PubMed1969
The expectation that microorganisms would be exposed to very high visible light intensity in a space environment led to the investigation of the killing of microorganisms by visible light. The carotenoid-containing yeast Rhodotorula glutinis was one of the several organisms investigated. It was found that despite the carotenoid content of the organism it can be killed by endogenous photosensitized oxidation. While the effects of various parameters such as culture age, temperature, etc., were studied, particular emphasis was placed upon the development of action spectrum, determination of sites of action and the mechanism by which cells are killed. The portion of the spectrum responsible for lethality lies between 300 and 400 nm with an apparent maximum response around 390 nm. The nucleus appeared to be damaged, as evidenced by a mutagenic effect when cells were radiated with light above 300 nm. Previously the only reported mutagenic responses from light of these wavelengths have been noted when cells have been irradiated in the presence of added sensitizing dyes. Increased leakage of irradiated cells indicated permeability membranes have undergone damage. Overall respiration of the cells is also decreased at a rate corresponding to the loss in viability. Such random damage to cells indicates that whatever the damaging species is, it must be quite non-specific. The sensitizing agent could be a free radical and evidence has been developed to support such a proposal. The sulfhydryl-containing compounds cysteine and gluthathione are capable of protecting the cells from death. Since these compounds are known to protect the organisms from ionizing radiation by the trapping of free radicals, it is logical to presume that the damaging species is a free radical. Little or no protection is offered to the microorganism by fat-soluble antioxidants such as tocopherol and related species. This would indicate that the primary lethal site must be in the water phase. It is likely that dehydration of the cells such as could be expected in a spacial environment would initially increase their susceptibility to irradiation, but upon intense dehydration could also result in their increased resistance.