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Membrane permeabilities to CO2and HCO3−constrain the function of CO2concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO2but effectively impermeable to HCO3−. Here, CO2and HCO3−membrane permeabilities were measured using an18O‐exchange technique on two species of haptophyte algae,
Emiliania huxleyi andCalcidiscus leptoporus , which showed that the plasma membranes of these species are also highly permeable to CO2(0.006–0.02 cm · s−1) but minimally permeable to HCO3−. Increased temperature and CO2generally increased CO2membrane permeabilities in both species, possibly due to changes in lipid composition or CO2channel proteins. Changes in CO2membrane permeabilities showed no association with the density of calcium carbonate coccoliths surrounding the cell, which could potentially impede passage of compounds. Haptophyte plasma‐membrane permeabilities to CO2were somewhat lower than those of diatoms but generally higher than membrane permeabilities of green algae. One caveat of these measurements is that the model used to interpret18O‐exchange data assumes that carbonic anhydrase, which catalyzes18O‐exchange, is homogeneously distributed in the cell. The implications of this assumption were tested using a two‐compartment model with an inhomogeneous distribution of carbonic anhydrase to simulate18O‐exchange data and then inferring plasma‐membrane CO2permeabilities from the simulated data. This analysis showed that the inferred plasma‐membrane CO2permeabilities are minimal estimates but should be quite accurate under most conditions.
