Synthesis of C8 ‐Vinyl Chlorophylls d and f Impairs Far‐Red Light Photoacclimation and Growth Under Far‐Red Light

The inducible biosynthesis of chlorophylls d and f enables a subset of specialised cyanobacteria to perform oxygenic photosynthesis under far‐red light—in the absence of visible wavelengths—via a process termed far‐red light photoacclimation. These pigments, like the more common chlorophylls a and b , typically carry an ethyl substituent at the C8 position of the macrocycle, formed by reduction of a vinyl group by an 8‐vinyl reductase enzyme. Here, we disrupted the gene encoding BciB, an 8‐vinyl reductase found in the majority of cyanobacteria, in Chroococcidiopsis thermalis PCC 7203, a model organism for studying far‐red light photoacclimation. Disruption of bciB results in the synthesis of 8‐vinyl chlorophyll a when cells are grown in white light; upon switching to far‐red light, 8‐vinyl forms of chlorophylls d and f , which have not been detected in nature, are synthesised in this strain. The bciB mutant exhibits sensitivity to high irradiance under both light regimes. Pigment analysis and whole‐cell absorption and fluorescence spectroscopy reveal decreased synthesis of far‐red absorbing chlorophylls, reduced photosystem assembly and an impaired acclimation to far‐red light, and transmission electron microscopy demonstrates altered thylakoid membrane morphology in the mutant when compared to the wild type. These results demonstrate the importance of 8‐vinyl group reduction for acclimation to far‐red light.