The Reddest Fluorescence of Photosystem I from Halomicronema hongdechloris Comes from Chlorophyll-f Dimer

Chlorophyll-f (Chl-f) is the most red-shifted Chl ever found in nature. It was first found in the cyanobacterium Halomicronema (H.) hongdechloris. This cyanobacterium was found inside rocks called stromatolites. It produces Chl-f to utilize far-red light when grown under far-red light (>700 nm), which is the only light available in a stromatolite. A structural analysis on photosystem I of the H. hongdechloris grown under far-red light showed that among ca. 100 antenna Chls-a, seven are replaced with Chl-f. Three fluorescence peaks at 746, 753, and 810 nm observed at 77 K are attributed to Chl-f. However, it has been unknown which Chl-f is responsible for each red-shifted fluorescence band. Furthermore, the mechanism and pathway of the extreme uphill excitation energy transfer from Chl-f to RC, which was shown to be still at 700 nm, have been a mystery. Here, we employed time-resolved fluorescence spectroscopy, single-PSI fluorescence polarization measurements, and numerical calculations based on the structure of the PSI to assign each far-red emission peak to the Chl-f in the known structure of PSI. Importantly, the reddest emission band at 810 nm was assigned to a dimeric Chl-f composed of f826A and f827A (nomenclature according to Kato Nat. Commun. 2020, 11, 238) located near the RC.