Carbon and hydrogen isotope fractionation associated with the aerobic microbial oxidation of methane, ethane, propane and butane

Carbon isotope fractionation factors associated with the aerobic consumption of methane (C1), ethane (C2), propane (C3), and nbutane (C4) were determined from incubations of marine sediment collected from the Coal Oil Point hydrocarbon seep field, located offshore Santa Barbara, CA. Hydrogen isotope fractionation factors for C1, C2 and C3 were determined concurrently. Fresh sediment samples from two seep areas were each slurried with sea water and treated with C1, C2, C3 or C4, or with mixtures of all four gases. Triplicate samples were incubated aerobically at 15 degrees C, and the stable isotope composition and headspace levels of C1-C4 were monitored over the course of the experiment. Oxidation was observed for all C1-C4 gases, with an apparent preference for C3 and C4 over Cl and C2 in the mixed-gas treatments. Fractionation factors were calculated using a Rayleigh model by comparing the delta C-13 and delta D of the residual C1-C4 gases to their headspace levels. Carbon isotope fractionation factors (reported in epsilon or (alpha-1) x 1000 notation) were consistent between seep areas and were -26.5 parts per thousand +/- 3.9 for Cl, -8.0 parts per thousand +/- 1.7 for C2, -4.8 parts per thousand +/- 0.9 for C3 and -2.9 parts per thousand +/- 0.9 for C4. Fractionation factors determined from mixed gas incubations were similar to those determined from individual gas incubations, though greater variability was observed during C1 consumption. In the case of C1 and C3 consumption, carbon isotope fractionation appears to decrease as substrate becomes limiting. Hydrogen isotope fractionation factors determined from the two seep areas differed for C1 oxidation but were similar for C2 and C3. Hydrogen isotope fractionation factors ranged from -319.9 parts per thousand to -156.4 parts per thousand for C1 incubations, and averaged -61.9 parts per thousand +/- 8.3 for C2 incubations and -15.1 parts per thousand +/- 1.9 for C3 incubations. The fractionation factors presented here may be applied to estimate the extent of C1-C4 oxidation in natural gas samples, and should prove useful in further studying the microbial oxidation of these compounds in the natural environment. (c) 2006 Elsevier Inc. All rights reserved.