Direct uptake of HCO3- in the marine angiosperm Posidonia oceanica (L.) Delile driven by a plasma membrane H+ economy

Seagrasses access HCO3- for photosynthesis by 2 mechanisms, apoplastic carbonic anhydrase-mediated dehydration of HCO3- to CO2 and direct HCO3- uptake. Here, we have studied plasma membrane energization and the mechanism for HCO3- 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 HCO3-. The photosynthetic response to HCO3- 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 HCO3- is transported into the cells by an H+-symport. Initial cytosolic acidification is followed by an alkalinization, suggesting an internal dehydration of HCO3-. The lack of cytosolic Na+ and Cl- responses rules out the contribution of these ions to HCO3- transport. The energetics of nH(+)/HCO3- symport allows, for n=1, an estimate of cytosolic accumulation of 0.22 mM HCO3-. Because this transporter could permit accumulation of HCO3- up to 100 times above the equilibrium concentration, it would be a significant component of a carbon-concentrating mechanism in this species.