Origin and fate of methane in the Eastern Tropical North Pacific oxygen minimum zone.. Methane sources and sinks in the ETNP OMZ

Oxygen minimum zones (OMZ) have been identified as major pools of oceanic methane but its origin and fate are poorly understood. Using high-resolution (<15m) water column profiles and sediment core incubations, we identified a benthic source of methane where the ETNP OMZ meets the continental shelf, 30-50 NM off the Guatemalan coast. At a seabed depth of 350-650m, where the bottom-water is anoxic, the sulphate rich surface (0-2 cm) sediments produce methane at a rate of 29 (±SE 4.6) nmol m-2 h-1. The methyl coenzyme M reductase alpha subunit (mcrA) gene deriving from this sediment layer revealed that 85% of the retrieved sequences clustered within the family Methanosarcinaceae (≥96% similarity to Methanococcoides sp.). These methylotrophic methanogens are capable of performing non-competitive methanogenesis, which is the most probable process given the high sulphate concentrations measured in the sediment porewater. A tongue of elevated methane (20-105 nM) and nitrite (0.2-1.9 nM) between 250 and 600 m, is evident at least 360 NM offshore. Short term (<15 days) incubations with 13C-CH4 showed potential for substrate limited aerobic and anaerobic methane oxidation in the waters within and above the OMZ. Analysis of the particulate methane monooxygenase (pmoA) gene indicated the presence of both aerobic and anaerobic methanotrophs, with the former being related to Type I methanotrophs and the latest with the lineage of the Candidatus Methylomirabilis oxyfera, known to perform nitrite-dependent anaerobic methane oxidation (N-DAMO).