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The algal flagellates have been subjected to relatively few radiobiological studies. In an early experiment, Holweck and Lacassagne (1, 2) irradiated the colorless chlamydomonad Polytoma uvella with a-particles. Ralston (3) studied the effects of X-rays on Dunaliella salina. Although both these organisms are presumably haploid (4), both exhibit multiple-hit survival curves, as does the colorless euglenid Astasia longa (5). Pandorina (6) shows a cumulative lethal effect without recovery. Nybom (7) has reported nonexponential survival curves in the green algae Chlamydomonas eugametos, C. moewusii, and C. reinhardi after exposure to X-rays. Since Nybom's irradiations were performed in conjunction with genetic experiments, the haploid condition of these cells seems reasonably certain. If more than one quantum must be absorbed to inactivate a single target, the last absorption can be thought of as the lethal event, and the preceding ones as sensitizing events. Reversibility of the "sensitizing" process would be manifested by an increased yield of survivors of a given dose when the exposure is interrupted, and sufficient time permitted to elapse before it is resumed. Sax (8) found such a dose fractionation effect in the production of two-hit chromosome aberrations in Tradescantia.

Evidence for Recovery from X-Ray Damage in Chlamydomonas