Photosystem I is indispensable for photoautotrophic growth, CO₂ fixation, and H₂ photoproduction in Chlamydomonas reinhardtii

Certain Chlamydomonas reinhardtii mutants deficient in photosystem I due to defects in psaA mRNA maturation have been reported to be capable of CO₂ fixation, H₂ photoevolution, and photoautotrophic growth (Greenbaum, E., Lee, J. W., Tevault, C. V., Blankinship, S. L., and Mets, L. J. (1995) Nature 376, 438-441 and Lee, J. W., Tevault, C. V., Owens, T. G.; Greenbaum, E. (1996) Science 273, 364-367). We have generated deletions of photosystem I core subunits in both wild type and these mutant strains and have analyzed their abilities to grow photoautotrophically, to fix CO₂, and to photoevolve O₂ or H₂ (using mass spectrometry) as well as their photosystem I content (using immunological and spectroscopic analyses). We find no instance of a strain that can perform photosynthesis in the absence of photosystem I. The F8 strain harbored a small amount of photosystem I, and it could fix CO₂ and grow slowly, but it lost these abilities after deletion of either psaA or psaC; these activities could be restored to the F8-psaAΔ mutant by reintroduction of psaA. We observed limited O₂ photoevolution in mutants lacking photosystem I; use of ¹⁸O₂ indicated that this O₂ evolution is coupled to O₂ uptake (i.e. respiration) rather than CO₂ fixation or H₂ evolution. We conclude that the reported instances of CO₂ fixation, H₂ photoevolution, and photoautotrophic growth of photosystem I-deficient mutants result from the presence of unrecognized photosystem I.