Certain Chlamydomonas reinhardtiimutants deficient in photosystem I due to defects in psaAmRNA 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 eitherpsaA or psaC; these activities could be restored to the F8-psaAΔ mutant by reintroduction ofpsaA. 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.