The nonlinear phototaxis signaling network of Chlamydomonas investigated by observing ciliary responses of individual cells to green and red light

The unicellular alga Chlamydomonas samples environmental conditions (light, chemical, temperature, touch, gravity) with multiple receptors. Sensory information is processed and integrated to control motion of its two cilia that propel the cell through its aquatic environment. The cell tracks light with green-light sensitive receptors concentrated in its eyespot and makes decisions to swim toward (positive phototaxis), away from (negative phototaxis), or orthogonal (diaphototaxis) to the light. The phototaxis signaling pathway was investigated by a holding single cell on a micropipette and illuminating its fixed photoreceptors while monitoring the ciliary motion. The cilia, beating in a planar "breast stroke" motion, were imaged on a quadrant photodiode array. Modulated green light stimulated the photoreceptors while detailed motion of each cilium was recorded over each beat cycle. Measures of ciliary responses determined from detector signals included beat frequency, stroke velocity, and relative phase or synchrony. Recordings from 302 individual cells were analyzed to investigate the phototaxis signaling network. Each measure displayed nonlinear response to green square-wave stimulation. Green light modulated by a pseudorandom noise pattern correlated with each measure produced linear and nonlinear response functions. This compact organization allows efficient response comparisons under various experimental conditions and with different mutants. Each measure has duration of a few hundred milliseconds; however each exhibits a unique delay, shape, and response sign. In addition, the two cilia respond differently. Modeling suggests a common signaling pathway for each measure from photoreceptor to cilia. Each cilium modifies the signal differently to provide phototaxis. Red light stimulates different photoreceptors than does green light, with different time scale responses. Long-period red-light step experiments show responses persisting for several seconds. Beat frequency increased with exponential saturation when red light is turned on and decreased exponentially when red light is turned off. Additionally, beat frequency undergoes a large amplitude transient decrease when red light is turned off. Results with simultaneous green and red light stimulation indicate that red light is involved in phototaxis direction decision-making. Single fixed-cell response modeling was extended to predict light modulation sensed by a free-swimming cell.