Toxin partitioning and sxtA4 expression in Alexandrium catenella under excess nitrogen supply

The frequency and intensity of harmful algal blooms (HABs) associated with nutrient loadings of water bodies are on the rise worldwide in recent decades. Among these HAB-forming species, the dinoflagellate genus Alexandrium which can produce paralytic shellfish toxins (PSTs) is notorious. This study focuses on the effects of different nutrient supply on toxin partitioning and the expression of sxtA4 core gene involved in saxitoxin biosynthesis pathway in Alexandrium catenella. Especially, extracellular toxins in the culture medium were measured, followed by intracellular toxins in A. catenella to achieve the quantification of total PST yield. The results showed that the maximum amounts of intra- and extracellular PSTs under high nitrogen supply reached 109.2 fmol cell^-1 and 152.9 nmol L^-1, respectively. It was noticed that extracellular toxins had a low proportion in total yield (< 10%) under different nutrient supply. Compared with low nitrogen treatment, high nitrogen supply not only enhanced toxin synthesis with a peak value of toxicity (24.0 pg STX eq. cell^-1), but also promoted the quantities of extracellular toxins. GTX4 with high toxicity had a predominant contribution for toxin composition in three experimental groups throughout the culture. At the gene expression level, our findings exhibited a strong link between sxtA4 expression and toxin yield in A. catenella. Excess nitrogen promoted the expression levels of sxtA4 core gene that had a positive correlation with total yield. In particular, there was a clear discrepancy on the correlation between sxtA4 expression levels and extracellular toxins (the quantities of toxins in extracellular pools were unified in each cell) in three experimental groups, and it was found a strong positive correlation between them in the low-N treatment. However, the correlation analysis showed that sxtA4 expression was not significantly correlated with PST concentrations in the culture medium under either low-N or high-N conditions (P > 0.05). The collective analysis of intra- and extracellular toxins as well as sxtA4 gene expression would provide comprehensive insights into PST biosynthesis and toxin partitioning response of the toxigenic Alexandrium under excess nitrogen supply.