Photodegradation of natural organic matter: Implications for dissolved methane cycling in the ocean

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Methane (CH4) is the second most potent greenhouse gas in the atmosphere (after carbon dioxide). Aquatic ecosystems, including rivers, lakes, oceans, wetlands, and saltmarshes, provide an important source of atmospheric CH4. It is commonly accepted that CH4 in natural waters almost exclusively originates from methane production by certain bacteria strictly in the absence of oxygen. This long-established belief, however, contradicts the CH4 distribution in the ocean water column where CH4 concentrations are mostly supersaturated and highest in the oxic surface waters. The presence of this CH4 maximum in the surface ocean is called the “oceanic methane paradox (OMP)”. ******Current explanations for the OMP point to CH4 production 1) in anoxic microenvironments inside digestive tracts of zooplankton and particulate organic matter (POM), 2) as a by-product of microbial decomposition of methylphosphate in phosphate-depleted oxic waters, and 3) by certain phytoplankton species. My lab has recently conducted preliminary experiments showing that photochemical degradation of dissolved organic matter (DOM) is potentially a significant CH4 production term not only in DOM-rich freshwaters but also in DOM-poor oceanic surface waters. These same tests also strongly suggest that POM is likely a substrate for significant CH4 photoproduction as well. ******This proposed research aims to assess the contribution of photodegradation of DOM and POM to CH4 production in marine waters, particularly its implication for the OMP. We will collect water samples from diverse natural water bodies covering estuaries, marginal seas, and open oceans, and will determine the rates and efficiencies of CH4 photoproduction in these samples. We will also evaluate the effects of DOM and POM color fading (i.e. photobleaching), temperature, pH, and salinity on CH4 photoproduction. Results from these experiments will be extrapolated to regional and global scales using coupled optical-photochemical modeling. Different model chemical compounds will be tested as potential CH4-forming precursors for elucidating the mechanisms of CH4 photoproduction.******By exploring a heretofore little explored area with potentially great climatic and biogeochemical importance, this proposed research is expected to create new research niches interrelating among chemical oceanography, marine ecology, and ocean optics. It will fill in gaps in our understanding of the roles of POM and DOM photochemistry in the cycling of trace gases in marine waters and hence of their impacts on the climate and the functioning of aquatic ecosystems. In particular, it will help decipher the OMP from an abiotic point of view.