Cyanobacterial pigments -biosynthesis and absorption spectra. (a) Phycobiliprotein and Chlorophyll biosynthesis. The enzymes Fe-chelatase, Mg-chelatase and Heme oxygenase play important regulatory roles in chlorophyll and bilin synthesis. The enzymes PebS synthase and PcyA synthase catalyse key steps in phycoerythrobilin and phycocyanobilin synthesis, respectively, and are either NAD(P)H-or ferredoxin-dependent bilin reductases. During chlorophyll biosynthesis, Mg-chelatase catalyses the insertion of Mg 2+ into protoporphyrin IX at the branch point between bilin synthesis and chlorophyll biosynthesis [35]. (b) Carotenoid biosynthetic pathway via the Methyl-Erythritol 4-Phosphate (MEP) pathway [44]. Phytoene synthase and phytoene desaturase (red dotted boxes) are both important enzymes in carotenoid biosynthesis. The carotenes and xanthophyll pathways are highlighted by the orange and yellow boxes, respectively. (c) Absorption spectra of major cyanobacterial pigments of commercial interest -Chlorophyll (Chlorophyll a), Carotenoids (β-carotene, lutein, fucoxanthin, astaxanthin) and Phycobiliproteins (phycocyanin)

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... phycoerythrin (PE), phycocyanin (PC), allophycocyanin (APC) and their chromophores [42,43]. Their chromophores (phycocyanobilin and phycoerythrobilin) are synthesised from glutamic acid, which is converted to aminolevulinic acid (ALA), two molecules of which form porphobilinogen and ultimately protoporphyrin IX by the action of three enzymes (Fig. 2a). The enzyme Fe-chelatase catalyses the formation of protoheme from protoporphyrin IX. Subsequently, this protoheme is converted to biliverdin IX, from which phycocyanobilin and phycoerythrobilin are ...

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... functions: PE, PC and APC absorb radiation in regions of the visible spectrum in which Chl has a low absorptivity (Fig. 2, 470-620 nm). Photosynthetic organisms typically have antenna systems that are tuned to their environmental conditions to best capture the light energy that they require. For example at the illuminated surface of a water column (euphotic zone) PAR in the 400-700 nm range is abundant, while below this (disphotic zone) less red, yellow and green ...

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... algae including diatoms, brown algae and dinoflagellates [63]. Chl d has been reported in certain cyanobacteria, for example in the cyanobacterium Acaryochloris marina it makes up 99% of the chlorophyll [64]. Chl f was found in extracts from stromatolytes, layered sedimentary formations which are rich in cyanobacteria [65]. Chlorophyll synthesis (Fig. 2a) involves the reduction of protochlorophyllide. Two pathways exist for chlorophyll biosynthesis, one taking place in darkness (using the enzyme dark-operative protochlorophyllide oxidoreductase) and the other requiring continuous light (light-dependent protochlorophyllide ...

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... carotenoids are typically synthesised from isopentenyl pyrophosphate (IPP) via the methylerythritol-4-phosphate (MEP) pathway in cyanobacteria and in chloroplasts of microalgae and higher plants (Fig. 2a) and via the mevalonic acid (MVA) pathway in the cytosol of bacteria and fungi [77]. Two important enzymes which regulate the first committed steps towards carotene biosynthesis are phytoene synthase and phytoene desaturase. Silencing the genes encoding these enzymes is reported to completely eliminate carotenoid production ...

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... functions: Carotenoids are indispensable components of chlorophyll/ carotenoid binding photosystems (Fig. 2a) of photoautotrophs (e.g. cyanobacteria, eukaryotic algae and plants) but also have other roles including the protection of membranes from oxidation [79,84]. In photosynthesis carotenoids have three key roles: Structural stabilisation of the photosystems [85], regulation of light capture [86] and supporting energy dissipation and ...