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, H-2 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 O-2 or H-2 (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-psaA Delta mutant by reintroduction of psaA, We observed limited O-2 photoevolution in mutants lacking photosystem I; use of O-18(2) indicated that this O-2 evolution is coupled to O-2 uptake (i.e. respiration) rather than CO2 fixation or H-2 evolution. We conclude that the reported instances of CO2 fixation, H-2 photoevolution, and photoautotrophic growth of photosystem I-deficient mutants result from the presence of unrecognized photosystem I.