Catalyzes the opening of the heme ring with the release of iron. Key enzyme in the synthesis of the chromophoric part of the photosynthetic antennae (By similarity). Upon overexpression in E.coli with PCB:ferredoxin oxidoreductase, CpeS and either CpcB or PecB permits synthesis of phycocyanin-coupled CpcB or PecB. heme b + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = biliverdin IXalpha + CO + Fe(2+) + H(+) + 3 H2O + 3 oxidized [NADPH--hemoprotein reductase] Belongs to the heme oxygenase family.