Interactive effects of temperature and salinity on the growth and cytotoxicity of the fish-killing microalgal species Heterosigma akashiwo and Pseudochattonella verruculosa

Fish-killing blooms of Heterosigma akashiwo and Pseudochattonella verruculosa have been devastating for the farmed salmon industry, but in Southern Chile the conditions that promote the growth and toxicity of these microalgae are poorly understood. This study examined the effects of different combinations of temperature (12, 15, 18 degrees C) and salinity (10, 20, 30 psu) on the growth of Chilean strains of these two species. The results showed that the optimal growth conditions for H. akashiwo and P. verruculosa differed, with a maximum rate of 0.99 day-1 obtained at 15 degrees C and a salinity of 20 psu for H. akashiwo, and a maximum rate of 1.06 day-1 obtained at 18 degrees C and a salinity of 30 psu for P. verruculosa. Cytotoxic assays (2 x 101 - 2 x 105 cell mL-1; cells, filtrates, and cell lysates) performed at salinities of 20 and 30 psu showed a 100% reduction in the viability of embryonic fish cells exposed to intact cells of H. akashiwo and a 39% reduction following exposure to culture filtrates of P. verruculosa. Differences in the fish-killing mechanisms (direct cell contact vs. extracellular substances) and physiological traits of H. akashiwo and P. verruculosa explain the recent occurrence of very large blooms under contrasting (cold-brackish vs. hot-salty) extreme climate conditions in Chile.

Automated summary

This study investigated the growth of Chilean strains of Heterosigma akashiwo and Pseudochattonella verruculosa under different temperature and salinity conditions, revealing optimal growth rates and mechanisms for these two species. The research also demonstrated the varying effects of these microalgae on fish cells and attributed the recent large blooms in Chile to differences in climate conditions and physiological traits of the two species.