Oxygenic Photoautotrophic Growth Without Photosystem I

Abstract

Contrary to the prediction of the Z-scheme model of photosynthesis, experiments demonstrated that mutants of Chlamydomonas containing photosystem II (PSII) but lacking photosystem I (PSI) can grow photoautotrophically with O2 evolution, using atmospheric CO2 as the sole carbon source. Autotrophic photosynthesis by PSI-deficient mutants was stable both under anaerobic conditions and in air (21 percent O2) at an actinic intensity of 200 microeinsteins per square meter per second. This PSII photosynthesis, which was sufficient to support cell development and mobility, may also occur in wild-type green algae and higher plants. The mutants can survive under 2000 microeinsteins per square meter per second with air, although they have less resistance to photoinhibition.

Arabidopsis thaliana Plants Lacking the PSI-D Subunit of Photosystem I Suffer Severe Photoinhibition, Have Unstable Photosystem I Complexes, and Altered Redox Homeostasis in the Chloroplast Stroma

“…This mutant is still able to fix CO 2 (20% of wild-type) and grow photoautotrophically, although at a reduced rate (49,48). The mutant, like the PSI-D-less plants, shows increased sensitivity to photoinhibition under high light conditions (50).…”

Photosystem I Is Indispensable for Photoautotrophic Growth, CO2 Fixation, and H2 Photoproduction inChlamydomonas reinhardtii

“…Introduction of this gene into either B4 or B4-psaA⌬ confers photoautotrophic growth to the transformants. 2 Thus, photosynthesis in B4 also correlates with the presence of PS I.…”

“…The amount of CO 2 fixed by these mutant strains was roughly equivalent to that measured in wild-type (WT) cells. Although CO 2 fixation in the PS I-deficient strains initially appeared to be limited to anaerobic conditions (1), photoautotrophic growth was later reported in the presence of O 2 (2). Several control experiments were carried out by the authors to eliminate the possibility of trace amounts of PS I. Irradiation with far red light ( Ͼ 700 nm), which excites PS I but not PS II, produced a small amount of H 2 evolution in WT cells but not in the B4 mutant (1).…”

“…Several control experiments were carried out by the authors to eliminate the possibility of trace amounts of PS I. Irradiation with far red light ( Ͼ 700 nm), which excites PS I but not PS II, produced a small amount of H 2 evolution in WT cells but not in the B4 mutant (1). Photobleaching experiments also failed to detect P 700 in thylakoid membranes from these mutants (2).…”

Photosynthetic Hydrogen and Oxygen Production by Green Algae

“…It has recently been reported (Greenbaum et al, 1995;Lee et al, 1996) that mutants of the green alga Chlamydomonas reinhardtii that lack detectable levels of the Photosystem I (PS I) reaction center are capable of sustained simultaneous photoevolution of H, and 0,, CO, reduction and photoautrophic growth. The data indicated that under some circumstances PS 11 alone is sufficient to generate reductant capable of driving hydrogen evolution and CO, fixation, not of course that the 2-scheme is universally wrong.…”