Bacterial resistance to ultraviolet irradiation under anaerobiosis: implications for pre-phanerozoic evolution.

The concept that low concentrations of atmospheric oxygen and consequent unattenuated ultraviolet irradiation limited the emergence of Phanerozoic life, the Berkner-Marshall hypothesis, is no longer tenable. Anaerobic bacteria, which probably evolved far earlier than Metazoa, were irradiated in a special chamber under strictly anaerobic conditions. Both intrinsic resistance and photoreactivation by visible light were discovered in obligately and facultatively anaeroboc microbes. Atmospheric scientists have shown that small amounts of oxygen would have limied pre-Phanerozoic surface ultraviolet irradiation to fluxes well below those used in the anaerobic experiments described. Since adequate ultraviolet protection mechanisms evolved early, the late Proterozoic appearance of Metazoa probably was not related to high fluxes of solar ultraviolet radiation.