Chlorophyll-a photochemistry using red light (680-700 nm) is near-universal and is sometimes considered to define the energy limit for oxygenic photosynthesis, the so-called red limit. We recently reported [1] the isolation and biophysical study of photosystems from a cyanobacterium grown in far-red light (750 nm). The few long-wavelength chlorophyll-f molecules and the single chlorophyll-d present are well resolved from each other and from the majority pigment, chlorophyll-a. Charge separation in Photosystem I and II uses chlorophyll-f at ~745 nm and chlorophyll-f (or -d) at 727 nm, respectively. Each photosystem also has a few even longer-wavelength chlorophylls-f that collect light and pass excitation energy up-hill to the photochemically active pigments. Thus by using a only few far-red pigments (~8 in PS1, ~5 in PS2) in specific locations, these photosystems can do oxygenic photosynthesis but nominally using ~ 110 meV less energy for the primary charge separation. These studies provide several new insights including: i) the nature of charge separation and excitation transfer in the standard chlorophyll-a-containing photosystems, ii) a new slant on photosystem evolution; and iii) the energy limits on oxygenic photosynthesis [1].

[1] Nürnberg D.J., Morton J., Santabarbara S., Telfer A., Joliot P., Antonaru L.A., Ruban A.V., Cardona T., Krausz E., Boussac A., Fantuzzi A, Rutherford A.W. (2018) Science 360, 1210- 1213 Photochemistry beyond the red limit in chlorophyll f–containing photosystems