A study on the killing by light of photosensitized cells of halobacterium salinarium

Summary

The present study aimed at a demonstration of a protecting effect of carotenoids against photokilling of Halobacterium salinarium in the presence of exogenous photosensitizer. In comparative experiments with a colorless mutant and the carotenoid-containing wild type the use of toluidine blue as photosensitizer gave erratic results. It seems probable that the erratic results were due to the varying ability of cells from one culture to another to reduce toluidine blue to a leuco form which is inactive as photosensitizer. The use of phenosafranine as photosensitizer gave consistent results; this dye has a lower oxidation-reduction potential than toluidine blue and is less easily reduced to a leuco form by the cells. When aerated cultures were exposed to light in the presence of phenosafranine the colorless cells were killed; the carotenoid-containing cells were not so affected. It can therefore be concluded that carotenoids protect the cells against photokilling under these conditions.

The killing of the colorless cells was closely parallelled by an extensive lysis of the cells; the structure of the carotenoid-containing cells was barely affected by the exposure. There are reasons to believe that the carotenoids are located in or on the cell envelope. It thus seems likely that the cell envelope is a site of the photochemically induced damage.