phycobilisomes

Phycobilisomes are large, extrinsic light-harvesting complexes found on the photosynthetic membranes of cyanobacteria, red algae, and some glaucophyte algae. They capture light energy and funnel it to chlorophyll a within the photosystems, enhancing photosynthesis in aquatic environments where light quality and quantity vary. The core of a phycobilisome is composed mainly of allophycocyanin, while rods extending from the core contain phycocyanin and, in many species, phycoerythrin. The phycobiliproteins are pigment–protein complexes with covalently bound phycobilin pigments (such as phycoerythrobilin, phycocyanobilin, and allophycobilin) that broaden the spectrum of light absorbed. Linker polypeptides help assemble the rods and core and attach the phycobilisome to the thylakoid membrane.

Structurally, phycobilisomes display a rod–core architecture, with multiple rods radiating from a central core. The exact composition varies among organisms, enabling different absorption properties. In cyanobacteria, phycobilisomes are attached to the photosynthetic membranes via anchor proteins, ensuring proximity to photosystems II and I for efficient energy transfer. Phycobilisomes can also be tuned in response to environmental light conditions by adjusting the relative abundance of phycoerythrin, phycocyanin, and allophycocyanin, enabling optimization of light harvesting under different spectral qualities.

Functionally, phycobilisomes expand the usable light spectrum, particularly in the green to orange range, and redirect absorbed energy toward chlorophyll a through a well-organized energy transfer cascade. This system is a key adaptation for aquatic photosynthesis, contributing to the success of cyanobacteria and red algae in diverse light environments. Genetic and structural studies highlight gene clusters encoding phycobiliproteins (e.g., cpc and apc families) and their assembly factors that regulate phycobilisome formation.