Langley, JulienMorton, JenniferPurchase, RobinTian, LShen, L.Han, G.Shen, Jian-RenKrausz, Elmars2021-12-092021-12-090300-3604http://hdl.handle.net/1885/255064We identified and characterised the deep red state (DRS), an optically-absorbing charge transfer state of PSII, which lies at lower energy than P680, in the red algae Cyanidioschyzon merolae by means of low temperature absorption and magnetic circular dichroism spectroscopies. The photoactive DRS has been previously studied in PSII of the higher plant Spinacia oleracea, and in the cyanobacterium Thermosynechococcus vulcanus. We found the DRS in PSII of C. merolae has similar spectral properties. Treatment of PSII with dithionite leads to reduction of cytochrome (cyt) b559 and the PsbV-based cyt c550 as well as the disassembly of the oxygen-evolving complex. Whereas the overall visible absorption spectrum of PSII was little affected, the DRS absorption in the reduced sample was no longer seen. This bleaching of the DRS is discussed in terms of a corresponding lack of a DRS feature in D1D2/cyt b559 reaction centre preparations of PSII.Australian Research Council through grants DP110104565 and DP 150103137application/pdfen-AU© 2018 The Author(s)https://creativecommons.org/licenses/by-nc-nd/4.0/optical spectraphotosynthesis201810.1007/s11099-017-0760-x2020-11-23Creative Commons CC BY-NC-ND license