Effect of vitamins B 1 and B 12 on bloom dynamics of the harmful brown tide alga, Aureococcus anophagefferens (Pelagophyceae)

Most harmful algae require B vitamins. We investigated vitamin use by the B 1 and B 12 auxotroph, Aureococcus anophagefferens , a harmful alga that dominates plankton communities during dense “brown tides” in North America, Africa, and Asia. B 12 ‐depleted cultures of A. anophagefferens (clone CCMP1984) adapted to lower ambient B 12 concentrations by reducing half‐saturation constants (K s ) of B 12 uptake and increasing maximum uptake rates (V max ) compared to vitamin‐replete cultures. In contrast, V max of vitamin B 1 was higher in replete compared to the depleted cultures, whereas the K s values were similar for both. K s values for B 12 (5.0–21 pmol L −1 ) were similar to or higher than concentrations measured during brown tides, suggesting that B 12 may restrict the growth of this alga in the field. Over the course of a dense brown tide (> 10 6 cells mL −1 ) in Quantuck Bay, New York, vitamin B 1 and B 12 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 B 1 and B 12 and 14 C‐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 B 1 uptake but shared vitamin B 12 uptake with smaller picoplankton (< 1 µm). Vitamin uptake rates during the brown tide were capable of turning over standing stocks of vitamin B 12 in 15 h, whereas B 1 depletion was slower with maximal turnover times of 2.8 d. As the brown tide intensified and vitamin B 12 levels declined, the experimental enrichment of brown tide water with vitamin B 12 significantly enhanced the growth rates of A. anophagefferens. Collectively, this study demonstrates that vitamin B 12 can influence the intensity of harmful algal blooms caused by A. anophagefferens.