Direct uptake of HCO 3 − in the marine angiosperm Posidonia oceanica (L.) Delile driven by a plasma membrane H + economy

Seagrasses access HCO 3 − for photosynthesis by 2 mechanisms, apoplastic carbonic anhydrase‐mediated dehydration of HCO 3 − to CO 2 and direct HCO 3 − uptake. Here, we have studied plasma membrane energization and the mechanism for HCO 3 − import in Posidonia oceanica . Classical electrophysiology and ion‐selective microelectrodes were used to measure the membrane potential, cytosolic pH, and the cytosolic concentrations of Na + and Cl − upon the addition of HCO 3 − . The photosynthetic response to HCO 3 − and to inhibitors was also measured. Results indicate that the primary pump of P. oceanica plasma membrane is a fusicoccin‐sensitive H + ‐ATPase. Bicarbonate depolarizes the plasma membrane voltage and transiently acidifies the cytosol, indicating that HCO 3 − is transported into the cells by an H + ‐symport. Initial cytosolic acidification is followed by an alkalinization, suggesting an internal dehydration of HCO 3 − . The lack of cytosolic Na + and Cl − responses rules out the contribution of these ions to HCO 3 − transport. The energetics of n H + /HCO 3 − symport allows, for n  = 1, an estimate of cytosolic accumulation of 0.22 mM HCO 3 − . Because this transporter could permit accumulation of HCO 3 − up to 100 times above the equilibrium concentration, it would be a significant component of a carbon‐concentrating mechanism in this species.