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. In this work, we have investigated the effect that the enrichment of HCO3− has on the cytosolic homeostasis of H+, Na+, and Cl− in the marine angiosperm Posidonia oceanica, to identify the inorganic carbon species that crosses the plasma membrane and the driving ion that fuels such transport. The enrichment of natural seawater with HCO3− evokes a transient acidification of cytosolic pH followed by an alkalinization that lasts as long as the HCO3− pulse lasts. This result, along with the sensitivity of the cytosolic alkalinization to ethoxyzolamide, suggests the operation of an H+/HCO3− symport and an internal dehydration of this ion. Na+ and Cl− do not have an effect on the transport. A kinetic and energetic characterization for the HCO3− transport and its effect on a putative carbon concentration mechanism is also discussed. To our knowledge, this is the first direct evidence for an active HCO3− transport in plants, and it can be a relevant part of the strategy of inorganic carbon use in seagrasses.