Effect of light quality on carotenogenic and non-carotenogenic species of the genus Dunaliella under nitrogen deficiency

Abstract

Changes in growth, pigment content, and photosynthetic response under different light qualities were monitored for two species of the green alga Dunaliella: the carotenogenic D. salina Teodoresco and the non-carotenogenic D. tertiolecta Butcher. Log-phase cultures were exposed to 100% red, 100% blue, and a mix of 50% red and 50% blue at 300 μmol photon m−2 s−1. Cell density, biomass concentration, pigment content and photosynthetic efficiency were recorded during stationary phase, once nitrogen had been depleted from the culture medium. For both species biomass concentration was maximal in red-light, while the lowest biomass concentration was found with blue-light. No significant effect of light quality on cell density was observed for any of the species. The photosynthetic response was completely different between D. salina and D. tertiolecta. In D. tertiolecta nitrate starvation was accompanied by a reduction in photosynthetic efficiency and an increase in non-photochemical quenching processes, regardless of the light condition. This species activated a clear mechanism of energy dissipation, probably related to a xanthophyll cycle. On the other hand, D. salina massively accumulated secondary carotenoids as a response to the same disruptive conditions. Carotenoids protected D. salina cells mainly against blue light, where Fv/Fm remained high. In D. salina, pigment content changed with respect to light quality. Blue light enhanced total carotenoid content, but a mix of red and blue increased total carotenoid concentration in the culture, by stimulating a more balanced increase in dry weight and carotenoid accumulation. The application of specific light spectrum for the production of microalgal biomass with different carotenoid content that has a potentially different market value, is confirmed an important tool for the optimization of artificial-light microalgae cultivation systems.