Adaptation of Spirulina platensis to salinity-stress

Spirulina platensis cells, growing photoautotrophically in optimal media under 100 or 200 μmol m^-2 s^-1 photon flux densities (PFD), were exposed to different concentrations of sodium chloride, up to 0.75 M. After an initial acclimation phase, in which growth rate, photosynthetic activity and endogenous respiration were inhibited, a new steady state was established and a recovery in the photosynthetic activity was observed. Furthermore an increase in the respiration rate took place, exceeding the initial rate of the non-stressed cells. Photosynthetic light-response curves (P-I) of stressed cells showed that the light compensation points were increased and light saturation values were decreased under the different salinity-stress conditions. Photoinhibition of photosynthesis was significantly enhanced under salinity-stress. Photosystem II activities of cells substantially decreased after a salt-shock. The results show that, cells grown in higher PFD are less tolerant to salinity-stress than those grown in lower PFD.