Plays a role in phycoerythrobilin biosynthesis, the red pigment chromophore photosynthetically active biliproteins of the host cyanobacteria. Uses a four-electron reduction to carry out the reactions catalyzed by two enzymes (EC 1.3.7.2 and EC 1.3.7.3) in host. (3Z)-phycoerythrobilin + 2 oxidized 2[4Fe-4S]-[ferredoxin] = biliverdin IXalpha + 4 H(+) + 2 reduced 2[4Fe-4S]-[ferredoxin] Belongs to the HY2 family.