Charasomes are not essential for photosynthetic utilization of exogenous HCO₃^- in Chara corallina

Cells of Chara corallina grown under high CO₂ culture conditions were able to utilize exogenous HCO₃^- to give appreciable rates of net photosynthesis. Since these rates of photosynthesis could be detected within 10 min of being transferred from high-CO₂ to normal HCO₃^- (pH 8.2) culture conditions, it would appear that the HCO₃^--accumulating system of Chara is not fully repressed under these high CO₂ culture conditions. The membrane potential of these cells also responded to light/dark treatments in a manner consistent with the operation of a HCO₃^- acquisition system. With prolonged exposure (2-6 days) to CPW/B, net photosynthesis continued to increase towards the expected control rate and, in parallel, the electrical responses elicited by light/dark treatments converged towards those obtained on control (CPW/B-grown)Chara cells. Charasomes were absent in CPW/CO₂-grown Chara, but redeveloped in mature cells once the culture was returned to CPW/B conditions; a minimum period of 7 days in CPW/B was required before charasomes were detected in tissue examined in the transmission electron microscope. As the above-detailed physiological and electrophysiological features were observed with both axial and whorl cells of Chara in which charasomes were completely absent, we conclude that this specialized organelle is not an essential component for photosynthetic utilization of exogenous HCO₃^- in this species.