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 platensis 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 variable and maximum fluorescence (F_v/F_m) during the day even when the culture was grown at higher oxygen concentration (about 7% lower in the evening than in the morning). Vice-versa, when the photochemical efficiency of PSII (photon yield, Φ_c) was measured, a reduction of about 20% was observed. Neither the F_v/F_m ratio nor the Φ_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 cultures bubbled with air or pure oxygen under continuous light showed a similar response; i.e., a smaller decrease in F_v/F_m (17%) than in the Φ_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 photoinhibition and photooxidation, two traditionally linked terms, although often closely associated under similar environmental conditions, may comprise two types of stress with different sites of inhibition.