Phycobilin
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The phycobilins are a group of photosynthetic pigments found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads (though not in green algae and higher plants). The phycobilins serve to capture light energy, which is then passed on to chlorophylls for photosynthesis. They are unique among the photosynthetic pigments in that they are bonded to certain water-soluble proteins, known as phycobiliproteins.
The phycobilins are especially efficient at absorbing red, orange, yellow and green light, wavelengths which are not well absorbed by chlorophyll a. Organisms growing in shallow waters tend to contain phycobilins that can capture yellow/red light, while those at greater depth often contain more of the phycobilins that can capture green light, which is more abundant there.
Two important classes of phycobilins are the bluish phycocyanins (good at absorbing red light) and the reddish phycoerythrins (good at absorbing green light). The former is more common in cyanobacteria, the latter is more common in red algae.
Chemically, phycobilins consist of an open chain of four pyrroles (tetrapyrrole) and are structurally similar to the bile pigment bilirubin. (Bilirubin's structure is also affected by light, a fact used for the phototherapy of jaundiced newborns.) Phycobilins are also closely related to the light-detecting plant pigment phytochrome: a chain of four pyrroles bonded to a protein. Chlorophylls are composed of four pyrroles as well, but there they are arranged in a ring and contain a metal atom in the center.
The phycobilins fluoresce at a particular wavelength, and are therefore often used in research as chemical tags, e.g. by binding phycobiliproteins to antibodies in a technique known as immunofluorescence.
The name phycobilin comes from the Greek phyco- for "algae", and bilin referring to bile.