Picoplanktonic methane production in eutrophic surface waters

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Abstract. Over the past decade, extensive research has delved into the methane (CH4) paradox, which involves aerobic CH4 production. We present noteworthy observations of CH4 oversaturation within the surface layer of the central Chile upwelling zone (36° S, 73° W) over two consecutive seasonal cycles (2018–2021). Complementing these observations, CH4 cycling experiments were conducted, utilizing distinct plankton fractions (encompassing the natural planktonic community, fractions < 150, < 3 and < 0.2 µm), in different productivity periods of phytoplanktonic production and composition throughout the year. Our findings underscore the pivotal role of picoplankton (< 3 µm) in CH4 production on the ocean surface, contrasting with the limited contribution of larger microorganisms (< 150 µm). Notably, incubations with methylated substrates, such as methylphosphonic acid (MPn) and trimethylamine (TMA), induce heightened CH4 production within the picoplanktonic fraction. This phenomenon is consistently observed during both upwelling (austral spring–summer) and non-upwelling (winter) seasons, with significance in the latter period, when Synechococcus sp. exhibits notably high relative abundance. Long-term microcosm experiments highlight the crucial roles played by heterotrophic bacteria and cyanobacteria in methylotrophic methanogenesis. This process enhances CH4 production, facilitated by the recycling of dissolved organic carbon (DOC). Picoplankton emerges as a pivotal factor influencing the recycling of methylated substrates, and it is responsible for maintaining CH4 supersaturation. These findings provide valuable insights into the biogeochemical processes driving CH4 dynamics, particularly in highly productive upwelling areas.