Tuning biflagellar dominance in a robotic model of phototactic algae

Microscopic algae use their flagella to generate diverse locomotive behaviors, such as goal-directed motion. In particular, phototaxis is a light-triggered response during which algae swim toward a light source to carry out photosynthesis. While the biological mechanism regarding the algae’s photoreceptor is well understood, less is known about how the flagellar dynamics lead to phototaxis. In Chlamydomonas, one possible mechanism of phototactic turning is achieved via either differences in beating frequency or amplitude between the cis (closest to the eyespot) and trans flagellum (farthest from the eyespot) (Witman, 1996). To test this, we developed a macroscopic motor-driven robophysical model that swims in a viscous fluid (glycerin, 1,100 cSt) to replicate low Reynolds number swimming. We implemented turning via differential-beating with varying flagellar (1) frequencies and (2) amplitudes. Differential-beating frequencies led to greater net turning with a maximum turning rate of 3.75±0.33 degrees per cycle (deg/cyc), compared to differential-beating amplitude with a maximum of 2.08±0.06 deg/cyc. Further, we observed that turning performance was sensitive to the flagellar waveforms. Based on these insights, we propose a mechanistic model of sensorimotor coupling in which phototaxis is achieved by controlling the functional asymmetry between the two flagella in response to light.