Phototaxis in Euglena—ii. Physical Factors Determining the Rate of Phototactic Response*

A novel approach to the measurement of phototaxis is described. It is based on recording the change in optical density which results when phototactic microorganisms collect in a beam of actinic light. The instrument allows the reproducible measurement of rate and degree of phototaxis. Using this device, we have carried out a systematic study of the influence of physical parameters on the phototactic response in Euglena gracilis, z‐strain. When Euglena is grown under conditions of artificial day/night cycles, circadian rhythms of phototactic ability are observed which parallel similar rhythms previously observed in the photosynthetic capacity of other algae. Maintenance of a light‐grown culture in complete darkness results in a loss of the phototactic response after about 72 hr. A dark‐grown culture develops chlorophyll and phototactic capacity simultaneously. These results suggest a relation between phototaxis and photosynthesis. From the results of light intensity dependence studies of phototaxis, it is concluded that the actinic light triggers rather than powers the response. The upper limit for the quantum threshold of phototaxis is estimated to be 100 quanta per photoreceptor at 460 mμ. Measurements of the dependence of phototaxis on the wavelength of the actinic light yield an action spectrum which agrees very well with the absorption spectrum of the eyespot pigments. The rate of phototaxis vs. temperature closely resembles the temperature dependence of enzymatic activity.