Poster: Photosynthesis

Abs # 365: The Chlamydomonas reinhardtii cia3 mutant lacking a thylakoid lumen-localized carbonic anhydrase is limited by CO₂ utilization by Rubisco and not PSII function in vivo

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. To resolve the role of this lumenal CA, we re-examined the physiology of the cia3 mutant. We confirmed and extended previous gas exchange analyses by using membrane-inlet mass spectrometry to monitor ¹⁶O₂, ¹⁸O₂, and CO₂ fluxes in vivo. The results demonstrate that CO₂ utilization of photosynthetic electrons and not PSII electron transport is limited at low inorganic carbon (Ci). We also measured metabolite pools sizes and showed that RuBP regeneration is not limited in the mutant at low CO₂. 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 HCO^3- to CO₂ and H₂O.