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Experiments were carried out to demonstrate the effects of low-level chronic irradiation on Paramecium tetraurelia proliferation. Biological effects were strongly dependent on the bacterial density of culture medium and more exactly on the catalase content of the medium. Significant growth stimulation was found under60 Co chronic irradiation at a dose rate of 2 rad/year when paramecia were grown in a medium containing a high bacterial concentration (2. 5× 107 cells/ ml) or supplemented with catalase (300 U/ml). In a medium with a low bacterial density (1 10^6 cell/ ml) or supplemented with a catalase activity inhibitor, growth stimulation was preceded by a transitory inhibiting effect which could be correlated with extracellularly radioproduced H2 O2. H2 O2 addition appeared to be able to simulate the biological effects of chronic irradiation. A possible mechanism is discussed. We propose that the stimulating effects were the result of intracellular enzymatic sca. . .
Croute et al. (Wed,) studied this question.