Little blue or red light reaches algae which reside at a depth of 1 metre or more in seawater, as this light is absorbed by seawater and fluorescent pigments of photosynthetic organisms above. A phycobilisome is a light-harvesting protein complex present in Cyanobacteria, Glaucocystophyta, and Red Algae. Phycobilins, fluorescent pigments which are linked to the peptide chain, absorb green light or red light. Other pigments which are present in the bacterial photosynthetic reaction centers, like bacteriochlorophyll and bacteriopheophytin do not absorb light in these regions. The fluorescent pigments which are present in the phycobilisome, such as phycocyanobilin and phycoerythrobilin re-emit the green light in regions which the other photosynthetic pigments can absorb.

The geometrical arrangement of a phycobilisome is very elegant and results in 95% effiency of energy transfer. There is a central core of allophycocyanin which sits above the photosynthetic reaction center. There are phycocyanin and phycoerythrin subunits which radiate out from this center like thin tubes. This increases the surface area of the absorbing section and helps focus and concentrate light energy down into the reaction center. The energy transfer from exited electrons absorbed by pigments in the phycoercythrin subunits at the periphery of these antennas appears at the reaction centre in less than 100 ps.

Biology