Influence of Temperature and CO 2 On Plasma‐membrane Permeability to CO 2 and HCO 3 − in the Marine Haptophytes Emiliania huxleyi and Calcidiscus leptoporus (Prymnesiophyceae)

Membrane permeabilities to CO 2 and HCO 3 − constrain the function of CO 2 concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO 2 but effectively impermeable to HCO 3 − . Here, CO 2 and HCO 3 − membrane permeabilities were measured using an 18 O‐exchange technique on two species of haptophyte algae, Emiliania huxleyi and Calcidiscus leptoporus , which showed that the plasma membranes of these species are also highly permeable to CO 2 (0.006–0.02 cm · s −1 ) but minimally permeable to HCO 3 − . Increased temperature and CO 2 generally increased CO 2 membrane permeabilities in both species, possibly due to changes in lipid composition or CO 2 channel proteins. Changes in CO 2 membrane 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 CO 2 were 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 interpret 18 O‐exchange data assumes that carbonic anhydrase, which catalyzes 18 O‐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 simulate 18 O‐exchange data and then inferring plasma‐membrane CO 2 permeabilities from the simulated data. This analysis showed that the inferred plasma‐membrane CO 2 permeabilities are minimal estimates but should be quite accurate under most conditions.