Phycobilins

Phycobilins are complex photoreceptor pigments – open-chain tetrapyrroles that are structurally related to mammalian bile pigments. Phytochromes are phycobilin-protein pigments involved in floral induction. There are two classes of phycobilins and they occur only in Cyanobacteria and Rhodophyta. The phycobilin component is similar to the porphyrins without a metallic atom. Water-soluble phycobilin pigments are found in the stroma of the chloroplast. In at least two groups of algae, phycobiliproteins are aggregated in a highly ordered protein complex called a phycobilisome (PBS), making these phycobilins unique among photosynthetic pigments.

Phycobilisomes are attached to the cytosol (stromal) face of the thylakoid. Extending into the cytosol, the phycobilisomes consist of a cluster of phycobilin pigments including phycocyanin (blue) and phycoerythrin (red) attached by their phycobiliproteins. These particles serve as light-energy antennae for photosynthesis. Phycobilisomes preferentially funnel light energy into photosystem II for the splitting of water and generation of oxygen. While many photosynthetic eubacteria possess photosystem I to oxidize reduced molecules such as H2S, only Cyanobacteria have photosystem II. The evolution of photosystem II apparently occured in Cyanobacteria.

The bluish pigment phycocyanin is found in Cyanobacteria, giving them their misleading common name of "blue-green algae". Different species of cyanobacteria possess differing ratios of phyocyanin and phycoerythrin. Cyanobacteria such as Hammatoidea, Heterohormogonium, Albrightia, Scytonematopsis, Thalopophila, Myxocarcina and Colteronema confer colors from red to purple on thermal springs and geyser pools. The ratio of phycocyanin and phycoerythrin can be environmentally altered. Cyanobacteria which are raised in green light typically develop more phycoerythrin and become red. The same Cyanobacteria grown in red light become bluish-green. This reciprocal color change has been named 'chromatic adaptation’.

Phycoerythrin is an accessory photoreceptor pigment found in the Rhodophyta ("red algae"). Phycoerythrin is associated with chlorophyll in the Rhodophyta, and enables them to be photosynthetically efficient in deep water where blue light predominates. The longer wavelength red portion of the spectrum that activate green chlorophyll pigments do not penetrate the deeper water of the photic zone, so green algae cannot survive at depth where red algae thrive.

Three major classes of photosynthetic pigments occur among the algae: chlorophylls, carotenoids (carotenes and xanthophylls) and phycobilins. The phycobilins and the carotenoid peridinin are water soluble. In the Cryptophyta, the phycobilin pigments are found in the spaces between the thylakoids, not in phycobilisomes as they are in the Cyanobacteria and Rhodophyta. Alpha-carotene, and the xanthophyll, diatoxanthan, combine with the proteinaceous phycobilin pigments phycoerythrin and phycocyanin. Chlorophyll-a and chlorophyll-c1 are the main photosynthetic pigments of the Cryptophyta, and chlorophyll-b is never present.

The photosynthesizing ability of eukaryotes was made possible by one or more endosymbiotic associations between heterotrophic eukaryotes and photosynthetic prokaryotes (or their descendents). Several primary endosymbioses occurred between eukaryotes and blue green algae. In one of the lineages, the photosynthetic organism lost much of its genetic independence and became functionally and genetically integrated as plastids – chloroplasts within the host cell. At least two types of protists – chloroarachniophytes and cryptomonads –acquired 'plastids' by forming symbioses with eukaryotic algae. Such acquisitions are referred to as secondary symbioses.

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External links Phycobilins : Phycobiliproteins :

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