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

甲烷（CH 4）是大气中第二大温室气体（仅次于二氧化碳）。包括河流、湖泊、海洋、湿地和盐沼在内的水生生态系统是大气CH 4的重要来源。人们普遍认为，天然沃茨中的甲烷几乎完全来源于某些细菌严格在无氧条件下产生的甲烷。然而，这一长期以来的信念与海洋水柱中甲烷的分布相矛盾，海洋水柱中甲烷浓度大多过饱和，并且在含氧表面沃茨中最高。海洋表层甲烷最大值的存在被称为“海洋甲烷悖论"。 目前对外膜磷的解释是：1）甲烷产生于浮游动物和颗粒有机物消化道内的缺氧微环境; 2）甲烷是磷酸盐耗竭的好氧沃茨中磷酸甲酯微生物分解的副产品; 3）某些浮游植物物种产生的。我的实验室最近进行了初步的实验表明，溶解有机物（DOM）的光化学降解是一个潜在的显着的CH 4生产长期不仅在DOM丰富的新鲜沃茨，但在DOM贫乏的海洋表面沃茨。这些相同的测试也强烈表明POM也可能是显著的CH 4光产生的底物。 本研究旨在评估DOM和POM光降解对海洋沃茨CH 4产生的贡献，特别是其对OMP的影响。我们将从不同的自然水体中收集水样，包括河口，边缘海和开放的海洋，并将确定这些样品中CH 4光产生的速率和效率。我们还将评估DOM和POM褪色（即光漂白），温度，pH值和盐度对CH 4光生产的影响。这些实验的结果将外推到区域和全球范围内使用耦合光学-光化学建模。不同的模型化合物将被测试为潜在的甲烷形成的前体，阐明甲烷光生产的机制。 通过探索一个迄今为止很少探索的地区，具有潜在的巨大的气候和海洋地球化学的重要性，这项拟议中的研究预计将创建新的研究利基之间的化学海洋学，海洋生态学和海洋光学。它将填补我们对POM和DOM光化学在海洋沃茨中痕量气体循环中的作用以及它们对气候和水生生态系统功能的影响的理解方面的空白。特别是，它将有助于从非生物的角度解读OMP。

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.