Prokaryotic and Eukaryotic Community Succession and Potential Parasitic Interactions During Two Alexandrium pacificum Blooms in Aotearoa New Zealand.

Harmful algal blooms (HABs), caused by the dinoflagellate Alexandrium pacificum, are increasingly frequent in the Marlborough Sounds, an important aquaculture region in Aotearoa New Zealand. Alexandrium pacificum produces paralytic shellfish toxins and blooms cause significant economic and ecological disruptions through contamination of edible shellfish. High-throughput sequencing of prokaryotic and eukaryotic communities was used to investigate community dynamics during bloom events across two consecutive summers. Distinct successional shifts were observed, with prokaryotic communities dominated by Rhodobacterales and Flavobacteriales during blooms, and increased abundance of the SAR11 clade (Pelagibacterales) post-bloom. Eukaryotic diversity was dominated by Alexandrium species (Gonyaulacales) during the bloom, and subsequently shifted towards Syndiniales, Gymnodiniales, and Peridiniales as blooms collapsed. Significant correlations indicated potential ecological roles for these taxa in bloom regulation, particularly Syndiniales, which could indicate parasitic interactions. Depth profiles revealed consistent microbial composition throughout the water column, validating depth-integrated sampling strategies for community studies. This research describes changes in the composition of microbial communities during two A. pacificum blooms, suggesting that species interactions (e.g., via parasitism) may play a role shaping bloom dynamics. Further studies incorporating environmental parameters, especially nutrient dynamics linked to anthropogenic activities, are necessary to better understand the drivers of blooms in this important aquaculture region.