Active uptake of CO₂ during photosynthesis in the green alga Eremosphaera viridis is mediated by a CO₂-ATPase

Abstract

Mass spectrometry was used to investigate the uptake of CO₂ in Eremosphaera viridis DeBary. Upon illumination, cells preincubated at pH 7.5 with 100 μM dissolved inorganic carbon (DIC) rapidly depleted almost all the free CO₂ from the medium. Rapid equilibrium between HCO ^-₃ and CO₂ occurred upon addition of bovine carbonic anhydrase (CA) to the medium, showing that CO₂ depletion resulted from a selective uptake of CO₂ rather than an uptake of all inorganic carbon species. Glycolaldehyde (10 mM) completely inhibited CO₂ fixation but had little effect on CO₂ transport. Transfer of glycolaldehyde-treated cells to the dark caused a rapid efflux of CO₂ from the unfixed intracellular DIC pool which was found to be at least threeto sixfold higher in concentration than that of the external medium. These results indicate that E. viridis actively transports CO₂ against a concentration gradient. No external CA was detected in these cells either by potentiometric or mass-spectrometric assay. In the absence of external CA, the rate of photosynthetic O₂ evolution in the pH range 7.5 to 8.0 did not exceed the calculated rate of CO₂ supply, indicating a limited capacity for HCO₂ uptake in these cells. Electrophysiological measurements indicate that CO₂ uptake is electrically silent and thus is not a consequence of H⁺-CO₂ symport activity. Microsomal membranes isolated from Eremosphaera showed ATPase activity which was enhanced by CO₂. These results indicate that active CO₂ uptake is mediated by an ATPase.