Toxin production in bloom-forming, harmful alga Alexandrium pacificum (Group IV) is regulated by cyst formation-promoting bacteria Jannaschia cystaugens NBRC 100362.

Harmful algal blooms (HABs) caused by dinoflagellates like Alexandrium pacificum pose significant ecological and public health risks due to their production of paralytic shellfish toxins (PSTs). Bacterial populations, particularly Alexandrium cyst formation-promoting bacteria (Alex-CFPB), are known to significantly influence growth, encystment, toxin synthesis, the composition of toxic components, and bloom dynamics of these dinoflagellates. However, the role of Alex-CFPB in Alexandrium toxin synthesis and the mechanisms thereof are still unclear. Here, we show that co-culturing A. pacificum with cyst formation-promoting bacteria Jannaschia cystaugens significantly increases total intracellular PSTs content in the late stationary phase (including more cysts and less vegetable cells compared with axenic group). Our results demonstrate that the presence of J. cystaugens alters metabolic pathways in A. pacificum by upregulating key paralytic shellfish toxins synthesis genes and inducing downregulation of sulfotransferase sxtN (related to PSTs sulfation) which decreases sulfated PSTs components (low-toxicity), leading to an increase in high-toxicity PSTs content at a single-cell level. Furthermore, bacterial oxidative stress signals, nutrient competition, and quorum sensing contribute to increased toxin synthesis. These results provide insights into the major role of bacteria in modulating growth, physiology, and toxin production in bloom-forming algae, and the complex regulatory mechanisms therein. This study thus defines the critical function of microbial associations in bloom formation and toxin production with implications for managing HABs and mitigating their impacts.