What is "phycobilin"

n. (context biochemistry English) any of a group of highly coloured pigments found in some photosynthetic algae; they are open-chain tetrapyrroles related to mammalian bile pigments

n. water-soluble proteinaceous pigments found in red algae and cyanobacteria

Phycobilins (from Greek: (phykos) meaning "alga", and from Latin: bilis meaning "bile") are light-capturing bilanes found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads (though not in green algae and plants). Most of their molecules consist of a chromophore which makes them colored. They are unique among the photosynthetic pigments in that they are bonded to certain water-soluble proteins, known as phycobiliproteins. Phycobiliproteins then pass the light energy to chlorophylls for photosynthesis.

The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that 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 relatively more abundant there.

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.