Carbon and hydrogen isotope fractionation during uncultured aerobic oxidation of short-chain alkanes that discharged from a natural hydrothermal system
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Aerobic oxidation of short-chain alkanes was observed in gas samples from the Lutao intertidal hydrothermal vents in Taiwan, during storage without adding strains and replenishing substrates at 20 <sup>o</sup>C up to 29 months. The carbon isotope fractionation factors (<em>ε<sub>C</sub></em>) of methane (C<sub>1</sub>), ethane (C<sub>2</sub>), and propane (C<sub>3</sub>), were calculated using the Rayleigh fractionation equation to be -37.1 ± 7.5‰, -14.8 ± 4.8‰, and -4.7 ± 5.2‰, respectively. The hydrogen isotope fractionation factor (<em>ε<sub>H</sub></em>) of methane was determined to be -281 ± 187‰. DNA sequencing of the 16sRNA gene in the vent fluids suggests that aerobic oxidation is dominated by methanotrophs of the genera <em>Methylomicrobium</em> and <em>Methylophaga,</em> which use the ribulose monophosphate pathway (RuMP). The degrees of isotope fractionation (<em>ε<sub>C</sub></em> and <em>ε<sub>H</sub></em> values) herein are larger than previously reported values, possibly due to the limited O<sub>2</sub> supply and low abundance of aerobic methane-oxidizing bacteria in the experiments. Since the fractionation factor of methane is higher than those of ethane and propane, the aerobic oxidation of thermogenic or microbial alkanes could produce carbon isotope reversal, which is frequently noted as a trait of abiotic hydrocarbons. This work demonstrates that in addition to anaerobic microbial oxidation, aerobic oxidation with a low cell density can also produce significant isotope fractionation of alkanes in geological closed/semi-closed environments that are characterized by moderate temperatures and a limited supply of substrates and O<sub>2</sub>; these environments include cold seeps, mud volcanoes, and low-temperature hydrothermal aquifers/reservoirs.
