Phycobilins

Phycobilins are proteinaceous, water-soluble accessory pigments that differ from both chlorophylls and carotenoids in one key structural respect: they have an open (rather than closed-ring) tetrapyrrole structure, and they contain neither a magnesium ion nor a phytol tail. Being water-soluble sets them apart functionally too, since chlorophylls and carotenoids are both lipid-soluble and embedded in the thylakoid membrane, while phycobilins are typically organised into separate protein-pigment complexes (phycobilisomes) that sit on the membrane surface. There are two phycobilin pigments: phycocyanin, a blue pigment found in cyanobacteria, and phycoerythrin, a red pigment found in rhodophycean (red) algae. Because they absorb strongly in the green-yellow part of the spectrum where chlorophyll absorbs only weakly, phycobilins are especially valuab …