Effect of certain substituted naphthoquinones on growth and respiration of Rhodospirillum rubrum

The effect of dichlone (2,3-dichloro-1,4-naphthoquinone) on photoautotrophic, photoheterotrophic and heterotrophic growth, and respiration of Rhodospirillum rubrum (a nonsulfur purple bacterium) was studied to elucidate the mechanism of action of this toxicant on photosynthetic bacteria. The photosynthetic growth with malate or with hydrogen and CO₂ was inhibited by dichlone. Light respiration of photoheterotrophically grown cells, unlike their dark respiration, was found to be insensitive to dichlone. Although dichlone caused an inhibition of the respiration of dark-grown cells, such cells were able to grow in the presence of dichlone after a lag. Light-dependent ¹⁴C-substrate incorporation by photoheterotrophic or photoautotrophic cells was found to be relatively more sensitive to dichlone than oxidative substrate incorporation by heterotrophic cells. Short-term exposure of the light-grown cells to dichlone resulted in an irreparable loss of their ability to grow photosynthetically and photoassimilate ¹⁴C-substrates. Menadione (2-methyl-1,4-naphthoquinone), a synthetic vitamin K, failed to affect these reactions to a significant extent at comparable concentrations. The findings suggest that dichlone causes an irreversible damage to some primary photosynthetic reaction in chromatophores, whereas the damage caused to the dark heterotrophic metabolism of the cell is less severe and repairable. The inhibitory action of dichlone does not appear to be via the formation of semiquinone free radicals.