Methane production in oxic seawater of the western North Pacific and its marginal seas

The oceans are natural sources of atmospheric methane (CH4), but the origin of excess CH4 at the surface remains enigmatic. Incubation experiments were conducted in the western North Pacific (WNP) and its marginal seas (i.e., Yellow Sea and South China Sea [SCS]) to identify the degradation of methylphosphonate (MPn) to CH4 in the oceans and the microbes associated with MPn-driven CH4 production. In the coastal seawater of the Yellow Sea, CH4 was observed to accumulate after MPn enrichment with a high MPn to CH4 conversion efficiency (approximately 60%). Dissolved inorganic phosphorus (Pi) did not effectively restrict the microbial utilization of MPn in the eutrophic coastal waters. The results of 16S rRNA gene sequencing showed that Vibrio spp. were the dominant bacteria in the MPn-amended treatments. Moreover, several Vibrio isolates isolated from the coastal waters were found to produce CH4 while growing in culture using MPn as the sole P source, thereby indicating that Vibrio spp. might be the major contributors to MPn-dependent CH4 production. In oligotrophic areas, such as the SCS and WNP, CH4 production from MPn metabolism was also observed in the surface seawater. In contrast to coastal waters, this pathway in oligotrophic areas is regulated by dissolved Pi availability. This work confirms that aerobic CH4 formation from MPn degradation can occur both in eutrophic coastal waters and oligotrophic oceans driven by MPn-utilizing microorganisms (especially heterotrophic bacteria), which may have a significant impact on our understanding of the CH4 and P cycles in global oceans.