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

Abstract

Microbial aerobic oxidation is known to be a significant sink of marine methane (CH 4 ), contributing to the relatively minor atmospheric release of this greenhouse gas over vast stretches of the ocean. However, the chemical kinetics of aerobic CH 4 oxidation are not well established, making it difficult to predict and assess the extent that CH 4 is oxidized in seawater following seafloor release. Here we investigate the kinetics of aerobic CH 4 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 CH 4 sink. The goals of this investigation were to determine the response or lag time following CH 4 release until more rapid oxidation begins, the reaction order, and the stoichiometry of reactants utilized (i.e., CH 4 , oxygen, nitrate, phosphate, trace metals) during CH 4 oxidation. The results for both Hudson Canyon and MC118 environments show that CH 4 oxidation rates sharply increased within less than one month following the CH 4 inoculation of seawater. However, the exact temporal characteristics of this more rapid CH 4 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 CH 4 oxidation begins. Plain Language SummaryIn 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 to oxidize one mole of methane • After a lag time lasting days to weeks, methane was rapidly oxidized in a few days following first-order chemical kinetics • These results appear consistent between different oceanic environments, despite regional variabilities Supporting Information:• Supporting Information S1

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

“…However, this conclusion is rather uncertain (k max = 0.44 ± 3.9 day −1 ) when the uncertainties in both the slope and y intercept of the geometric mean regression (k = (−9.3 ± 2.7)α + (9.8 ± 2.8)) are propagated. Nonetheless, it is interesting to note the similarities between this maximum rate constant which would produce no isotopic fraction and the rate constants measured during and after the DWH blowout (Figure ; Chan et al, , Figure ).…”

“…Full details of the sample collection, analysis, and calibration procedures can be found in the companion paper (Chan et al, 2019). In addition, the specific details and validation tests…”

“…All information relating to the seawater sample collection, incubation, and analysis can be found in the companion paper (Chan et al, ). To briefly summarize, waters directly influenced by, or adjacent to, known CH 4 seep activity were chosen to examine CH 4 oxidation kinetics.…”

“…Full details of the sample collection, analysis, and calibration procedures can be found in the companion paper (Chan et al, ). In addition, the specific details and validation tests for the system used to incubate and analyze these mesocosms are presented in Chan et al ().…”

Ideas and perspectives: A strategic assessment of methane and nitrous oxide measurements in the marine environment

“…Natural stable isotopes have been used to inform spatial and temporal changes in dissolved CH 4 concentrations (e.g. Pack et al, 2011;Mau et al, 2012;Weinstein et al, 2016;Leonte et al, 2017;Chan et al, 2019) and incubation experiments with added stable isotopes and radiotracers have helped elucidate how oxidation (anaerobically in sediments and aerobically in the water column), ebullition (where CH 4 pore water partial pressure exceeds sediment hydrostatic pressure), and subsequent bubble dissolution in the water column interact to mitigate CH 4 emissions to air (Steinle et al, 2015;Jordan et al, 2020). The information deriving from these various approaches is inherently different but complementary.…”