Protistan grazer-mediated regulation of domoic acid: interactions between Noctiluca scintillans and Pseudo-nitzschia reshape toxin fate and risk in coastal waters.

Domoic acid (DA)-producing Pseudo-nitzschia and the heterotrophic dinoflagellate Noctiluca scintillans frequently co-occur during coastal harmful algal blooms, yet their interactions and consequences for toxin cycling remain poorly understood. Here, controlled co-cultures were conducted to examine grazer-induced defense, grazing dynamics, and DA accumulation-depuration. Sustained 3-d exposure stimulated a 3-4-fold increase in cellular DA production, without detectable changes in cell size or frustule silicification, indicating a primarily chemical defense response. Cellular DA induction exceeded dissolved-pool enhancement during sustained co-culture. Conversely, short-term 24-h exposure increased dissolved DA concentrations by 1.73-fold relative to grazer-free controls, whereas cellular DA was not significantly induced. Mass-balance calculations revealed that grazer excretion accounted for only 15.4% of this dissolved surplus, indicating grazer-induced extracellular DA release by P. fukuyoi. These patterns suggest a potential shift from early extracellular chemical deterrence to sustained intracellular toxin-based defense. N. scintillans ingested all prey strains, but prey toxicity suppressed ingestion rates and drove negative grazer growth via post-ingestive toxin accumulation. Feeding on P. fukuyoi generated high cellular DA burdens in the grazer, with an apparent accumulation efficiency ∼300%, but depuration was rapid, removing ∼90% of accumulated DA within 6 h. This rapid clearance resulted in a low trophic transfer efficiency (3.69%), with only 12.52% released as dissolved DA, while most parent toxin became undetectable, suggesting internal transformation or degradation. These findings characterize N. scintillans as a transient DA reservoir and active toxin recycler that substantially modifies DA partitioning and dampens its upward trophic transfer potential in marine food webs.