LIGHT AND OXYGEN STRESS IN SPIRULINA-PLATENSIS (CYANOBACTERIA) GROWN OUTDOORS IN TUBULAR REACTORS

As the effects of light and oxygen stress in algae on mass culture has not been intensively studied, we investigated them in Spirulina plate nsis under outdoor conditions in controlled tubular reactors where the respective roles of each stress can be distinguished. It was observed that exposure of this cyanobacterium at two oxygen concentrations (ca 20 and 53 mg 1(-1)) caused very little change in the ratio between va riable and maximum fluorescence (F-v/F-m) during the day even when the culture was grown at higher oxygen concentration (about 7% lower in t he evening than in the morning). Vice-versa, when the photochemical ef ficiency of PSII (photon yield, Phi(c)) was measured, a reduction of a bout 20% was observed. Neither the F-v/F-m ratio nor the Phi(c) of the culture grown at the lower oxygen concentration changed significantly during the day. The daily productivity of the culture exposed to the higher oxygen concentration was reduced by about 20%. Laboratory cultu res bubbled with air or pure oxygen under continuous light showed a si milar response; i.e., a smaller decrease in F-v/F-m (17%) than in the Phi(c) (56%) after 4 h. After 32 h of culture in pure oxygen, a total lysis of the cells occurred. Our results support the hypothesis that p hotoinhibition and photooxidation, two traditionally linked terms, alt hough often closely associated under similar environmental conditions, may comprise two types of stress with different sites of inhibition.