The effect of respiration on the phototactic behavior of the purple nonsulfur bacterium Rhodospirillum centenum.

The effect of respiration on the positive phototactic movement of swarming agar colonies of the facultative phototroph Rhodospirillum centenum was studied. When the electron flow was blocked at the bc1 complex level by myxothiazol, the oriented movement of the colonies was totally blocked. Conversely, inhibition of respiration via the cytochrome c oxidase stimulated the phototactic response. No phototaxis was observed in a photosynthesis deficient mutant (YB707) lacking bacteriochlorophylls. Analyses of the respiratory activities as monitored by a oxygen microelectrode in single agar colonies during light/dark transitions showed a close functional correlation between the photosynthetic and respiratory apparatuses. The respiratory chain of Rsp. centenum was formed by two oxidative pathways: one branch leading to a cytochrome c oxidase inhibited by low cyanide concentrations and a second pathway formed by an oxidase less-sensitive to cyanide that also catalyzes the light-driven respiration. These results were interpreted to indicate that (1) there is a cyclic electron transport, and (2) photoinduced cyclic electron flow is required for the phototactic response of Rsp. centenum. Furthermore, under oxic conditions in the light, reducing equivalents may switch from photosynthetic to respiratory components so as to reduce both the membrane potential and the rate of locomotion.