Investigations of Aerobic Methane Oxidation in Two Marine Seep Environments: Part 2—Isotopic Kinetics

• Published in: Journal of geophysical research. Oceans 2019-11, Vol.124 (11), p.8392-8399
• Main Authors: Chan, E. W., Shiller, A. M., Joung, D. J., Arrington, E. C., Valentine, D. L., Redmond, M. C., Breier, J. A., Socolofsky, S. A., Kessler, J. D.
• Format: Article
• Language: English
• Subjects: Blooms; Canyons; Chemical analysis; chemical kinetics; Cultivation; Fractionation; Geophysics; Incubation period; Isotope fractionation; Isotope ratios; Isotopes; Kinetics; Leases; Marine environment; Mesocosms; Methane; methane oxidation; Microorganisms; Natural environment; Ocean floor; Oxidation; Ratios; Reaction kinetics; Seawater; Stable isotopes; Substrates; Water analysis
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2019JC015603

During aerobic oxidation of methane (CH4) in seawater, a process which mitigates atmospheric emissions, the 12C‐isotopologue reacts with a slightly greater rate constant than the 13C‐isotopologue, leaving the residual CH4 isotopically fractionated. Prior studies have attempted to exploit this systematic isotopic fractionation from methane oxidation to quantify the extent that a CH4 pool has been oxidized in seawater. However, cultivation‐based studies have suggested that isotopic fractionation fundamentally changes as a microbial population blooms in response to an influx of reactive substrates. Using a systematic mesocosm incubation study with recently collected seawater, here we investigate the fundamental isotopic kinetics of aerobic CH4 oxidation during a microbial bloom. As detailed in a companion paper, seawater samples were collected from seep fields in Hudson Canyon, U.S. Atlantic Margin, and atop Woolsey Mound (also known as Sleeping Dragon) which is part of lease block MC118 in the northern Gulf of Mexico, and used in these investigations. The results from both Hudson Canyon and MC118 show that in these natural environments isotopic fraction for CH4 oxidation follows a first‐order kinetic process. The results also show that the isotopic fractionation factor remains constant during this methanotrophic bloom once rapid CH4 oxidation begins and that the magnitude of the fractionation factor appears correlated with the first‐order reaction rate constant. These findings greatly simplify the use of natural stable isotope changes in CH4 to assess the extent that CH4 is oxidized in seawater following seafloor release. Plain Language Summary The aerobic oxidation of methane in seawater is a process that prevents methane produced in the oceanic environment from being emitted to the atmosphere. During this process, isotopic forms of methane are oxidized at slightly different rates leading to changes in the natural methane isotope ratios with the extent of oxidation. While these changes in isotope ratios would seem to be a proxy for the extent of methane oxidation, laboratory‐based studies involving pure cultures have shown that these isotope ratio changes vary as a microbial population blooms in response to an increase in substrates. This study systematically measured the stable isotope changes that are associated with aerobic methane oxidation in recently collected seawater collected from regions of active seafloor methane release along the U.S. Atlantic m