Temperature controls the toxicity of the ichthyotoxic dinoflagellate Cochlodinium polykrikoides

Blooms of the harmful dinoflagellate Cochlodinium polykrikoides occur across temperate and tropical zones of the Northern Hemisphere and have lethal impacts on marine life, including fish, coral, and shellfish. We report a series of laboratory-and ecosystem-based experiments examining the effects of temperature on the growth and lethal effects of C. polykrikoides. Growth rates of this alga were positive from 15 to 30 degrees C and were maximal at 24 to 27 degrees C, with cultures cultivated at warmer temperatures (> 20 degrees C) yielding significantly higher percentages of cells in chains. In contrast to growth, lethality of clonal isolates and bloom populations of C. polykrikoides to both larval fish and larval shellfish significantly decreased as temperatures progressively increased from 16 to 28 degrees C. Clonal populations comprising primarily single cells were significantly more toxic than cell-equivalent densities of populations in chains at the same temperature, suggesting temperature-induced changes in cell concatenation influenced lethal effects. The enzyme catalase was capable of mitigating the lethality of C. polykrikoides to fish at higher, but not lower, temperatures, suggesting that higher rates of enzyme activity by the alga and/or target organisms at higher temperatures may mitigate toxicity associated with reactive oxygen species. Collectively, these findings suggest that (1) as blooms in temperate zones progress from summer to fall, their lethality may increase, (2) blooms occurring at tropical latitudes may be less toxic than those in temperate regions, and (3) as global temperatures increase, Cochlodinium spp. blooms may expand into new, higher-latitude regions, where they may be highly lethal.