Physiological responses of Ostreopsis ovata to changes in N and P availability and temperature increase

Ostreopsis ovata is a benthic dinoflagellate that produces palytoxin and ovatoxins. Blooms of O. ovata causing human health problems and mortality of benthic fauna have been reported from many tropical and temperate marine waters. In the present study we examined the combined effects of temperature and different nutrient conditions on the biochemical composition, growth, toxicity and carbohydrate production of an O. ovata strain originating from the Tyrrhenian Sea. O. ovata cultures with N:P ratios of 1.6, 16 and 160 (N deficient, NP sufficient and P deficient, respectively) were grown at 20 °C and 30 °C. Biomass accumulation, growth rates, cell volumes, biochemical composition, cell toxicity and carbohydrate production in each treatment were studied. Results indicated that under nutrient sufficiency O. ovata biomass accumulation increased significantly compared to N and P deficiency and also that N limitation severely affected growth. The highest growth rates were recorded at 30 °C. Cellular contents and the atomic ratios of C, N and P were higher in the cells grown at 20 °C than in those grown at 30 °C. O. ovata cell volumes increased at 20 °C. N deficiency significantly increased cell toxicity. Toxicity per cell was higher at 20 °C, but per carbon was highest at 30 °C. The highest carbohydrate production was found in conditions of N deficiency and at the lower temperature.Our study suggests that temperature increases due to global warming and nutrient enrichment of coastal waters stimulate the proliferation of O. ovata, particularly for the strains that have become adapted to warm temperate waters.

► We studied physiological response of O. ovata to three NP conditions at 20 and 30 °C. ► Growth rates and biomass were highest at 30 °C and in NP sufficiency, respectively. ► N deficiency decreased growth, but increased toxicity and carbohydrate production. ► Cell quotas and atomic ratios of C, N and P were higher in the cells grown at 20 °C. ► Global warming and nutrient enrichment might be behind the proliferation of O. ovata.