Photosynthesis and photorespiration in freshwater green algae

Abstract

Photosynthesis in freshwater green algae has been studied for years, but interest has increased in the last decade, largely owing to the demonstration that unicellular green algae avoid photorespiration through the use of a CO 2-concentrating system. Biochemically, photosynthesis and photorespiration in the green algae are similar to C 3 higher plants. There is a difference, however, in the mechanism of inorganic carbon uptake prior to the actual fixation of CO 2. The unicellular green algae can accumulate inorganic carbon through active transport, although it is not clear whether CO 2 or HCO 3 − is transported, nor is it clear across which membrane the transport occurs. Some unicellular green algae can use bicarbonate as a source of inorganic carbon, but it is thought that the bicarbonate is only used indirectly, after conversion to CO 2. The active transport of inorganic carbon results in a substantial increase in the intracellular CO 2 concentration, which inhibits ribulose bisphosphate (RuBP) oxygenase activity, suppressing photorespiration. This inorganic carbon transport is also inducible, apparently being expressed only when inorganic carbon concentrations are low or inorganic carbon is limiting for growth. The filamentous green algae, represented by charophytes are apparently able to use bicarbonate as a source of inorganic carbon, although there is some controversy over whether the bicarbonate is directly or indirectly transported into the cells. Unlike the situation in microalgae, there is no evidence as yet that the charophytes concentrate OC 2 intracellularly. The ecological significance of the inorganic carbon-acquisition systems of the green algae is not completely clear. Although it has been suggested that inorganic carbon is rarely limiting in natural systems, the argument can be made that it might well be limiting in the absence of these specialized inorganic carbon uptake systems, even though inorganic carbon might not be limiting in their presence.