Strong seasonality of marine microbial communities in a shallow man-made lagoon in the United Arab Emirates, and first detection of the toxin domoic acid

The marine planktonic community (eukaryotic, bacterial and archaeal) has been little investigated in the Persian/Arabian Gulf (PAG); however, the composition and function of these microbial communities are critical for the healthy functioning of biogeochemical cycling and ecology of this subtropical ecosystem, which in the 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 several beach closures. In this study, we surveyed the microbial community by analyzing 18S ribosomal RNA (rRNA) eukaryotic, 16S rRNA bacterial and archaeal gene sequences, alongside measuring environmental parameters over the course of a full annual cycle. We found strong temporal seasonality between temperate winter conditions and the extreme high temperatures and salinities of summer. Mirroring these seasonal changes, phytoplankton and bacterial diversity and community composition were significantly different in both winter and summer. Dinoflagellates dominated the phytoplankton community in terms of 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 the neurotoxin domoic acid, for the first time in the region, highlighting the need for understanding environmental and molecular drivers of toxin production in UAE coastal waters. This study provides a baseline for understanding how environmental and anthropogenic factors influence HABs and microbial dynamics in the Persian/Arabian Gulf.