Direct Evaluation of in Situ Biodegradation in Athabasca Oil Sands Tailings Ponds Using Natural Abundance Radiocarbon

Compound-specific stable (delta C-13) and radiocarbon (Delta C-14) isotopes of phospholipid fatty acids (PLFAs) were used to evaluate carbon sources utilized by the active microbial populations in surface sediments from ponds. were at three the four sites investigated, and delta C-13(PLFA) values were generally within,similar to 3 parts per thousand of that reported for oil sands bitumen (similar to-30 parts per thousand), suggesting that the microbial communities growing on petroleum constituents were dominated by aerobic heterotrophs. Delta C-14(PLFA) values ranged from -906 to -586 parts per thousand and pointed to significant uptake of fossil carbon, particularly in PLFAs (e.g., cy17:0 and Athabasca oil sands tailings Algal-specific PLFAs absent of cy19:0) often associated with petroleum hydrocarbon degrading bacteria. The comparatively heavier Delta C-14 values found in other, less specific PLFAs (e.g., 16:0) indicated the preferential uptake of younger organic matter by the general microbial population. Since the main carbon pools in tailings sediment were essentially."radiocarbon dead" (i.e., Delta C-14 similar to -1000 parts per thousand), the principal source for this relatively modern carbon is considered to be the Athabasca River, which provides the bulk of the water used in the bitumen extraction process. The preferential utilization of the minor amount of younger and presumably more labile material present in systems otherwise dominated by petroleum carbon has important implications for remediation strategies, since it implies that organic contaminants may persist long after reclamation has begun. Alternatively, this young organic matter could play a vital and necessary role in supporting the microbial utilization of fossil carbon via cometabolism or priming processes.