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Growing attention in aquatic ecology is focusing on biogeographic patterns in microorganisms and whether these potential patterns can be explained within the framework of general ecology. The long-standing microbiologist’s credo ‘Everything is everywhere, but, the environment selects’ suggests that dispersal is not limiting for microbes, but that the environment is the primary determining factor in microbial community composition. Advances in molecular techniques have provided new evidence that biogeographic patterns exist in microbes and that dispersal limitation may actually have an important role, yet more recent study using extremely deep sequencing predicts that indeed everything is everywhere. Using a long-term field study of the ‘invasive’ marine haptophyte Prymnesium parvum, we characterize the environmental niche of P. parvum in a subtropical impoundment in the southern United States. Our analysis contributes to a growing body of evidence that indicates a primary role for environmental conditions, but not dispersal, in the lake-wide abundances and seasonal bloom patterns in this globally important microbe.

The rapid range expansion of the toxigenic marine haptophytic alga Prymnesium parvum in inland aquatic systems across the southern USA and beyond has prompted great interest in the ecology and evolutionary biology of this invasive bloom-forming species. Researchers have speculated that increased toxicity and heterotrophy in suboptimal environments allow blooms to develop in these new inland habitats that seem to represent extremes relative to P. parvum's perceived optimal niche. We used a laboratory-based study to elucidate the roles of salinity and nutrient availabilities in P. parvum growth and toxicity under environmental conditions representative of hypereutrophic reservoirs of the southwestern USA in which P. parvum blooms are now common. We found evidence that nutrient conditions favoring toxigenesis in P. parvum are suboptimal for growth and bloom formation, whereas conditions conducive to high growth rates are less favorable for toxigenesis. In contrast, both growth and toxicity scaled positively with salinity. Taken in the context of the documented biogeography of P. parvum and that toxigenesis in P. parvum probably evolved in support of heterotrophic nutrient acquisition, our results suggest that P. parvum's apparent range expansion into and across the USA is driven by increasing availabilities of salty and nutrient-rich systems, rather than by its toxigenic abilities.