Spheroidene monooxygenase (, CrtA, acyclic carotenoid 2-ketolase, spirilloxantin monooxygenase, 2-oxo-spirilloxanthin monooxygenase) is an enzyme with systematic name spheroidene, reduced-ferredoxin:oxygen oxidoreductase (spheroiden-2-one-forming). This enzyme catalyses the following chemical reaction
The enzyme is involved in spheroidenone biosynthesis and in 2,2'-dioxospirilloxanthin biosynthesis.