Involved in the biosynthesis of the acyclic C50 carotenoid bacterioruberin (BR). CrtD is involved in the desaturation reactions that form double bonds at C-3,4 of dihydroisopentenyldehydrorhodopin (DH-IDR) and C-3',4' of dihydrobisanhydrobacterioruberin (DH-BABR) to yield isopentenyld ehydrorhodopin (IDR) and bisanhydrobacterioruberin (BABR), respectively. A + dihydroisopentenyldehydrorhodopin = AH2 + isopentenyldehydrorhodopin A + dihydrobisanhydrobacterioruberin = AH2 + bisanhydrobacterioruberin Carotenoid biosynthesis. Cells lacking this gene accumulate DH-IDR and DH-BABR. Belongs to the carotenoid/retinoid oxidoreductase family.