Allophycocyanin B (AP-B) is one of two terminal emitters of phycobilisomes (PBS),the photosynthetic light-harvesting complexes in cyanobacteria and red algae.Among all phycobiliproteins in PBS,AP-B exhibits the lowest energy level with the most red-shifted absorption (λmax = 656 and 668 nm) and fluorescence emission (675 nm).AP-B is located in the core of PBS and is spatially close to the reaction center.