Energy transfer from chlorophyll f to the trapping center in naturally occurring and engineered Photosystem I complexes

Certain cyanobacteria can thrive in environments enriched in far-red light (700–800 nm) due to an acclimation process known as far-red light photoacclimation (FaRLiP). During FaRLiP, about 8% of the Chl a molecules in the photosystems are replaced by Chl f and a very small amount of Chl d. We investigated the spectroscopic properties of Photosystem I (PSI) complexes isolated from wild-type (WT) Synechococcus sp. PCC 7335 and a chlF mutant strain (lacking Chl f synthase) grown in white and far-red light (WL–PSI and FRL–PSI, respectively). WT–FRL–PSI complexes contain Chl f and Chl a but not Chl d. The light-minus dark difference spectrum of the trapping center at high spectral resolution indicates that the special pair in WT–FRL–PSI consists of Chl a molecules with maximum bleaching at 703–704 nm. The action spectrum for photobleaching of the special pair showed that Chl f molecules absorbing at wavelengths up to 800 nm efficiently transfer energy to the trapping center in FRL–PSI complexes to produce a charge-separated state. This is ~ 50 nm further into the near IR than WL–PSI; Chl f has a quantum yield equivalent to that of Chl a in the antenna, i.e., ~ 1.0. PSI complexes from Synechococcus 7002 carrying 3.8 Chl f molecules could promote photobleaching of the special pair by energy transfer at wavelengths longer than WT PSI complexes. Results from these latter studies are directly relevant to the issue of whether introduction of Chl f synthase into plants could expand the wavelength range available for oxygenic photosynthesis in crop plants.