The quantitative determination of the spectral distribution of phototactic sensitivity in the purple bacterium Rhodospirillum rubrum

By using a compensation method, the action spectrum (spectral distribution of stimulating efficiency in a quantitative measure) of phototaxis in the purple bacterium Rhodospirillum rubrum (Esmarch) Molisch Strain 4 was determined. Two differently coloured adjacent small light fields were projected in a slide chamber containing a (dilute) suspension of moving spirilla. One of the fields acted as a standard, whereas in the other one the intensity as well as the wave-length of the light could be varied. The principle of the determination of the spectral sensitivity in the spirilla was based upon the adjustment to equivalence of the two fields for different wave-lengths. Shock reactions of the spirilla at the boundary between the fields were used as an “indicator” as to how far the fields were adjusted. A mathematical derivation was given. It showed that the action spectrum of phototaxis is identical with the absorption spectrum of the light acceptor active in the phototactical reaction. The action spectrum shows maxima at about 8800, 6900, 5900, 5250, 4900, 4600, and at about 4000 A