Photosynthetic carbon assimilation and the suppression of photorespiration in the cyanobacteria

The physiology and biochemistry of photosynthetic carbon assimilation in cyanobacteria is discussed in relation to its apparent suppression of photorespiration in these organisms. Experimental evidence indicates that CO 2 fixation in cyanobacteria is mediated by the C 3 pathway and although C 4 acids are among the initial products of photosynthesis, these organisms do not have the enzymatic capacity for the initial products of photosynthesis, these organisms of not have for the biosynthesis of amino acidsm necessitated by the absence of a complete TCA cycle in these organisms. The ribulose-1,5-bisphosphate carboxylase isolated from cyanobacteria has a low CO 2 affinity and is markedly inhibited by O 2 but it may be protected from O 2 inhibition by sequestration in carboxylasomes. There is no evidence that glycolate, the photo-respiratory substrate, is produced in air-grown cyanobacteria and a glycolate oxidation pathway, similar to that C 3 plants, appears to be lacking in these organisms. Excretion of glycolate by cyanobacteria is an artifact of growth on high CO 2 concentrations, or of experimental manipulation, and is not a normal consequence of photosynthesis. Cyanobacterial photosynthesis is not inhibited by O 2 and the cells have a high apparent affinity for CO 2 and very low CO 2 compensation points. No photorespiratory CO 2 release has been detected in cyanobacterial cells maintained at CO 2 concentrations above the CO 2 compensation point. Cyanobacteria take up extracellular HCO 3- by a Na + and energy-dependent active transport mechanism which results in the intracellular accumulation of inorganic carbon 500–1000-fold the concentration outside the cells. There is also some evidence of a small component of active CO 2 uptake by cyanobacterial cells. The active intracellular accumulation of inorganic carbon may be sufficient to inhibit ribulose-1,5-bisphosphate oxygenase activity in cyanobacterial cells and therefore to suppress photorespiration. The evidence for bicarbonate uptake by cyanobacteria in nature is sparse. Laboratory studies suggest, however, that cyanobacteria growing under conditions of DIC depletion and high O 2 tension have the ability to take up bicarbonate and that photorespiration is suppressed under these conditions.