CAROTENOIDS PROTECT PHAFFIA-RHODOZYMA AGAINST SINGLET OXYGEN DAMAGE

The only known habitat of the astaxanthin-containing Phaffia rhodozyma is in slime fluxes of deciduous trees at high altitudes. In this habitat, the function II of carotenoids in P. rhodozyma is probably to provide protection against photogenerated antifungal substances in the tree flux such as singlet oxygen (O-i(2)). To investigate the role of carotenoids in P. rhodozyma, genetic selections were employed to determine if carotenogenic yeast strains of P. rhodozyma have enhanced ability to quench O-i(2). Singlet oxygen was generated in liquid culture by the interaction of visible light (lambda = 550 nm) with the photosensitizer rose bengal or by the activation of alpha-terthienyl with ultraviolet light (lambda = 366 nm). In each case the treatments selected for growth of pigmented strains of P. rhodozyma. Albino (carotenoid-less) or yellow (beta-carotene producing) strains grew less well in media containing O-i(2). Addition of the O-i(2) quencher sodium azide to the medium with alpha-terthienyl allowed growth of non-pigmented strains. Since the ecological niche of P., rhodozyma is highly specific, we investigated whether extracts of birch trees (Betula), the original source of P. rhodozyma, contained a compound that would select for pigmented populations of the yeast. When P. rhodozyma strains were exposed to ethyl acetate extracts of Betula papyrifera excited with 366 nm ultraviolet light, only pigmented cells were able to grow. These results suggest that carotenogenesis developed in P. rhodozyma in response to the presence of photoactivatable antifungal compounds produced by the host tree.