DOC dynamics and bacterial community succession during long-term degradation of Ulva prolifera and their implications for the legacy effect of green tides on refractory DOC pool in seawater

Under climate warming and coastal eutrophication, outbreaks of green tides have increased in recent decades; e.g., the world's largest green tide caused by Ulva prolifera has occurred in the Yellow Sea for 13 consecutive years. The massive assemblage of macroalgae absorbs large amounts of atmospheric CO2 and converts it into biomass. After the green tide, millions of tons of the macroalgal biomass sink to the seabed to be degraded eventually; this inevitably has a significant impact on the coastal organic carbon pool and microbial community. However, this impact is poorly understood. Here, the degradation of Ulva prolifera over 520 days revealed that relatively sufficient degradation of the macroalgae occurred at ca. 7 months. The rapid release of dissolved organic carbon (DOC) mainly occurred in the first week, which not only increased the size and diversity of the DOC pool in a short time but also promoted the rapid growth of bacteria and led to hypoxia and acidification of the seawater. After that, the labile portion of DOC was gradually used up by bacteria within one month, while the degradation of semi-labile or semi-refractory DOC occurred in half a year. The remaining DOC existed in the form of refractory DOC (RDOC), resisting bacterial consumption and remaining stable for 10 months. During the long-term degradation process, bacterial community structure and metabolic function showed obvious successional characteristics, driving the gradual transformation of DOC from labile to refractory through the microbial carbon pump mechanism. After the long-term degradation, the remaining RDOC accounted for approximately 1.6% of the macroalgal carbon biomass. As RDOC can maintain long-term stability, we propose that the frequent outbreaks of green tides not only affect microbial processes but also may have an important cumulative effect on the coastal RDOC pool. (C) 2020 The Author(s). Published by Elsevier Ltd.