Optimal growth conditions of the haptophyte Chrysochromulina leadbeateri causing massive fish mortality in Northern Norway

•Chrysochromulina leadbeateri formed fish-killing blooms in Northern Norway in 1991 and 2019.•Our study showed maximal growth rate of C. leadbeateri at salinity 28–30 and temperature 13–15 °C.•The upper temperature limit of C. leadbeateri was 17–19 °C.•Several ribotypes of C. leadbeateri were identified and the blooms were caused by a cold-water type. The haptophyte Chrysochromulina leadbeateri formed the most devastating fish-killing algal bloom ever recorded in Norway, in May and June 2019. The bloom resulted in the death of 14,500 tons of farmed salmon in Nordland and Troms Counties and large economic losses to the aquaculture industry in the region. Fish mortalities due to blooms of this species have occurred before in this region in 1991. Environmental conditions promoting bloom formation and growth of C. leadbeateri are, however, still poorly understood. Here we investigated growth as a function of temperature, salinity and irradiance in combinations using a high throughput experimental set-up. Three strains of C. leadbeateri isolated from the 2019 event and an earlier bloom in 1991 were examined. The highest maximal specific growth rate was found at salinities 28–30 and temperatures between 13 and 15 °C, with growth rate generally increasing with irradiance. The upper temperature tolerance for growth for all strains was at 17–19 °C. Further, analyses of the geographical distribution of C. leadbeateri in previous DNA-based studies compiled in the metaPR2 database revealed several ribotypes, and that a cold-water ribotype of C. leadbeateri caused both the 1991 and 2019 blooms.