Revealing the Hidden Risk of Neurotoxin Domoic Acid in Tropical Seawaters: From Sampling Bias to Climate-Driven Risk

Neurotoxin domoic acid (DA) poses threats to marine ecosystems and human health, yet its true levels and dynamics remain poorly understood in tropical seawaters. This study first revealed the methodological bias of the conventional sampling method using a phytoplankton net (20 μm) in previous phytotoxin assessments, which underestimated particulate DA (pDA) by approximately 90% due to cell loss while artificially elevating dissolved DA (dDA) due to cell rupture. Subsequently, we conducted a year-long monthly survey using in situ seawater in the Chinese tropical Daya Bay. The results revealed widespread DA contamination, with detection rates of 82% and 93% for pDA and dDA, respectively. Higher levels were concentrated between April and June, with peaks of 5059 ng L-1 for pDA and 13220 ng L-1 for dDA in May, orders of magnitude higher than previously reported. DA was detected in 76.3% of shellfish samples, with scallops showing the highest tissue burdens (144-26075 μg kg-1), primarily concentrated in the digestive glands (p < 0.01). The majority (92.9%) of elevated pDA levels (exceeding 200 ng L-1) occurred at temperatures between 24.9 and 30.8 °C, implying a warm-temperature adaptation in toxigenic Pseudonitzschia. Dissolved silicate (DSi) showed a significant negative correlation with environmental DA, indicating that silicate limitation under nitrogen-replete conditions may enhance DA synthesis. A recent decadal monitoring data set revealed a warming trend and a declining DSi/DIN ratio during spring, reinforcing the emergence of conditions favorable to DA production. These findings revealed underestimated DA levels and environmental drivers in tropical seawaters, suggesting that warming and nutrient imbalance may exacerbate DA-related risks.