The physiological role of the carotenoid pigments of halobacterium salinarium

Summary

A colorless mutant of the extremely halophilic bacterium, Halobacterium salinarium, has been isolated. In comparative experiments with the carotenoid-containing wild type, it was shown that neither type was killed upon exposure to bright sunlight under aerobic conditions. Exposure to tungsten light of an incident intensity comparable to that of bright sunlight inhibited strongly the growth of the colorless mutant; the growth of the carotenoid-containing wild type was unaffected.

In their natural habitats, the extremely halophilic bacteria are usually exposed to bright sunlight. It seems likely that such light will affect the growth of these bacteria as does tungsten light. It is therefore inferred that in their natural habitats, carotenoid-containing forms will develop more freely than colorless forms, because the pigments protect the cells against an inhibitory effect of light upon growth. Carotenoid-containing forms have therefore come to dominate the population of extremely halophilic bacteria in nature.