The epipelagic macroalgae of Ulva prolifera and Sargassum are the primary contributors to widespread seaweed tides globally. Both ocean plants release large amounts of chromophoric dissolved organic matter (CDOM) into the surrounding seawater. The photochemical reactivity of this CDOM, however, has not been adequately addressed. In this study, we extracted CDOM from Ulva prolifera and Sargassum, examined their ultraviolet (UV)-visible absorption characteristics, and quantified their broadband apparent quantum yields (AQY) of absorbance photobleaching and photomineralization (in terms of CO2, CO, and CH4 photoproduction). On a per-unit-weight basis, Sargassum leached 3.5 times more CDOM than did Ulva prolifera in terms of the absorption coefficient averaged over 254–500 nm. Both Ulva prolifera and Sargassum CDOM were characterized by quasi-exponential decay absorption spectra, with Sargassum CDOM exhibiting a distinct shoulder over 310–350 nm suggestive of mycosporine amino acids. The Sargassum CDOM had a higher photobleaching AQY but lower photomineralization AQYs compared to Ulva prolifera CDOM. The photobleaching and photomineralization AQYs of both macroalgal CDOM are, however, orders of magnitude higher than those of CDOM in various natural waters. Potential photoproduction rates of CO2 and CO from the Ulva prolifera CDOM and Sargassum CDOM during the bloom periods are several times to orders of magnitude higher than the air-sea fluxes of these gases in the absence of the macroalgae. This study demonstrates that CDOM released by Ulva prolifera and Sargassum is extremely prone to photobleaching and photomineralization, rendering floating mats of these plants in oceans as potential “hotspots” of greenhouse gas emissions to the atmosphere. This photochemical feedback should be considered when assessing ocean afforestation as a CO2 removal approach to mitigate climate warming.

中文翻译：

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源自石莼和马尾藻的发色溶解有机物具有高光反应性


浮生石莼和马尾藻的表层大型藻类是全球广泛海藻潮汐的主要贡献者。这两种海洋植物都会向周围海水中释放大量发色溶解有机物（CDOM）。然而，这种 CDOM 的光化学反应性尚未得到充分解决。在这项研究中，我们从石莼和马尾藻中提取了 CDOM，检查了它们的紫外 (UV)-可见光吸收特性，并量化了它们的吸收光漂白和光矿化的宽带表观量子产率 (AQY)（以 CO2、CO 和 CH4 光生产量计） ）。以单位重量计算，马尾藻浸出的 CDOM 比石莼多 3.5 倍，平均吸收系数超过 254-500 nm。石莼和马尾藻 CDOM 均具有准指数衰减吸收光谱特征，其中马尾藻 CDOM 在 310-350 nm 处表现出明显的肩部，表明存在菌孢素氨基酸。与石莼 CDOM 相比，马尾藻 CDOM 具有较高的光漂白 AQY，但光矿化 AQY 较低。然而，两种大型藻类 CDOM 的光漂白和光矿化 AQY 比各种天然水中的 CDOM 高几个数量级。在藻华盛期期间，石莼 CDOM 和马尾藻 CDOM 产生的 CO2 和 CO 的潜在光生产率比没有大型藻类时这些气体的海气通量高几倍到几个数量级。这项研究表明，石莼和马尾藻释放的 CDOM 极易发生光漂白和光矿化，使这些植物在海洋中的漂浮垫成为向大气排放温室气体的潜在“热点”。 在评估海洋造林作为缓解气候变暖的二氧化碳清除方法时，应考虑这种光化学反馈。