Cochlodinium polykrikoides is a globally distributed, ichthyotoxic, bloom-forming dinoflagellate. Blooms
of C. polykrikoides manifest themselves as large (many km2) and distinct patches with cell densities
exceeding 103 ml1 while water adjacent to these patches can have low cell densities (100 pmol1 L1 d1) in bloom samples, resulting in turn-over times of hours
during blooms. This high vitamin demand likely led to the vitamin B12 limitation of C. polykrikoides
observed during nutrient amendment experiments conducted with bloom water. Collectively, this study
revealed that C. polykrikoides blooms fundamentally change microbial communities and accelerate the
cycling of carbon, some nutrients, and vitamin B12.