CHLOROFLEXUS-AURANTIACUS AND ULTRAVIOLET-RADIATION - IMPLICATIONS FORARCHEAN SHALLOW-WATER STROMATOLITES

The phototrophic growth of Chloroflexus aurantiacus under anoxic condi tions was determined as a function of continuous UV irradiance. Cultur es grown under an irradiance of 0.01 Wm-2 exhibited a slightly depress ed yield over the non-irradiated control. Yields decreased further wit h increasing irradiance. Inhibition was severe at an irradiance of 0.6 6 WM-2. Growth of E. coli cultures was severely depressed at UV-C irra diances that permitted good growth of C aurantiacus. Low levels of Fe3 + provided a very effective UV absorbing screen. The apparent UV resis tance of Chloroflexus and the effectiveness of iron as a UV-absorbing screen in sediments and microbial mats are suggested to be likely mech anisms of survival of early phototrophs in the Precambrian in the abse nce of an ozone shield.