Investigations of Aerobic Methane Oxidation in Two Marine Seep Environments: Part 1—Chemical Kinetics

Microbial aerobic oxidation is known to be a significant sink of marine methane (CH4), contributing to the relatively minor atmospheric release of this greenhouse gas over vast stretches of the ocean. However, the chemical kinetics of aerobic CH4 oxidation are not well established, making it difficult to predict and assess the extent that CH4 is oxidized in seawater following seafloor release. Here we investigate the kinetics of aerobic CH4 oxidation using mesocosm incubations of fresh seawater samples collected from seep fields in Hudson Canyon, U.S. Atlantic Margin and MC118, Gulf of Mexico to gain a fundamental chemical understanding of this CH4 sink. The goals of this investigation were to determine the response or lag time following CH4 release until more rapid oxidation begins, the reaction order, and the stoichiometry of reactants utilized (i.e., CH4, oxygen, nitrate, phosphate, trace metals) during CH4 oxidation. The results for both Hudson Canyon and MC118 environments show that CH4 oxidation rates sharply increased within less than one month following the CH4 inoculation of seawater. However, the exact temporal characteristics of this more rapid CH4 oxidation varied based on location, possibly dependent on the local circulation and biogeochemical conditions at the point of seawater collection. The data further suggest that methane oxidation behaves as a first‐order kinetic process and that the reaction rate constant remains constant once rapid CH4 oxidation begins. Plain Language Summary In and below the seafloor resides the largest global reservoir of methane, a potent greenhouse gas. Following the release of methane from the seafloor, a significant fraction dissolves in the overlying seawater and is oxidized by indigenous microorganisms, helping to prevent its atmospheric release. However, the timing and chemical requirements for this process to occur are not well established, making it difficult to predict and assess the efficiency of methane oxidation following seafloor release. This study systematically measured the chemical changes that are associated with aerobic methane oxidation in seawater using water collected from regions of active seafloor methane release along the U.S. Atlantic margin and the Gulf of Mexico. These results help to refine our understanding of how quickly and how much methane can typically be oxidized in seawater. Key Points Aerobic methane oxidation was investigated and showed that two moles of oxygen are not required