Aerobic methane production in the western Tropical North Atlantic

<p>The surface waters of the open ocean are mostly oversaturated with methane and thus act as a net source to the atmosphere. <em>In situ</em> production from methylated compounds or inorganic carbon has been proposed to act as a potential source of methane in the oxygenated surface waters. However, the distribution and importance of aerobic methane production in different marine regions remains poorly constrained. We investigated the processes and microorganisms involved in aerobic methane production in the surface waters of the western Tropical North Atlantic off Barbados. Using stable isotope incubation experiments, we showed that within 24 hours methane was readily produced from methylphosphonate (MPn) but not from dissolved inorganic carbon. MPn-derived methane production reached up to ca. 8 nmol l^-1 d^-1, with the highest rates measured in surface waters above the deep chlorophyll maximum (DCM). Additions of inorganic phosphate resulted in the suppression of methane production in the surface waters but not below the DCM. Additional controlling factors of MPn-derived methane production, both physicochemical (depth, nutrients) as well as biological (primary production, nitrogen fixation), were also investigated. Our metagenomic and metatranscriptomic analyses revealed that various microbial groups, including <em>Trichodesmium</em> and <em>Alphaproteobacteria</em>, had the capacity to utilise MPn <em>in situ</em>, making them potential contributors to methane production in this region. Overall, our results highlight the importance of MPn-derived methane production in the phosphate-limited western Tropical North Atlantic and identify the controlling factors that may regulate aerobic methane production and thereby exert control over marine methane emissions.</p>