Co2 and phosphate availability control the toxicity of the harmful bloom dinoflagellate Karlodinium veneficum.

We demonstrated that the toxicity of the harmful bloom dinoflagellate Karlodinium veneficum is regulated by both CO 2 concentrations and phosphate availability. Semi-continuous cultures were grown in a factorial experiment under all combinations of 3 CO 2 levels (230, 430 and 745 ppm) and 2 phosphate conditions (0.5 and 20 uM). After steady-state acclimation was achieved, karlotoxin cellular quotas and growth rates were determined in all 6 treatments. This strain produced both types of karlotoxin, KmTx-1 and KmTx-2. Chlorophyll a-normalized production of the 2 types of karlotoxins was much higher in P-limited cultures compared with P-replete ones under the same CO 2 conditions. Increasing CO 2 strongly stimulated production of KmTx-1 and decreased production of KmTx-2 in both treatments, but especially in P-limited cultures. Because the KmTx-1 toxin is an order of magnitude more potent than KmTx-2, total cellular toxicity was increased dramatically at high pCO 2 , particularly in P-limited cultures. Specific growth rates were accelerated by enriched CO 2 in P-replete cultures, but not in P-limited treatments. Growth rates or toxicity of K. veneficum could increase substantially in the future with high CO 2 levels in the ocean, depending on P availability, and so interactions between rising CO 2 and eutrophication could cause major shifts in present day patterns of harmful algal toxin production. These results suggest that over the coming decades, rising CO 2 could substantially increase karlotoxin damage to food webs in the often P-limited estuaries where Karlodinium blooms occur.