The bacterium Rhodospirillurn rubrum is capable of a photosynthetic reduction of CO2 similar to plant photo synthesis. A decrease in the illumination of R. rubrum induces it to reverse its direction of swimming; an investigation of the probability of this phototactic response as a function of the parameters of the stimulus, such as the intensity of illumination and its time pattern, should enhance our understanding of biological irritability.
The present investigation is preliminary to such an undertaking. It was attempted to learn more about the responding system through studies of the interaction between the pigments, phototaxis, and metabolism of R. rubrum. A determination was made of the relative spectral effectiveness of light in promoting a phototactic response; this measurement provided evidence that in addition to bacteriochlorophyll the predominant carotenoid pigment of rubrum absorbs phototactically active light. It has been suggested that the tactic response of R. rubrum is associated directly with a decrease in its metabolic rate; studies of the tactic response to oxygen yielded evidence that this hypothesis must be abandoned.
The remarkable adherence to the Weber Law reportedly displayed in the phototaxis of R. rubrum was reinvestigated; an adherence over a much smaller range of intensities was observed than had been reported earlier. The masking, under certain conditions of a potentially close Weber Law adherence was discussed.
