Photosynthesis, photorespiration and ecophysiological interactions in marine macroalgae

Abstract

Although macroalgae play a significant role in the productivity of marine ecosystems, the marine environment presents constraints to the achievement of maximum photosynthesis and productivity. Many macroalgae have developed strategies to minimize these constraints, which include low saturating irradiance requirements and bicarbonate use for photosynthesis, morphological features to reduce desiccation and morphological and biochemical modifications to enhance photosynthetic carbon fixation. In many marine macroalgae, fixation occurs solely by ribulose bisphosphate carboxylase/oxygenase (RuBPCO) and the photosynthetic carbon reduction cycle, and in some, considerable photorespiration is evident. But, in others photorespiration is suppressed, and studies are only now beginning to elucidate the methods that accomplish this effect. A C 4-like system based on phosphoenolpyruvate carboxykinase appears to operate in the green alga Udotea flabellum (Ellis & Solander) Lamouroux, while in the red alga, Gracilaria corticata J. Agardh, phosphoenolpyruvate carboxylase is used. Among brown algae CAM-type systems or bicarbonate uptake may reduce photorespiration. Bicarbonate uptake in association with carbonic anhydrase activity may be responsible for low photorespiration in some red algae. Regardless of the mechanism, reduced photorespiration is proposed to be achieved by concentrating CO 2 at the RuBPCO fixation site. However, elevated internal CO 2 levels have yet to be demonstrated in any marine macroalga. Although photorespiration seems suppressed in many marine macroalgae, further studies are needed to comprehend fully the mechanisms involved.