Temperature-Driven Intraspecific Diversity in Paralytic Shellfish Toxin Profiles of the Dinoflagellate Alexandrium pacificum and Intragenic Variation in the Saxitoxin Biosynthetic Gene, sxtA4.

Alexandrium pacificum, a globally distributed dinoflagellate, is well-known for causing harmful algal blooms and producing Paralytic Shellfish Toxins (PSTs), a threat to marine life and human health. The frequency and intensity of Alexandrium blooms have increased in recent decades, driven, in some cases, by increasing temperatures. Here, we investigated the temperature-dependent (15 °C, 20 °C, 25 °C, and 30 °C) growth rates and paralytic shellfish toxin profiles of eight A. pacificum strains while concurrently examining differences in sequences of the saxitoxin biosynthetic gene, sxtA4. While maximum cell densities were lowest at 30 °C, toxin production per cell was highest at higher temperatures that inhibited growth, with greater diversity of toxin analogs peaking at 30 °C, as confirmed by the higher Shannon's diversity index obtained for the toxin profiles with the increasing temperatures. Furthermore, genetic analysis of the sxtA4 gene showed that greater genetic diversity-quantified by nucleotide diversity (π) ranging from 9.91 to 30.21 across strains-was positively correlated with this wider array of toxin analogs (Shannon's diversity index; p < 0.0001). Conserved regions within the gene were identified, suggesting that these regions may play important structural or functional roles in the saxitoxin biosynthetic pathway. These findings highlight the role of temperature, genetic diversity, and sxtA4 conserved regions in influencing toxin production and profiles in Alexandrium. Further research into the genetic mechanisms underlying saxitoxin biosynthesis will improve our understanding of Alexandrium's adaptability to changing temperatures. Such insights are essential for effective ecosystem management and safeguarding public health.