Nonheme diiron monooxygenase involved in the biosynthesis of astaxanthin. Hydroxylates beta-ring of beta-carotene and catalyzes the conversion of canthaxanthin to astaxanthin. Uses ferredoxin as an electron donor. all-trans-beta-carotene + 4 H(+) + 2 O2 + 4 reduced [2Fe-2S]-[ferredoxin] = all-trans-zeaxanthin + 2 H2O + 4 oxidized [2Fe-2S]-[ferredoxin] Not detected in vegetative cells. Up-regulated during cyst cell formation. Up-regulated by moderate light in correlation with the redox state of the photosynthetic electron transport. The histidine box domains may contain the active site and/or be involved in iron binding. Belongs to the sterol desaturase family.