Extensive remodeling of the photosynthetic apparatus alters energy transfer among photosynthetic complexes when cyanobacteria acclimate to far-red light

Abstract

Some cyanobacteria remodel their photosynthetic apparatus by a process known as Far-Red Light Photoacclimation (FaRLiP). Specific subunits of the phycobilisome (PBS), photosystem I (PSI), and photosystem II (PSII) complexes produced in visible light are replaced by paralogous subunits encoded within a conserved FaRLiP gene cluster when cells are grown in far-red light (FRL; λ = 700–800 nm). FRL-PSII complexes from the FaRLiP cyanobacterium, Synechococcus sp. PCC 7335, were purified and shown to contain Chl a, Chl d, Chl f, and pheophytin a, while FRL-PSI complexes contained only Chl a and Chl f. The spectroscopic properties of purified photosynthetic complexes from Synechococcus sp. PCC 7335 were determined individually, and energy transfer kinetics among PBS, PSII, and PSI were analyzed by time-resolved fluorescence (TRF) spectroscopy. Direct energy transfer from PSII to PSI was observed in cells (and thylakoids) grown in red light (RL), and possible routes of energy transfer in both RL- and FRL-grown cells were inferred. Three structural arrangements for RL-PSI were observed by atomic force microscopy of thylakoid membranes, but only arrays of trimeric FRL-PSI were observed in thylakoids from FRL-grown cells. Cells grown in FRL synthesized the FRL-specific complexes but also continued to synthesize some PBS and PSII complexes identical to those produced in RL. Although the light-harvesting efficiency of photosynthetic complexes produced in FRL might be lower in white light than the complexes produced in cells acclimated to white light, the FRL-complexes provide cells with the flexibility to utilize both visible and FRL to support oxygenic photosynthesis.