A thermodynamic analysis of the anaerobic oxidation of methane in marine sediments

Anaerobic oxidation of methane (AOM) in anoxic marine sediments is a significant process in the global methane cycle, yet little is known about the role of bulk composition, temperature and pressure on the overall energetics of this process. To better understand the biogeochemistry of AOM, we have calculated and compared the energetics of a number of candidate reactions that microorganisms catalyse during the anaerobic oxidation of methane in (i) a coastal lagoon (Cape Lookout Bight, USA), (ii) the deep Black Sea, and (iii) a deep-sea hydrothermal system (Guaymas basin, Gulf of California). Depending on the metabolic pathway and the environment considered, the amount of energy available to the microorganisms varies from 0 to 184 kJ mol(-1). At each site, the reactions in which methane is either oxidized to HCO3-, acetate or formate are generally only favoured under a narrow range of pressure, temperature and solution composition - particularly under low (10(-10) m) hydrogen concentrations. In contrast, the reactions involving sulfate reduction with H-2, formate and acetate as electron donors are nearly always thermodynamically favoured. Furthermore, the energetics of ATP synthesis was quantified per mole of methane oxidized. Depending on depth, between 0.4 and 0.6 mol of ATP (mol CH4)(-1) was produced in the Black Sea sediments. The largest potential productivity of 0.7 mol of ATP (mol CH4)(-1) was calculated for Guaymas Basin, while the lowest values were predicted at Cape Lookout Bight. The approach used in this study leads to a better understanding of the environmental controls on the energetics of AOM.