Active uptake of CO2 during photosynthesis in the green alga Eremosphaera viridis is mediated by a CO2-ATPase |
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Authors: | Caterina Rotatore Roger R Lew Brian Colman |
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Institution: | (1) Department of Biology, York University, M3J 1P3 North York, Ontario, Canada |
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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 M dissolved inorganic carbon (DIC) rapidly depleted almost all the free CO2 from the medium. Rapid equilibrium between HCO
3
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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.Abbreviations BTP
1,3-bistris(hydroximethyl)-methylamino]-propane
- CA
carbonic anhydrase
- Chl
chlorophyll
- DIC
dissolved inorganic carbon
- 14C]DMO
5,5-dimethyl-2-14C]-oxaz-didine-2,4-dione
- WA
Wilbur-Anderson units
This work was supported by grants to B.C. and R.R.L. from the Natural Sciences and Engineering Research Council of Canada. We thank the Department of Biology, Queen's University, Kingston, Ontario for the use of the mass-spectrometer facility. We are indebted to A.G. Miller for his expert advice on operating the mass spectrometer and to Ms. Shahebina Samji for running the Bradford assays. |
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Keywords: | ATPase Bicarbonate transport Carbon dioxide Carbonic anhydrase Eremosphaera Inor ganic carbon (dissolved) |
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