Green Tides Significantly Alter the Molecular Composition and Properties of Coastal DOC and Perform Dissolved Carbon Sequestration

Abstract

Despite green tides (or macroalgal blooms) having multiple negative effects, it is thought that they have a positive effect on carbon sequestration, although this aspect is rarely studied. Here, during the world’s largest green tide (caused by Ulva prolifera) in the Yellow Sea, the concentration of dissolved organic carbon (DOC) increased by 20–37% in intensive macroalgal areas, and thousands of new molecular formulas rich in CHNO and CHOS were introduced. The DOC molecular species derived from U. prolifera constituted ∼18% of the total DOC molecular species in the seawater of bloom area, indicating the profound effect that green tides have on shaping coastal DOC. In addition, 46% of the macroalgae-derived DOC was labile DOC (LDOC), which had only a short residence time due to rapid microbial utilization. The remaining 54% was recalcitrant DOC (RDOC) rich in humic-like substances, polycyclic aromatics, and highly aromatic compounds that resisted microbial degradation and therefore have the potential to play a role in long-term carbon sequestration. Notably, source analysis showed that in addition to the microbial carbon pump, macroalgae are also an important source of RDOC. The number of RDOC molecular species contributed by macroalgae even exceed (77 vs 23%) that contributed by microorganisms.

Legacy Effects of Late Macroalgal Blooms on Dissolved Inorganic Carbon Pool through Alkalinity Enhancement in Coastal Ocean

“…The photosynthetic growth of massive U. prolifera during the early stage of green tides removes substantial DIC from seawater to convert it into algal biomass. , Large amounts of DOC are concurrently released from growing macroalgae, which leads to a significant increase in DOC concentrations. , However, when the green tide enters the decline stage (i.e., late-bloom stage), the photosynthetic growth of macroalgae is significantly weakened or even ceases, and the microbial remineralization of macroalgae-derived organic carbon and macroalgal respiration release a large amount of DIC, which exceeds the absorption capacity of macroalgae, resulting in an increase in the seawater DIC concentration. ,, Therefore, the massive macroalgae at the late-bloom stage has a significant impact not only on the coastal organic carbon pool but also on the inorganic carbon pool. Several recent studies have observed an evident increase in the mean concentrations of DIC and TAlk in certain regions of the South Yellow Sea during the late-bloom period of green tides. , In fact, although the distribution area of U.…”

“…12,13 Large amounts of DOC are concurrently released from growing macroalgae, which leads to a significant increase in DOC concentrations. 5,10 However, when the green tide enters the decline stage (i.e., late-bloom stage), the photosynthetic growth of macroalgae is significantly weakened or even ceases, and the microbial remineralization of macroalgae-derived organic carbon and macroalgal respiration release a large amount of DIC, which exceeds the absorption capacity of macroalgae, resulting in an increase in the seawater DIC concentration. 2,14,15 Therefore, the massive macroalgae at the late-bloom stage has a significant impact not only on the coastal organic carbon pool but also on the inorganic carbon pool.…”

“…prolifera floated on the coastal ocean during the blooming stage, releasing large amounts of macroalgal organic carbon . The burial of macroalgal organic carbon in sediments and its transport into the deep sea, , as well as the generation of recalcitrant dissolved organic carbon (RDOC) by microorganisms after metabolizing macroalgal organic carbon, − jointly contribute to the long-term organic carbon sequestration. Indeed, the majority of macroalgae-released dissolved organic carbon (DOC) is labile DOC that can be rapidly transformed by microorganisms into dissolved inorganic carbon (DIC) .…”

Two treatment methods on Ulva prolifera bloom result in distinctively different ecological effects in coastal environment

“…However, the cell structure of pre-decomposed U. prolifera has been destroyed during the pretreatment process, and once it was thrown into water, almost all organic matter was released rapidly. Different concentrations and compositions of organic matter also affected microbial community succession (Liang et al, 2021;Li et al, 2022a), leading to different trends in FDOM components.…”

“…The dynamic changes in DOM composition was the driving force for the bacterial community (Liang et al, 2021;Li et al, 2022a). Proteobacteria, which widely existed in the environment, were always in relative high abundance in the degradation process of fresh and the pre-decomposed U. prolifera.…”