Interactive effects of irradiance and temperature on growth and domoic acid production of the toxic diatom Pseudo-nitzschia australis (Bacillariophyceae)

The influence of temperature and light on the growth, physiology and domoic acid (DA) production of the pennate toxic diatom Pseudo-nitzschia australis isolated from the English Channel was studied in semicontinuous culture. The effects of eight irradiances (35–400 mmol photons m2 s-1) and a temperature gradient (3.5–25.5 8C) were investigated. The highest growth rates (0.47–0.83 d-1) were observed between 18.6 and 13.5 8C, with an optimum irradiance for growth at 100 mmol photons m2 s-1. A decrease in the optimum growth temperature was observed with an increase in irradiance. Cell chlorophyll a content decreased at both low and high extreme temperatures and high irradiances. Stable and high values of Fv/Fm ratios below 21.8C highlight the great acclimation capacity of P. australis, which may explain its wide biogeographic distribution. The strain studied produced DA without nutrient limitation and during exponential growth although at lower levels than in other studies (0.15– 2.0 pg DA cell-1). Results underline the importance of light–temperature interactions for growth and DA production. Toxin production increased with increasing temperature and light, and DA production increased exponentially with the growth rate. These results show that DA is not only produced under nutrient stress or by unhealthy cells in P. australis. This study underlines the need to take the ability of P. australis to produce DA during the exponential growth into consideration to understand the link between toxin production and bloom dynamics. These results will not only help explore the processes involved, but also help parameterize future models of growth and DA production especially for P. australis.