Photoinhibition of photosynthesis in Macrocystis pyrifera (Phaeophyceae), Chondrus crispus (Rhodophyceae) and Ulva lactuca (Chlorophyceae) in outdoorculture systems

The effect of solar radiation on photosynthesis and chlorophyll fluorescenc e associated to photosystem II (PS II) was determined in the Phaeophyta Mac rocystis pyrifera, the Rhodophyta Chondrus crispus and the Chlorophyta Ulva lactuca by oxygen evolution and pulse-amplitude-modulated (PAM) fluorescen ce. The algae were maintained in 1.2 m(3) outdoor tanks with constant aerat ion and at 8, 26 and 100% incident irradiance (E-o). All three species show ed a decrease in DeltaF/F-m' values during solar noon compared to values in the morning and afternoon, suggesting a photoinhibition of photosynthesis. In general, photoinhibition was negatively correlated to increasing daily irradiance in all three species. Photoinhibition in C. crispus occurred in tissue incubated at 8, 26 and 100% E-o, while in M. pyrifera and U. lactuca a decrease in DeltaF/F-m' values was only observed in tissue incubated at 100% E-o. This suggests that species that naturally grow at greater depths might be more susceptible to excessive light when cultured in shallow water s compared to species that naturally inhabit shallower depths. In M. pyrife ra, DeltaF/F-m' values were lower in the afternoon than those in the mornin g, suggesting slower repair mechanisms of the photosystem II compared to th e other species. The results suggest that photoinhibition could be reduced by reducing incident irradiance to culture systems or increasing of biomass to promote self-shading. Gross oxygenic photosynthesis increased linearly at low electron transport rates after which it saturated in all three speci es. This suggests that chlorophyll fluorescence could be used as an indicat or of the physiological status of macroalgae maintained in dense aquacultur e systems. (C) 2000 Elsevier Science S.A. All rights reserved.