Anaerobic carotenoid biosynthesis in Rhodobacter sphaeroides 2.4.1: H2O is a source of oxygen for the 1-methoxy group of spheroidene but not for the 2-oxo group of spheroidenone.

Anaerobic biosynthesis of carotenoids in the purple facultative photosynthetic bacterium Rhodobacter sphaeroides was studied using mass spectrometry. We have demonstrated that (18)O from H2(18)O was incorporated into the 1-methoxy group of spheroidene and spheroidenone, the two major carotenoids produced by this bacterium during photosynthetic growth. Neither water nor CO2 was shown to provide an oxygen atom for the 2-oxo group of spheroidenone in R. sphaeroides 2.4.1 grown photosynthetically in the absence of molecular oxygen. Possible mechanisms for the anaerobic biosynthesis of spheroidenone in R. sphaeroides are discussed.