Carbon and Hydrogen Isotopic Fractionation during Biodegradation of Methyl tert-Butyl Ether

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Abstract
Carbon and hydrogen isotopic fractionation during aerobic biodegradation of MTBE by a bacterial pure culture (PM1) and a mixed consortia from Vandenberg Air Force Base (VAFB) were studied in order to assess the relative merits of stable carbon versus hydrogen isotopic analysis as an indicator of biodegradation. Carbon isotopic enrichment in residual MTBE of up to 8.1‰ was observed at 99.7% biodegradation. Carbon fractionation was reproducible in the PM1 and VAFB experiments, yielding similar enrichment factors (ε) of −2.0‰ ± 0.1‰ to −2.4‰ ± 0.3‰ for replicates in the PM1 experiment and −1.5‰ ± 0.1‰ to −1.8‰ ± 0.1‰ for replicates in the VAFB experiment. Hydrogen isotopic fractionation was highly reproducible for the PM1 pure cultures, with ε values of −33‰ ± 5‰ to −37‰ ± 4‰ for replicate samples. In the VAFB microcosms, there was considerably more variability in ε values, with values of −29‰ ± 4‰ and −66‰ ± 3‰ measured for duplicate sample bottles. Despite this variability, hydrogen isotopic fractionation always resulted in 2H enrichment of the residual MTBE of >80‰ at 90% biodegradation. The reproducible carbon fractionation suggests that compound-specific carbon isotope analysis may be used to estimate the extent of biodegradation at contaminated sites. Conversely, the large hydrogen isotopic fractionation documented during biodegradation of MTBE suggests that compound-specific hydrogen isotope analysis offers the most conclusive means of identifying in-situ biodegradation at contaminated sites.