Composition and presumed biosynthetic pathway of carotenoids in the astaxanthin-producing bacteriumAgrobacterium aurantiacum

The carotenoid composition of the astaxanthin-producing bacterium Agrobacterium aurantiacum was analysed under different culture conditions. Ten kinds of carotenoids, β-carotene, echinenone, β-cryptoxanthin, 3-hydroxyechinenone, canthaxanthin, 3′-hydroxyechinenone, zeaxanthin, adonirubin, adonixanthin and astaxanthin, were identified by HPLC and spectroscopical techniques. A. aurantiacum synthesized astaxanthin from β-carotene through two hydroxylation steps at C-3 and 3′, and oxidation steps at C-4 and 4′. The order of these reactions appeared to be controlled by the culture conditions. A new pathway for astaxanthin formation, different from that of other astaxanthin-producing microorganisms, is proposed.