Catalyzes the introduction of C-3,4 double bonds into 1-hydroxyneurosporene (1-HO-Neu) to yield demethylspheroidene (DMS). It prefer the acyclic carotenoids such as 1-hydroxylycopene, and 1-hydroxy-gamma-carotene, whereas 1-hydroxy-3,4-didehydrolycopene and 1,1-dihydroxylycopene are much less effective. A + rhodopin = (3E)-3,4-didehydrorhodopin + AH2 Carotenoid biosynthesis; spheroidene biosynthesis. The hydrogen acceptor in the C-3,4 desaturation reaction is molecular oxygen. NAD, NADP, and FAD have no influence on enzymatic activity. Another unknown electron acceptor may exist in Rhodobacter species that replaces molecular oxygen during anaerobic growth (PubMed:9393712). Belongs to the carotenoid/retinoid oxidoreductase family.