Recent results suggest that the presence of common nitrogen salts (sodium nitrite and nitrate) in the irradiation medium can markedly protect filamentous blue green algae from potentially lethal ultraviolet light irradiation. Our results as well as general biologica-l arguments as presented by Egami (above) support and extend Egami's original view that anaerobic respiratory pathways using nitrite and nitrate as terminal electron acceptors evolved prior to oxygen requiring aerobic respiratory pathways.