Microbial community dynamics and first quantification of the toxin domoic acid in a eutrophic bay in the United Arab Emirates

The marine planktonic community (eukaryotic, bacterial and archaeal) has been little investigated in the Persian/Arabian Gulf (PAG), despite its crucial role in sustaining biogeochemical cycles and the ecological health of this subtropical ecosystem, which during summer is the hottest marine body of water on Earth. This study explored the temporal changes in the planktonic community in a densely populated, semi-enclosed bay in the United Arab Emirates (UAE), in which seasonal succession of persistent phytoplankton blooms resulted in beach closures. We surveyed the microbial community by analyzing eukaryotic 18S ribosomal RNA (rRNA) and bacterial and archaeal 16S rRNA amplicon gene sequences, alongside measuring environmental parameters over the course of a year. Seasonal differences between cooler winter conditions and extreme high temperatures of summer were evident over the year. Mirroring these environmental changes, phytoplankton and bacterial diversity and community composition were significantly different in both winter and summer. Dinoflagellates dominated the phytoplankton community based on the relative abundance of 18S rRNA, though microscopy revealed a higher biomass contribution from diatoms. We observed several toxin producing dinoflagellates and diatoms in the community composition and seasonal co-occurrence networks. Notably, during a Pseudo-nitzschia bloom, we detected a concentration of 1.12 µg/L of the neurotoxin domoic acid for the first time in UAE coastal waters, highlighting the need for understanding environmental and molecular drivers of toxin production in the region. This study provides a baseline for understanding how environmental and anthropogenic factors influence HABs and microbial dynamics in the Persian/Arabian Gulf.