The effect of the aging processes of Ulva prolifera-derived dissolved organic nitrogen associated with green tide on the diatoms-dinoflagellates succession in the Southern Yellow Sea, China

Multitudinous dissolved organic nitrogen (DON) enters seawater from Ulva prolifera green tides impacting phytoplankton community structure in the Yellow Sea. Field investigations and cultural experiments on U. prolifera-derived DON aging revealed its regulatory effects. The green-tide outbreak area of the Southern Yellow Sea exhibits a seasonal cycle where dominance shifts from diatoms to dinoflagellates effected by young to aging U. prolifera-derived DON from summer to spring. Diatom abundance rose significantly following the summer green tide outbreak, fueled by young, protein-rich DON released by U. prolifera. After autumn and winter aging, U. prolifera-derived DON was dominant from protein-like to humic-like components, and dinoflagellates bloomed in the green tide outbreak area. An aging U. prolifera-derived DON adding culture experiment of Chaetoceros curvisetus and Prorocentrum minimum proved that old and young DON promoted dinoflagellate and diatom growth, respectively. Diatoms preferred 0 and 12 days-aged DON (DON0 and DON12) with tyrosine B and tryptophan T components, whereas dinoflagellate favored 80 days-aged DON (DON80) with humic acid E and UV fulvic acid A components. DON0 and DON12 can be absorbed by diatoms with higher uptake (kup) and growth (kG) constants than by dinoflagellates under low leucine aminopeptidase (LAP) conditions. Dinoflagellates absorb old DON with higher kup and kG values under higher LAP concentrations than diatoms. This was consistent with the loop of U. prolifera-derived DON turning over the seasonal succession of diatom dinoflagellates in the green-tide outbreak area. This study revealed mechanism of seasonal dinoflagellate-U. prolifera-diatom cycle, and provided new insights for impacts of U. prolifera green tides on phytoplankton regime shift, thus, acting as a basis for marine management strategies.