Effect of vitamins B₁ and B₁₂ on bloom dynamics of the harmful brown tide alga, Aureococcus anophagefferens (Pelagophyceae)

Most harmful algae require B vitamins. We investigated vitamin use by the B₁ and B₁₂auxotroph, Aureococcus anophagefferens, a harmful alga that dominates plankton communities during dense "brown tides" in North America, Africa, and Asia. B₁₂-depleted cultures of A. anophagefferens (clone CCMP1984) adapted to lower ambient B₁₂ concentrations by reducing half-saturation constants (K_s) of B₁₂ uptake and increasing maximum uptake rates (V_max) compared to vitamin-replete cultures. In contrast, V_max of vitamin B₁ was higher in replete compared to the depleted cultures, whereas the K_s values were similar for both. K_s values for B₁₂ (5.0-21 pmol L^-1) were similar to or higher than concentrations measured during brown tides, suggesting that B₁₂may restrict the growth of this alga in the field. Over the course of a dense brown tide (> 10⁶cells mL^-1) in Quantuck Bay, New York, vitamin B₁ and B₁₂ concentrations declined from > 100 pmol L^-1 to < 8 pmol L^-1, suggesting there was rapid uptake by A. anophagefferens and its associated microbial community. Experiments performed using radioisotope-labeled vitamins B₁ and B₁₂ and ¹⁴C-bicarbonate indicated that plankton in the size range of A. anophagefferens (1-5 μm) were responsible for the majority of primary production and the majority of vitamin B₁ uptake but shared vitamin B₁₂ uptake with smaller picoplankton (< 1 μm). Vitamin uptake rates during the brown tide were capable of turning over standing stocks of vitamin B₁₂ in 15 h, whereas B₁depletion was slower with maximal turnover times of 2.8 d. As the brown tide intensified and vitamin B₁₂ levels declined, the experimental enrichment of brown tide water with vitamin B₁₂significantly enhanced the growth rates of A. anophagefferens. Collectively, this study demonstrates that vitamin B₁₂ can influence the intensity of harmful algal blooms caused by A. anophagefferens.