Phycobilisomes: modular light-harvesting systems of cyanobacteria and red algae

Phycobilisomes (PBSs) are supramolecular pigment-protein complexes that function as the principal light-harvesting antennae in cyanobacteria, red algae (Rhodophyta), and glaucophytes, and to a lesser extent in cryptophytes. Because the intrinsic chlorophyll-based antennae of the photosystems absorb poorly in the green region of the spectrum, PBSs employ specialized bilin chromophores to capture complementary wavelengths that chlorophyll cannot efficiently harvest. Thus, PBSs broaden the photosynthetic active spectrum of oxygenic phototrophs and thereby increase their photosynthetic capacity. Structural studies using cryogenic electron microscopy and advanced spectroscopic techniques have revealed increasingly detailed insights into excitation energy-transfer pathways and the bilin microenvironments that support highly efficient energy funneling with minimal loss. These approaches have also uncovered previously unrecognized conformational dynamics, although the functional significance of these motions remains unknown. Together, these findings portray PBSs as both ancient and highly adaptable molecular machines, central to the ecological success of cyanobacteria and other phycobiliprotein-containing phototrophs, and still yielding new surprises as analytical techniques continue to improve. This review integrates foundational and recent advances in the structural organization, functional mechanisms, and emerging applications of PBSs. We examine PBS architectures and their distribution across lineages, energy-transfer and photoprotective processes, structural dynamics, evolutionary trajectories, and the expanding landscape of biotechnological uses. These insights establish a foundation for future efforts to link PBS structure with ecological function and biotechnological innovation.