Light harvesting pigment use by cyanobacteria for photosynthesis
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Cyanobacteria are a group of bacteria, which are able to perfom oxygenic photosynthesis (rely on oxygenic photosynthesis as a main energy source) to convert sun light into chemical energy. In addition to the photosystems, cyanobacteria employ phycobilisomes to enhance their light-harvesting abilities. Phycobilisomes consist of phycobiliproteins (mainly phycocyanin and phycoerythrin) with covalently attached open-chain tetrapyrroles (phycobilins) as light-harvesting pigments. These phycobilins are derived from heme. The first step of bilin synthesis is the ring opening reaction of heme into biliverdin IXα mediated by heme oxygenases. A set of different ferredoxin-dependent bilin reductases catalyse the reactions from biliverdin IXα to several phycobilins. These pigments are subsequently attached to conserved cysteine residues in the phycobiliproteins. In order to ensure the correct attachment of the phycobilins and the chromophore composition of the phycobiliproteins, the binding is mediated by phycobiliprotein-lyases. Recent studies showed that this machinery is not only present in cyanobacteria but also in phages which infect cyanobacteria. This chapter describes the biosynthesis and assembly of all components of functional phycobilisomes and their role in energy conversion as well as adaptations to changing environmental conditions.