Minisymposium 10: Photosynthesis: Light Adaptation

Abs # 22004: Does PSII activity or CO₂ availability limit photosynthesis when the thylakoid lumen-localized carbonic anhydrase is absent?

Presenter:	Hanson, David T., dthanson@unm.edu
Authors	Hanson, David T. (A)   Franklin, Linda A. (C)   Samuelsson, Goran  (D)   Badger, Murray R. (B)
Affiliations:	(A): University of New Mexico, Department of Biology (B): Australian National University, Research School of Biological Sciences (C): Smithsonian Environmental Research Center (D): University of Umeå, Department of Plant Physiology
Web Site:	http://www.unm.edu/~dthanson/

The Chlamydomonas reinhardtii cia3 mutant has a phenotype indicating it requires high-CO₂ levels for effective photosynthesis and growth. It was initially proposed that this mutant was defective in a carbonic anhydrase (CA) that was a key component of the photosynthetic CO₂ concentrating mechanism (CCM). However more recent identification of the genetic lesion as a defect in a lumenal CA associated with PSII has raised questions about the role of this CA in either the CCM or PSII function. Important questions have arisen from additional studies on purified thylakoid membranes that have shown mutant extracts require bicarbonate for maximum function of the water-oxidizing complex of PSII, and that mutant preparations were impaired in ATP synthesis compared with the wild-type. Other intriguing findings showed that cia3 cells have twice as many PSII complexes as the wild-type, but only enough Mn²⁺ to supply half of the PSII cores with complete Mn-clusters, and experiments on intact cells showed that the mutant is more sensitive to high light treatments. All of these results have renewed interest in the necessity of the lumenal CA in the CCM of C. reinhardtii. To resolve the role of this lumenal CA, we have re-examined the physiology of the cia3 mutant. We confirmed and extended previous gas exchange analyses by using membrane-inlet mass spectrometry to simultaneously monitor ¹⁶O₂, ¹⁸O₂, and CO₂ fluxes in vivo. This method allowed direct measurement of PSII activity in vivo (via ¹⁶O₂ evolution) and comparison with simultaneous measurements of CO₂ fluxes. The results demonstrate that CO₂ utilization of photosynthetic electrons (via Rubisco) and not PSII electron transport is limited at low Ci. We also measured metabolite pools sizes and have shown that RuBP regeneration is not limited in the mutant at low CO₂ which suggests that ATP is not limiting. Overall the results demonstrate that the mutant is impaired in the ability of Rubisco to fix CO₂ at low Ci and is not limited directly by any aspect of PSII function. We speculate that the thylakoid CA may play 2 roles: 1) it is primarily required for the proper functioning of the CCM at low Ci by providing an ample supply of CO₂ for Rubisco, 2) it aids the proper function of PSII at high light possibly by preventing an excessively low lumenal pH through rapid dehydration of HCO₃^- to CO₂ and H₂O.