Cyanobacterial carotenoids: their roles in maintaining optimal photosynthetic production among aquatic bloom forming genera.

Photoprotective and photosynthetic roles of carotenoid pigments (xanthophylls and β-carotene) were examined in the major bloom forming blue-green algal (cyanobacterial) genera, Anabaena, Aphanizomenon and Microcystis. Since these genera often reside as scums in surface waters, attention was given to the ability of carotenoids to counter potential photooxidation due to maximum near U.V. and visible radiation as well as O2 supersaturation, characterizing surface waters supporting blooms. In U.V.-transparent quartz incubation flasks it was shown that inhibition of carotenoid synthesis by diphenylamine led to rapid photooxidation among the above genera. When carotenoid synthesis was allowed to proceed, a high degree of resistance to photooxidation resulted. Prolonged exposure to near U.V. irradiation led to enhanced carotenoid synthesis relative to chlorophyll a, which extended viability. Carotenoid enhancement also increased chlorophyll a-specific photosynthetic O2 production. It is concluded that enhanced carotenoid synthesis observed during blooms serves at least two ecological functions, i) providing photoprotection and ii) increasing photosynthetic performance of surface cyanobacterial populations.