Demonstration of a biological effect of natural ionizing radiations.

The blue-green alga, Synechococcus lividus, was grown under various levels of radiations. Shielding of cultures with lead resulted in a lower cell growth rate: the reduction disappeared when a normal radiation level was restored in the lead chamber. Irradiations from a thorium source at a dose-rate 14 times higher than that of natural irradiation stimulated the growth of the algae. These results are a new demonstration of a biological effect of background radiations.