Pigments other than chlorophyll a found in photosynthetic cells. They include the carotenes and the xanthophylls, together known as the carotenoids, chlorophylls b, c, and d, and the phycobilins. The latter are found only in blue-green and red algae. The composition of accessory pigments in algae is used as a taxonomic character.
The accessory pigments function as secondary absorbers of light in regions of the visible spectrum not covered by chlorophyll a. The light energy that they absorb must be transferred to chlorophyll a before it can be used in the photosynthetic process. As energy transfer from one molecule to another can only occur from a shorter wavelength
absorbing form to a longer wavelength absorbing form, all accessory pigments have absorption maxima at shorter wavelengths than chlorophyll a. Energy is passed from chlorophyll a to the reaction centre pigments, P680 and P700. Some accessory pigments may have a protective function, preventing photoox-idation of the cell's chlorophyll at high light intensities.