A continuous densitostat culture of unicellular alga of Chlorella exposed to short-wave UV emission was used to demonstrate the nature of damage and ability of this biological system to self-restore after the removal of exposure. A facility for continuous densitostat cultivation of Chlorella for this kind of research has been described. A flow densitostat culture is a direct analogue of the natural selection. Dependence of damage on the emission dose is S-shaped. Specific growth rate with time after UV exposure is expressed analytically. The mathematical model has shown good agreement with experiment.