Growth and toxicity responses of Mediterranean Ostreopsis cf. ovata to seasonal irradiance and temperature conditions

The genus Ostreopsis includes several species capable of producing various palytoxin-like compounds which have harmful effects on humans and marine fauna. Species in this genus are regular members of the epiphytic community in tropical seas but their geographic range have shown an apparent expansion towards temperate regions in recent years. This is the case of the Mediterranean Sea, where Ostreopsis cf. ovata is responsible for intense blooms in summer and autumn. With the aim of describing the growth performances of O. cf. ovata, we carried out a multifactorial experiment in which strains isolated from 3 locations along the Italian coast were grown at different combinations of temperature, daylength and irradiance encompassing different seasonal conditions.Temperature constrained growth of O. cf. ovata within a window of 18–30 °C, while best performances were recorded at 22 and 26 °C. Growth was maximum at 12 h daylength, whereas it was limited by photon flux density at short daylength (9 h) and often showed photosaturation at the longest daylength (15 h). Cells with anomalous shape were found during the exponential phase at the lowest and highest temperatures, while cells with a reduced size were produced at all conditions except 18 °C. Total toxin concentration varied between the different environmental conditions and between exponential and post-exponential growth phases without a clear pattern, whereas toxin composition was less variable. Our results suggest that O. cf. ovata is adapted to intermediate temperatures and daylength conditions such as those recorded in the natural environment at the beginning of summer and/or at the beginning of autumn, when this dinoflagellate builds up its biomass along the coast of the Mediterranean Sea. Photosaturation instead occurs at the highest temperature and irradiance conditions, thus supporting the observations of generally lower cell abundances in late July–August.

► We describe the temperature-irradiance growth response of Ostreopsis cf. ovate from the Mediterranean.
► Temperature constrains growth between 18 and 30 °C, with best performances at 22 and 26 °C.
► Growth was maximum at 12 h light; light-limitation occurred at 9 h daylength and photosaturation at 15 h light and under the highest irradiance.
► Our results help explaining the seasonal pattern of this species along the Italian coast.
► The total toxin concentration varies with experimental conditions but toxin composition is rather constant.