Response of microbial eukaryote community to the oligotrophic waters of the Gulf of Mexico: a plausible scenario for warm and stratified oceans

In oligotrophic environments, interactions among eukaryotic microorganisms are highly complex. In the Gulf of Mexico (GoM), the Loop Current intensifies in summer and supplies the Gulf with warm and oligotrophic waters. However, mesoscale eddies within the GoM create favorable conditions for biological productivity by bringing nutrient-rich water to the subsurface layer. This study aimed to determine the structure, variability, and ecological roles of the protist in the mixed layer (ML) and deep chlorophyll maximum (DCM), representing the first V9-18S rRNA survey studying the protist community from the Southern GoM. Results revealed different assemblages between the ML and DCM. In the ML, species abundance was highly and positively correlated with temperature but negatively correlated with the nitrate concentration, whereas the opposite pattern was observed in the DCM. Alveolata represented [~]60% in both the ML and DCM, while Haptophytes and MAST dominated the ML, and Pelagophytes and Radiolarians dominated the DCM. Interestingly, Ostreococcus abundance increased under upwelling conditions suggesting that it may act as an indicator of the vertical nitrate flux and that picoeukaryotes respond to this instead of diatoms. Finally, our analyses revealed high levels of competition, parasitism, and predation with a high proportion of self-exclusion relationship (30%) in both depths.