(A) Method for calculation of quantum effectiveness of a particular wavelength of light in inducing phototaxis. Shown is a fluence rate-response curve for a particular wavelength. The fitted line is extrapolated to the criterion response. The fluence rate at the criterion response (*Fc*) is inversely proportional to the quantum effectiveness of the light used. Therefore, the inverse of *Fc* (1/*Fc*) is expressed as a percentage of the strongest response and then plotted against the wavelength to give the final action spectrum. (B) Action spectrum of positive phototaxis in wild-type (WT) *Synechocystis* sp. strain PCC6803 cells at low light intensity. (C) Action spectrum of negative phototaxis in wild-type *Synechocystis* sp. strain PCC6803 cells at high light intensity. At the wavelengths 394, 424, 523, 552, 574, and 595 nm, only positive phototaxis was observed at the highest possible fluence rates (maximum output from the illuminator) used, which were 16.29, 79.65, 422.1, 498.5, 304.9, and 1,046 μmol m^{−2} s^{−1}, respectively. Therefore, the negative phototactic responsiveness of cells at these wavelengths was considered to be 0.

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