浒苔绿潮暴发期间海水溶解有机碳的生物可利用性及其碳汇作用

自2007年以来，浒苔绿潮已经连续15年在南黄海暴发。浒苔（Ulva prolifera）作为主要肇事藻种，在暴发过程中向海水释放大量的溶解有机碳（DOC）。然而，这些藻源DOC能否长期保存在海洋中，主要取决于它们的生物可利用性，目前关于此方面的研究甚少。本研究在浒苔绿潮大规模暴发时期（2019年6月），分别在浒苔暴发海区和无浒苔海区各选择3个站位富集表层海水，在实验室进行长期（300 d）的DOC降解实验。结果发现，在60 d内，不同站位富集海水中的DOC浓度随着微生物的利用快速下降，微生物丰度也在第60天达到峰值，表明这些被消耗的DOC是生物可利用性高的活性DOC（LDOC）。60 d后，剩余的DOC可抵抗微生物的降解，在60~300 d内保持稳定，表明这些DOC是具有强稳定性的惰性DOC（RDOC）。最终发现，浒苔暴发海水的RDOC占富集DOC的46%，明显高于无浒苔海水的（36%）。并且，荧光溶解有机物（FDOM）中活性的类蛋白组分随着微生物的利用被快速消耗，惰性的类腐殖质组分逐渐积累，暗示了在降解过程中LDOC逐渐向RDOC转化。可见，浒苔绿潮暴发除了在短时间内增加海水中的DOC浓度，在长时间尺度上也可增加近海RDOC库的积累。

Bioavailability and carbon sequestration function of dissolved organic carbon in seawater during geen tide bloom ofUlva prolifera

Massive green tides caused by macroalgae Ulva prolifera are a periodic event in the South Yellow Sea of China in the past 15 years. As U. proliferagrows rapidly producing a large amount of organic biomass, it also releases substantial amounts of dissolved organic carbon (DOC) into the ambient seawater. However, the bio-availability of the released DOC, which is important for its fate, is poorly understood. During a massive macroalgal bloom in June 2016, three stations located in the U. prolifera bloom zone and three stations located the nonU. prolifera bloom zone were respectively selected to enrich the DOC of surface seawater and conduct a long-term (300 days) DOC degradation experiment in the laboratory. It revealed that in 60 days the DOC concentration of seawater enriched at different stations decreased rapidly associated with a sharp increasing microbial abundance, indicating the rapid consumption of labile DOC (LDOC). The remaining DOC was found to be resistant to microbial utilization and did not show any significant reduction during the experiment, suggesting that this DOC was recalcitrant DOC (RDOC) with strong stability. After 300 days of incubation, the RDOC accounted for 46% of the DOC in the U. prolifera bloom seawater, which was significantly higher than that in the nonU. proliferabloom zone (36%). Moreover, the labile protein-like components of fluorescent dissolved organic matter (FDOM) were rapidly consumed by the microorganisms and the recalcitrant humus-like component gradually accumulated, suggesting that LDOC was gradually transformed into RDOC in the process of degradation. Hence, we infer that the outbreak of U. prolifera green tides not only increases the concentration of DOC in seawater in a short-term, but also contributes to the long-term accumulation of RDOC in the coastal seawater.