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

甲烷（CH4）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CH4的重要来源。人们普遍认为，天然水体中的CH4几乎完全来自某些细菌在缺氧条件下产生的甲烷。然而，这一长期确立的信念与CH4在海洋水柱中的分布相矛盾，因为海洋水柱中CH4浓度大多过饱和，在含氧地表水中浓度最高。这种CH4最大值在海洋表面的存在被称为“海洋甲烷悖论”。目前对OMP的解释指出：1)在浮游动物和微粒有机物（POM）消化道内的缺氧微环境中产生CH4, 2)在贫磷含氧水域中甲基磷酸盐的微生物分解副产物，3)某些浮游植物产生CH4。我的实验室最近进行的初步实验表明，不仅在富含DOM的淡水中，而且在缺乏DOM的海洋地表水中，溶解有机物（DOM）的光化学降解可能是一个重要的CH4生产条件。这些同样的试验也有力地表明，POM也可能是产生大量CH4的底物。本研究旨在评估DOM和POM的光降解对海水中CH4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、边缘海和开阔的海洋，并将确定这些样品中CH4光合作用的速率和效率。我们还将评估DOM和POM褪色（即光漂白）、温度、pH和盐度对CH4光生产的影响。这些实验的结果将使用耦合光学-光化学模型外推到区域和全球尺度。不同的模型化合物将被测试作为潜在的CH4形成前体，以阐明CH4光产生的机制。通过探索迄今为止很少探索的具有潜在巨大气候和生物地球化学重要性的区域，本研究有望在化学海洋学，海洋生态学和海洋光学之间建立新的研究生态位。它将填补我们对POM和DOM光化学在海洋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.