Most harmful algal bloom species are vitamin B₁ and B ₁₂ auxotrophs

Eutrophication can play a central role in promoting harmful algal blooms (HABs), and therefore many HAB studies to date have focused on macronutrients (N, P, Si). Although a majority of algal species require exogenous B vitamins (i.e., auxotrophic for B vitamins), the possible importance of organic micronutrients such as B vitamins (B₁, B₇, B₁₂) in regulating HABs has rarely been considered. Prior investigations of vitamins and algae have examined a relatively small number of dinoflagellates (n = 26) and a paucity of HAB species (n = 4). In the present study, the vitamin B ₁, B₇, and B₁₂ requirements of 41 strains of 27 HAB species (19 dinoflagellates) were investigated. All but one species (two strains) of harmful algae surveyed required vitamin B₁₂, 20 of 27 species required B₁, and 10 of 27 species required B₇, all proportions higher than the previously reported for non-HAB species. Half-saturation (K_s) constants of several HAB species for B ₁ and B₁₂ were higher than those previously reported for other phytoplankton and similar to vitamin concentrations reported in estuaries. Cellular quotas for vitamins suggest that, in some cases, HAB demands for vitamins may exhaust standing stocks of vitamins in hours to days. The sum of these findings demonstrates the potentially significant ecological role of B-vitamins in regulating the dynamics of HABs.