Influence of Oxygen upon Genetic and Nongenetic Effects of Ionizing Radiation on Paramecium aurelia. ∗

Summary

The nongenetic effects of ionizing radiation upon Paramecium, when many bacteria are present during irradiation, are not influenced or perhaps are slightly increased by hypoxia. However, when few bacteria are present, these effects are decreased by hypoxia. The latter finding is in accord with the previously obtained evidence that much of the nongenetic damage produced when few bacteria are present is due to H₂O₂ formed outside the cell. The former finding suggests that H₂O₂ formed inside the cell is not of importance in producing nongenetic damage. On the other hand, genetic damage is consistently less under hypoxia. Previous evidence has shown that this kind of damage is not produced by H₂O₂ formed outside the cell. However, it seems quite possible that H₂O₂ or HO₂ produced locally within the nucleus is of importance in producing genetic damage; but the evidence from Paramecium does not rule out other interpretations; e.g., an effect of oxygen itself upon the recombination of broken chromosomes.