far has suggested that the metabolites and intermediates are polar in nature and has focused on finding them in the groundwater or in polar media. In our work on oil from sediments within the oil body (oil > 10% by weight) we found evidence for non-polar metabolites and a degradation progression for the n-alkanes that is quite different from what is generally recorded in the literature. Degradation proceeds from the high molecular weight end of the alkane distribution rather than the lower end as in aerobic weathering, and results in a transitory but absolute concentration increase of lower weight homologs. In addition to n-alkanes, two series of hydrocarbons with a homologous n-alkyl side chain, namely n-alkylcyclohexanes and n-alkylbenzenes (side chain length of 1-15 carbon atoms) show a similar loss from the high molecular weight end of the series and enhanced levels of shorter carbon chain homologs. This progression suggests that new lower molecular weight members of these series are non-polar metabolites being formed from microbial degradation of higher-end members. This degradation pathway is also seen in our work on a methanogenic diesel fuel spill in Mandan, ND.

Further work is necessary to reliably ascertain the specific mechanism(s) of alkane degradation under these methanogenic conditions. The loss of higher m.w. n-alkanes and n-alkylated ring compounds resulting in the formation of lower m.w. homologs as intermediates should be considered in the overall redox characterization of the biodegradation progress.