The synergistic effect of Selenium (selenite, -SeO₃^2-) dose and irradiance intensity in Chlorella cultures

Microalgae are able to metabolize inorganic selenium (Se) to organic forms (e.g. Se-proteins); nevertheless at certain Se concentration culture growth is inhibited. The aim of this work was to confirm the hypothesis that the limit of Se tolerance in Chlorella cultures is related to photosynthetic performance, i.e. depends on light intensity. We studied the relation between the dose and irradiance to find the range of Se tolerance in laboratory and outdoor cultures. At low irradiance (250 µmol photons m^-2 s^-1), the daily dose of Se below 8.5 mg per g of biomass (<20 µM) partially stimulated the photosynthetic activity (relative electron transport rate) and growth of Chlorella cultures (biomass density of ~1.5 g DW L^-1) compared to the control (no Se added). It was accompanied by substantial Se incorporation to microalgae biomass (~0.5 mg Se g^-1 DW). When the Se daily dose and level of irradiance were doubled (16 mg Se g^-1 DW; 500 µmol photons m^-2 s^-1), the photosynthetic activity and growth were stimulated for several days and ample incorporation of Se to biomass (7.1 mg g^-1 DW) was observed. Yet, the same Se daily dose under increased irradiance (750 µmol photons m^-2 s^-1) caused the synergistic effect manifested by significant inhibition of photosynthesis, growth and lowered Se incorporation to biomass. In the present experiments Chl fluorescence techniques were used to monitor photosynthetic activity for determination of optimal Se doses in order to achieve efficient incorporation without substantial inhibition of microalgae growth when producing Se-enriched biomass.