Differential responses and adaptive mechanisms of diatoms and dinoflagellates to changes in dissolved organic...

Dissolved organic nitrogen (DON) composition critically influences phytoplankton community dynamics, yet the species-specific regulatory mechanisms remain unclear. Combining cultivation experiments and transcriptomics, we reveal distinct growth responses of the diatom Chaetoceros curvisetus and dinoflagellate Prorocentrum minimum to three DON components: plant-derived peptides in labile DON (LDON_P) as well as animal manure-derived humic in semilabile DON (SDON_H) and fulvic acids in refractory DON (RDON_F). Diatoms exhibited superior LDON_P utilization via upregulated nitrogen metabolism genes (gdhA, GLT1, glnA) and peptide transporters (PTR), while dinoflagellates dominated SDON_H assimilation through endocytosis-associated gene activation (PLD, PIP5K) and enhanced photosystem efficiency. RDON_F inhibited both species but triggered energy reallocation to the TCA and Calvin cycles. Fluorescence spectroscopy further linked bioavailability to molecular structure, with diatoms efficiently utilizing protein-like components (T) and dinoflagellates preferentially absorbing humic-like components (E). This study proposed a genomic insight of diatoms and dinoflagellates growth into adaptation to DON regimes. Ecologically, the molecular mechanistic framework might predict phytoplankton succession under shifting DON regimes in coastal ecosystems. These findings provide strategies to mitigate harmful algal blooms in eutrophic coastal waters through DON source management.