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 asso- ciated 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 (δ 13 C, δ 15 N). 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 13 C enriched (1.4‰ to 3.0‰) 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 13 C enrichment of microbial biomass (8.5‰) relative to the DOC source. Overall, the δ 13 C of the DOC comprised of complex mixtures of NAs showed minimal change (−0.5‰ to −0.1‰) during the incubation period whereas the δ 13 C of the dissolved inorganic carbon (DIC) was more variable (−5.0‰ to +5.4‰). In tests where the concentration of available nitrogen was increased, the final biomass values were 15 N enriched (3.8‰ to 8.4‰) 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 15 N enrichment.