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.