Aerobic production of methane from dissolved water-column methylphosphonate and sinking particles in the North Pacific Subtropical Gyre

Supersaturation of methane in aerobic surface waters is a phenomenon widely observed in the ocean, albeit poorly understood. Recent studies have shown that degradation of methylphosphonate (MPn) can lead to the production of methane under aerobic conditions; however, the importance of this process has not been quantified under natural conditions. Here, we quantified the production of methane from dissolved MPn (d-MPn) in surface waters and observed that this process cannot account for the methane oversaturation observed in the oligotrophic waters of the North Pacific Subtropical Gyre. Degradation rates of d-MPn were, however, stimulated by the addition of low concentrations of nitrate and iron, suggesting regulation of the d-MPn degradation pathway by these nutrients. Degradation of d-MPn associated with sediment trap-collected particles was significantly faster than in the water column, even after normalizing to particulate organic carbon, particulate phosphorus, or heterotrophic bacterial numbers, suggesting that MPn degradation might be localized on sinking particles. We hypothesize that MPn is transformed to methane directly from sinking particles, without transitioning through the dissolved phase.