B-vitamin requirements and phycosphere interactions for the HAB-forming dinoflagellate, Pyrodinium bahamense var. bahamense

Pyrodinium bahamense is a saxitoxin-producing dinoflagellate found in tropical and subtropical estuarine waters globally, with the var. bahamense variety forming recurrent harmful algal blooms (HABs) in Florida estuaries. The intensity and duration of bloom peaks in Florida estuaries vary interannually, but life cycle transitions, estuarine water residence time, water temperature, salinity, and nutrient concentrations appear to contribute to bloom formation. However, the impact of B-vitamin availability on P. bahamense bloom formation has not been investigated, despite the knowledge that many HAB dinoflagellates require B-vitamins for growth and bloom development. In this study, we determined that of the three B-vitamins most commonly required by marine phytoplankton--biotin (B7), thiamine (B1), and cobalamin (B12)--P. bahamense strictly requires an exogenous B12 source to sustain growth. As bacterial interactions can fulfill phytoplankton B12 requirements, we tested whether bacterial communities recruited from two Florida estuaries could rescue P. bahamense from B12 limitation. We demonstrate that bacteria from the Indian River Lagoon (IRL) estuary collected during a P. bahamense bloom restored P. bahamense growth in media missing B12, while bacteria present in Old Tampa Bay (OTB) water collected when P. bahamense was not present did not restore P. bahamense growth in media missing B12. Putative bacterial B12 producers exclusively recruited from IRL included Enterobacteraceae genera that likely have anthropogenic sources. Our findings suggest B12 is essential for fueling P. bahamense blooms, and that specific bacterial communities may contribute to bloom development through B12 production.