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. 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.