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

Abstract

Most harmful algae require B vitamins. We investigated vitamin use by the B1 and B12 auxotroph, 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 (Ks) of B12 uptake and increasing maximum uptake rates (Vmax) compared to vitamin‐replete cultures. In contrast, Vmax of vitamin B1 was higher in replete compared to the depleted cultures, whereas the Ks values were similar for both. Ks values for B12 (5.0–21 pmol L−1) were similar to or higher than concentrations measured during brown tides, suggesting that B12 may restrict the growth of this alga in the field. Over the course of a dense brown tide (> 106 cells 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 B1 depletion 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 B12 significantly 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.