Phycobilins are water-soluble photosynthetic pigments found in the cytoplasm, or in the stroma of the chloroplast. There are two classes of phycobilins and they occur only in Cyanobacteria and Rhodophyta. The bluish pigment phycocyanin is found in Cyanobacteria and gives them their common name of "blue-green algae". The reddish pigment phycoerythrin is found only in Rhodophyta and gives them their common name of "red algae".
Phycobilins are not only useful to the organisms which use them for soaking up light energy; they have also found use as research tools. Both pycocyanin and phycoerythrin fluoresce at a particular wavelength. That is, when they are exposed to strong light, they absorb the light energy, and release it by emitting light of a very narrow range of wavelengths. The light produced by this fluorescence is so distinctive and reliable, that phycobilins may be used as chemical tags. The pigments are chemically bonded to antibodies, which are then put into a solution of cells. When the solution is sprayed as a stream of fine droplets past a laser and computer sensor, a machine can identify whether the cells in the droplets have been tagged by the antibodies. This has found extensive use in cancer research, for tagging tumor cells.