Optical Properties and Photochemical Transformation of the Dissolved Organic Matter Released by Sargassum
Optical Properties and Photochemical Transformation of the Dissolved Organic Matter Released by Sargassum
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DOI：
10.3389/fmars.2020.588287
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发表时间：
2020-11
期刊：
影响因子：
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通讯作者：
L. Powers;Rossana Del Vecchio;N. Blough;N. McDonald;P. Schmitt‐Kopplin;M. Gonsior
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作者：
L. Powers;Rossana Del Vecchio;N. Blough;N. McDonald;P. Schmitt‐Kopplin;M. Gonsior
有色溶解有机物（CDOM）是海洋中紫外线辐射的主要吸收体，但其在海洋中的来源及其化学组成仍然不确定。海洋CDOM的一个来源是马尾藻，这是一种常见于墨西哥湾、加勒比海和西北大西洋的上层海洋大型棕色藻类。此外，马尾藻含有根皮单宁，这是一类多酚，可能具有与陆地多酚相似的光学性质。在这里，我们分析了马尾藻CDOM的光学特性，从吸收光谱和荧光光谱的海藻渗出实验中收集的样品在海水坦克。为进一步研究马尾藻CDOM光学性质的结构基础，采用固相萃取法（SPE）提取马尾藻CDOM，通过pH滴定法和硼氢化钠还原法测定其化学组成。这些化学测试表明，马尾藻CDOM吸收光谱响应类似的pH滴定和硼氢化物还原相比，陆地来源的材料，但马尾藻CDOM具有独特的吸收峰的差异光谱中没有观察到的陆地来源的CDOM。这些吸光度特征与改性马尾藻根皮单宁的去质子化一致，这可能是高度相关的酚酸和多酚。马尾藻CDOM也更迅速的光降解相比，陆地CDOM，如苏瓦尼河天然有机物。与陆地DOM相似，高分辨率质谱分析表明，阳光降低了m/z离子和分子式的相对丰度，平均O/C比为0.6，平均H/C比为0.9，这表明优先光降解和/或光转化的缺氢和含氧化合物，如马尾藻phlorotannins。假设大部分的马尾藻CDOM在其快速光降解过程中迅速矿化为CO2，马尾藻在其年度生长周期中可能在海洋光化学碳矿化中发挥重要作用。
Chromophoric dissolved organic matter (CDOM) is the dominant absorber of ultraviolet radiation in the ocean, but its sources within the ocean, as well as its chemical composition, remain uncertain. One source of marine CDOM is Sargassum, an epipelagic marine macro brown alga common to the Gulf of Mexico, Caribbean, and Western North Atlantic. Furthermore, Sargassum contains phlorotannins, a class of polyphenols that may have similar optical properties to terrestrial polyphenols. Here, we analyze Sargassum CDOM optical properties, acquired from absorption and fluorescence spectra of filtered samples collected during Sargassum exudation experiments in seawater tanks. To further evaluate the structural basis of Sargassum CDOM optical properties, Sargassum CDOM was collected by solid phase extraction (SPE) and its chemical composition was tested by pH titration and sodium borohydride reduction. These chemical tests revealed that Sargassum CDOM absorption spectra respond similarly to pH titration and borohydride reduction when compared to terrestrially-derived materials, but Sargassum CDOM has unique absorbance peaks in difference spectra that have not been observed in terrestrially-derived CDOM. These absorbance features are consistent with the deprotonation of modified Sargassum phlorotannins, which are likely highly related phenolic acids and polyphenols. Sargassum CDOM was also more rapidly photodegraded when compared to terrestrial CDOM such as Suwannee River Natural Organic Matter. Similar to terrestrial DOM, ultrahigh resolution mass spectrometry revealed that sunlight decreases relative abundances of m/z ions and molecular formulas with an average O/C ratio of 0.6 and an average H/C ratio of 0.9, suggesting preferential photodegradation and/or phototransformation of hydrogen-deficient and oxygenated compounds, such as Sargassum phlorotannins. Assuming a large fraction of Sargassum CDOM is quickly mineralized to CO2 during its rapid photodegradation, Sargassum could play a major role in marine photochemical carbon mineralization during its annual growth cycle.