Variability in the photophysiology of the harmful alga Heterosigma akashiwo (Raphydophyceae).

The light-limited slope of a photosynthesis-irradiance curve and the light-saturated rate (Pbmax) usually show covariation if measured on diel time scale (Ek-independent variability). We measured PE curves on a culture of the red tide alga Heterosigma akashiwo during 24 hours cycles experiments, to verify the covariation between alfa and Pbmax and to better understand the underlying physiological mechanisms. When the culture is grown in a sinusoidal light mode, PE parameters show a three-phases pattern of variability: a phase of Ek-independence from midnight until after-dawn, followed by two phases of Ek-dependence (photoacclimation). Shifting the light from sinusoidal to continuous, PE parameters turn to an only two-phases pattern where alfa and Pbmax do not covary, meaning photoacclimation in progress. Both the two experiments show a general positive correlation between photoinibition (beta) and Pbmax: in sinusoidal light, this can be explained as an evidence of the xanthophylls cycle, while in continuous light it might be linked to alternative electron sinks. Finally, we measured PE parameters variations during an experiment of nitrogen starvation and recovery: Pbmax values measured both by evolved oxygen and PAM variable fluorescence recover quickly within a day from N-supply. All these findings may help to identify the environmental factors that promote raphidophycean growth and to explain the physiological mechanisms that support natural harmful blooms.