The natural design for harvesting far-red light: the antenna increases both absorption and quantum efficiency of Photosystem II

Cyanobacteria carry out photosynthetic light-energy conversion using phycobiliproteins for light harvesting and the chlorophyll-rich photosystems for photochemistry. While most cyanobacteria only absorb visible photons, some of them can acclimate to harvest far-red light (FRL, 700-800 nm) by integrating chlorophyll f and d in their photosystems and producing red-shifted allophycocyanin. Chlorophyll f insertion enables the photosystems to use FRL but slows down charge separation, reducing photosynthetic efficiency. Here we demonstrate with time-resolved fluorescence spectroscopy that charge separation in chlorophyll-f-containing Photosystem II becomes faster in the presence of red-shifted allophycocyanin antennas. This is different from all known photosynthetic systems, where additional light-harvesting complexes slow down charge separation. Based on the available structural information, we propose a model for the connectivity between the phycobiliproteins and Photosystem II that qualitatively accounts for our spectroscopic data. This unique design is probably important for these cyanobacteria to efficiently switch between visible and far-red light.