The enzymes from Rubrivivax gelatinosus and Rhodobacter sphaeroides prefer the acyclic carotenoids (e.g. 1-hydroxy-1,2-dihydroneurosporene, 1-hydroxy-1,2-dihydrolycopene) as substrates. The conversion rate for the 3,4-desaturation of the monocyclic 1'-hydroxy-1',2'-dihydro-gamma-carotene is lower [2,3]. The enzyme from the marine bacterium strain P99-3 shows high activity with the monocyclic carotenoid 1'-hydroxy-1',2'-dihydro-gamma-carotene . The enzyme from Rhodobacter sphaeroides utilizes molecular oxygen as the electron acceptor in vitro . However, oxygen is unlikely to be the natural electron acceptor under anaerobic conditions.