CHLOROFLEXUS-AURANTIACUS AND ULTRAVIOLET-RADIATION - IMPLICATIONS FOR ARCHEAN SHALLOW-WATER STROMATOLITES

The phototrophic growth of Chloroflexus aurantiacus under anoxic conditions was determined as a function of continuous UV irradiance. Cultures grown under an irradiance of 0.01 Wm-2 exhibited a slightly depressed yield over the non-irradiated control. Yields decreased further with increasing irradiance. Inhibition was severe at an irradiance of 0.66 WM-2. Growth of E. coli cultures was severely depressed at UV-C irradiances that permitted good growth of C aurantiacus. Low levels of Fe3+ provided a very effective UV absorbing screen. The apparent UV resistance of Chloroflexus and the effectiveness of iron as a UV-absorbing screen in sediments and microbial mats are suggested to be likely mechanisms of survival of early phototrophs in the Precambrian in the absence of an ozone shield.