Figure 4. Active turning of individual P. parasitica zoospores in water.

(A) Images of a zoospore changing direction. The two flagella cooperate to help the cell body rotate and steer to a new direction achieving a turning angle $\mathrm{\Delta }\theta$. (B) Trajectory of the zoospore during the turning event. Three red arrows represent 3 back and forth stroke-like motions. (C) The speed $U$ of the zoospore during the turning event. The turning starts when the speed begins to fluctuate with large magnitude and lower frequency, and lasts for a duration of ${\tau }_{\mathrm{s}}$ with a rotation of the cell body followed by steering to the new direction. (D) The moving directions θ and the body orientation ψ of the zoospore during the turning event. (E) Images of the anterior flagellum of a zoospore beats with power and recovery stroke, similar to C. reinhardtii’s in a temporal zoom corresponding to the "Rotate" step of the turning event (but not from the same video as (A)). (F) Schematic to describe the gait of the flagella during a turning event. (1-2) Power stroke 1, (2-3) recovery stroke 1, (3-4) power stroke 2.