Examining the Microbial Degradation of Naphthenic Acids Using Stable Isotope Analysis of Carbon and Nitrogen

Naphthenic acids (NAs) are a complex group of naturally occurring oil sands constituents that constitute a significant portion of the dissolved organic carbon (DOC) pool available for microbial degradation in the process-related waste water associated with oil sands mine sites. One approach to understanding the biological fate of oil sands process-derived carbon and nitrogen in aquatic reclamation of the mine sites involves the use of stable isotope analyses. However, for stable isotope analyses to be useful in such field-based assessments, there is a need to determine how microbial degradation of a complex mixture of NAs might change the stable isotope values (delta C-13, delta N-15). In batch cultures and semi-continuous laboratory microcosms, utilization of a commercial mixture of NAs by oil sands-derived microbial cultures resulted in microbial biomass that was similar or slightly C-13 enriched (1.4aEuro degrees to 3.0aEuro degrees) relative to the DOC source, depending on the length of incubation. Utilization of a NA-containing extract of oil sands processed water resulted in greater C-13 enrichment of microbial biomass (8.5aEuro degrees) relative to the DOC source. Overall, the delta C-13 of the DOC comprised of complex mixtures of NAs showed minimal change (-0.5aEuro degrees to -0.1aEuro degrees) during the incubation period whereas the delta C-13 of the dissolved inorganic carbon (DIC) was more variable (-5.0aEuro degrees to +5.4aEuro degrees). In tests where the concentration of available nitrogen was increased, the final biomass values were N-15 enriched (3.8aEuro degrees to 8.4aEuro degrees) relative to the initial biomass. The isotope trends established in this study should enhance our ability to interpret field-based data from sites with hydrocarbon contamination, particularly in terms of carbon source utilization and N-15 enrichment.