Tracking marine phycotoxins: spatial distribution and source elucidation of domoic acid and lipophilic shellfish toxins in the Beibu Gulf, South China Sea.

Harmful algal blooms (HABs) pose escalating threats to marine ecosystems and seafood safety worldwide, particularly in nearshore waters. The Beibu Gulf, located in the northwestern South China Sea, supports rich biodiversity and serves as a major fishing area. To comprehensively understand the current status of phycotoxin pollution, we conducted a basin-wide cruise survey in August 2021, collecting 60 phytoplankton samples to characterize the spatial distribution, compositional profile, and molecular origins of 12 target marine phycotoxins, along with their key environmental drivers. Domoic acid (DA), pectenotoxin-2 (PTX2), homo-yessotoxin (homoYTX), and gymnodimine A (GYM-A) were the predominant phycotoxins detected, whereas okadaic acid (OA), dinophysistoxin (DTX1), and yessotoxin (YTX) were found sporadically (< 6.7% detection rate). The DA was present in 20% of samples (mean 14,424 pg L-1, max 163,434 pg L-1), PTX2 in 36.7% (mean 508 pg L-1, max 2995 pg L-1), homoYTX in 43.3% (mean 1717 pg L-1, max 42,680 pg L-1), and GYM-A exhibited the highest detection rate at 68.3% (mean 37 pg L-1, max 902 pg L-1). Phycotoxin concentrations were markedly higher in the northern waters than in the southern gulf. Metabarcoding revealed 14 potential toxigenic microalgae, 12 of which belonged to Dinophyceae. Redundancy analysis indicated that concentrations of the four dominant phycotoxins were positively correlated with dissolved oxygen and chlorophyll a, but negatively correlated with salinity (p < 0.05). This suggests that low-salinity (28-31), resulting from terrestrial inputs and marine circulation, create favorable niches for toxigenic species. This study elucidates the phycotoxin profiles and their putative toxigenic microalgae in the Beibu Gulf, while highlighting environmental factors that promote phycotoxin production. These findings provide foundational insights into HAB dynamics in tropical estuarine systems, aiding future monitoring and management efforts in the region.