Ultrastructural alterations following X-ray and proton irradiation of dividing Chlamydomonas reinhardtii.

Light-dark synchronized Chlamydomonas reinhardtii are most sensitive to irradiation when exposed early in the dark period of a 12 hour light: 12 hour dark regime. At this time the algae are either dividing or preparing for division. Liquid cell cultures of the 137c wild type strain exposed to 9,000 R X-irradiation exhibit 69% mortality. Monolayer cultures on Millipore filters exposed to proton irradiation (1.5 MeV protons from a positive Van de Graff accelerator) exhibit 89% mortality following a dose of 9 kilorad, and 98% mortality following 18 kilorad. Ultrastructural examination of X-irradiated cells over five days following exposure documented chloroplast and nuclear alterations which included non-nuclear condensations, swollen and ruptured nuclear envelopes and disrupted pyrenoid structure. Following both types of irradiation, both live and dead daughter cells were present within the old mother cell wall. Following both types of irradiation, cells were also observed in which there were (1) profiles resembling division conformations, and (2) chromosomes and spindle fibers present at a time not characteristic for such events in synchronized control cultures. No changes distinct to either X-ray or proton irradiation were present, indicating that, after a given level of injury, similar ultrastructural manifestations of cell damage are evident in irradiated cells.