Chlorophyll f: Re-evaluating Photochemical Activity in Far-Red Light

Researchers claiming to have identified the structural basis for far-red photosynthesis argue that Chlorophyll f plays a direct photochemical role, contrary to prior assumptions. This laboratory study, focused exclusively on the cyanobacterium Chroococcidiopsis thermalis PCC 7203, relies on cryo-electron microscopy (cryo-EM) to map pigment locations.

Previous structural assessments failed to locate this specific pigment among photochemically active sites. Consequently, the prevailing view held that it served a passive or accessory function rather than driving the reaction itself. The current investigation challenges that limitation. The team produced two cryo-EM structures of the far-red photosystem I. These models facilitated the assignment of eight specific molecules, notably placing one at the A_-1B site—a position associated with redox activity.

Chlorophyll f and Far-Red Efficiency

To validate this structural assignment, the authors employed spectral simulations. These calculations indicate that the experimental absorption difference spectra can only be reproduced if a Chlorophyll f molecule is present at the active A_-1B site. This suggests that the pigment is not merely structural but functional.

The identification of conserved residues specific to far-red light absorption provides a potential roadmap for understanding how organisms manage low-energy photons. However, these findings rely heavily on the accuracy of the simulation models matching the physical structures. While the data implies a distinct paradigm for oxygenic photosynthesis in this specific organism, the broader applicability remains to be tested.