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.

中文翻译：

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马尾藻释放的溶解有机物的光学性质和光化学转化

发色溶解有机物 (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成分仍不确定。海洋 CDOM 的一个来源是马尾藻，这是一种常见于墨西哥湾、加勒比海和北大西洋西部的上层海洋大型褐藻。此外，马尾藻含有根皮单宁，这是一类多酚，可能与陆地多酚具有相似的光学特性。在这里，我们分析了马尾藻 CDOM 光学特性，这些特性是从海水罐中马尾藻渗出实验期间收集的过滤样品的吸收和荧光光谱中获得的。为了进一步评估马尾藻 CDOM 光学特性的结构基础，通过固相萃取(SPE)收集马尾藻CDOM，并通过pH滴定和硼氢化钠还原测试其化学成分。这些化学测试表明，与陆源材料相比，马尾藻 CDOM 吸收光谱对 pH 滴定和硼氢化物还原反应相似，但马尾藻 CDOM 在不同光谱中具有独特的吸收峰，这在陆源 CDOM 中未观察到。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。与陆地 CDOM（如 Suwannee 河天然有机物）相比，马尾藻 CDOM 的光降解速度也更快。类似于陆地 DOM，超高分辨率质谱显示，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，表明缺氢和氧化的优先光降解和/或光转化化合物，如马尾藻根皮单宁。假设大部分马尾藻 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。