Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
Environ Sci Process Impacts. 2020 May 1;22(5):1214-1223. doi: 10.1039/c9em00504h. Epub 2020 Feb 19.
In sunlit waters, photodegradation of dissolved organic matter (DOM) yields completely oxidized carbon (i.e., CO) as well as a suite of partially oxidized compounds formed from oxygen incorporation (i.e., partial photo-oxidation). Of these two groups of DOM photo-products, more studies focus on CO (a greenhouse gas) than on partially oxidized DOM, which is likely a diverse group of compounds with poorly constrained roles in aquatic carbon cycling or biogeochemistry. The objective of this study is to address knowledge gaps on the prevalence, products, and pathways of DOM partial photo-oxidation. Here we traced the photochemical incorporation of isotopically labelled O into DOM isolated from Alaskan Arctic surface waters using high-resolution mass spectrometry. Complete and partial photo-oxidation of DOM was also quantified as CO production and O consumption. The majority of O-containing partial oxidation photo-products were classified as carboxylic rich alicyclic molecules (CRAM) and overlapped in composition with previously reported photo-products known to result from the oxidation of DOM by singlet oxygen. These results support a previously proposed hypothesis that photo-oxidation by singlet oxygen may contribute to the formation of CRAM, a compound class of DOM ubiquitously observed in surface waters. The novel application of an isotopic tracer for oxygen incorporation with a mass balance approach to quantify complete and partial photo-oxidation of DOM revealed that less than one mol of O is required to produce one mol of CO. A sensitivity analysis based on this new knowledge demonstrated that the magnitude of DOM partial photo-oxidation may be underestimated by up to four-fold. Consequently, partial photo-oxidation likely plays a more prominent role in shaping DOM composition in sunlit waters of the Arctic than previously understood. Therefore, partial photo-oxidation should be increasingly incorporated into the experimental framework of studies focused on DOM composition in surface waters.
在阳光照射的水域中，溶解有机质（DOM）的光降解会产生完全氧化的碳（即 CO）以及一系列通过氧掺入形成的部分氧化化合物（即部分光氧化）。在这两组 DOM 光产物中，更多的研究关注 CO（一种温室气体）而不是部分氧化的 DOM，部分氧化的 DOM 可能是一组具有较差约束作用的化合物，在水生碳循环或生物地球化学中。本研究的目的是解决 DOM 部分光氧化的普遍性、产物和途径方面的知识空白。在这里，我们使用高分辨率质谱追踪了从阿拉斯加北极地表水中分离的 DOM 中同位素标记的 O 的光化学掺入。通过 CO 产量和 O 消耗来量化 DOM 的完全和部分光氧化。大多数含 O 的部分氧化光产物被归类为羧酸丰富的脂环分子（CRAM），并且在组成上与先前报道的由单线态氧氧化 DOM 产生的光产物重叠。这些结果支持了先前提出的假设，即单线态氧的光氧化可能有助于 CRAM 的形成，CRAM 是一种在地表水中普遍存在的 DOM 化合物类。本研究创新性地应用同位素示踪剂追踪 O 的掺入，并采用质量平衡方法来量化 DOM 的完全和部分光氧化，结果表明，产生 1 mol CO 只需不到 1 mol O。基于这一新知识的敏感性分析表明，DOM 部分光氧化的幅度可能被低估了四倍之多。因此，与之前的理解相比，部分光氧化可能在塑造北极阳光照射水域中 DOM 组成方面发挥了更为突出的作用。因此，部分光氧化应该越来越多地纳入到以地表水中 DOM 组分为重点的研究实验框架中。
