Figure 4.

Cells maintain their speed and polarize their motion toward the light during phototaxis. (A) Schematic of two cells with similar speeds, moving with positive bias (left) and zero bias (right). (B) The average speed of single cells was similar along the two axes, before and after the light was turned off. Data are mean ± SE of n = 4 independent inoculations. Asterisks represent statistically significant differences, with ^∗p < 0.05 and ^∗∗p < 10⁻⁴. There was a small but significant increase in the average speed of single cells upon restoration of light (t₍₈₎ = 4.0, p < 0.01; t₍₈₎ = 5.0, p < 0.01; t₍₈₎ = 4.7, p < 10⁻⁴; t₍₈₎ = 2.7, p < 0.05, respectively). Similar to the parallel bias (Fig. 2Bii), once the light was turned off, the parallel velocity decreased approximately to zero (t₍₈₎ = 11.6, p < 10⁻⁴). Upon renewal of light, the velocity reverted to a value similar to its initial value (t₍₈₎ = 11.6, p < 10⁻⁴). (C) The distributions of step angles under the respective light conditions (color scheme as in B) illustrate an increase in the fraction of steps taken toward the light when light is present.