Phycocyanin

Phycocyanin is a pigment-protein complex from the light-harvesting phycobiliprotein family, along with allophycocyanin and phycoerythrin. It is an accessory pigment to chlorophyll, found in cyanobacteria (also called blue-green algae). Because all phycobiliproteins are water-soluble, they cannot exist within the phospholipidcell membranes like carotenoids can. Instead, phycobiliproteins aggregate to form clusters that adhere to the membrane, called phycobilisomes.

The term "phycocyanin" come from the Greekphyco meaning "algae" and cyanin is from the English word "cyan", which conventionally means a shade of blue-green (close to aqua) and is also derived from the Greek "kyanos", which means a somewhat different color of dark color. Phycocyanin has a characteristic light blue color, absorbing orange and red light, particularly the 620 nm wavelength (depending on which specific type it is), and emits fluorescence at about 650 nm (also depending on which type it is). The related allophycocyanin absorbs and emits at longer wavelengths than phycocyanin C or phycocyanin R.

C-phycocyanin is often found in cyanobacteria that thrive around hot springs, as it can be stable up to around 70 °C (158 °F), with identical spectroscopic (light absorbing) behaviours at 20 °C (68 °F) and 70 °C. Thermophiles contain slightly different amino acid sequences making it stable under these higher conditions. The phycobiliproteins are made of two subunits (alpha and beta) having a protein backbone to which 1–2 linear tetrapyrrolechromophores are covalently bound, with a molecular weight around 30,000 Da. Stability of this protein in vitro at these temperatures has been shown to be substantially lower. Photo-spectral analysis of the protein after 1 min exposure to 65 °C conditions in a purified state demonstrated a 50% loss of tertiary structure.

Phycobiliproteins have fluorescent properties that are used in immunoassay kits. The phycocyanin produced by Aphanizomenon flos-aquae and Spirulina, for example, is used in the food and beverage industry as the natural 'Lina Blue' or 'EXBERRY Shade Blue' and is found in sweets and ice cream. In addition, fluorescence detection of phycocyanin pigments in water samples is a useful method to monitor cyanobacteria biomass.