Multi-bacterial influences on diatom bloom and toxigenesis

The association of marine diatoms with multiple bacteria is ubiquitous in nature but the influences of these bacteria on the physiology and ecology of the diatom host are not understood. Diatom blooms that occur in coastal environment are often caused by several environmental factors but it is currently unknown if bacteria also plays a role in this ecological phenomenon. Here, we determine the bacterial assemblages present before, during and after toxic and non-toxic Pseudo-nitzschia blooms to understand the patterns of bacterial succession in these events, as well as understand the roles of these bacteria on diatom bloom formation, bloom demise and toxigenesis. Bacterial community profiles were determined by 454 pyrosequencing and analyzed together with domoic acid levels and phytoplankton structure. We found that bacterial diversity is high during non-bloom events and low bacterial diversity is seen during bloom events. Bacterial diversity is also lower in bloom events with high domoic acid concentration, suggesting that domoic acid can further structure bacterial community. Bacterial groups belonging to Gamma-Proteobacteria are abundant during non-toxic blooms while Firmicutes bacteria dominate during toxic blooms. We experimentally manipulated the algal-bacteria association in culture to determine the single or combined effects of these dominant bacteria on Pseudo-nitzschia host fitness and domoic acid production. We found that multi-bacteria association can either have synergistic or dampening effects on diatom growth and toxin production, suggesting that multi-bacteria interactions play an important role on algal bloom and toxigenesis.