Excitable gait control of microswimming

Since the invention of the microscope, scientists have known that pond-dwelling algae can actually swim – powering their way through the fluid at several times their own body size per second using tiny limbs called cilia and flagella. Only recently has it become clear that these tiny structures are the very same cilia that fulfil many important physiological and developmental functions within the human body. Here, we report a novel, tripartite motility in an octoflagellate Prasinophyte alga, comprising a steady forward gait (run), a fast knee-jerk response with dramatic reversals in beat waveform (shock), and, remarkably, long quiescent periods (stop) within which the flagella continue to exhibit very small amplitude oscillations. Combining high-speed imaging with detailed tracking of the swimming dynamics, we estimate transition probabilities between these highly stereotypical states to reveal that gait control in these microorganisms exists far from equilibrium. Moreover, we will demonstrate that rapid bifurcations in locomotion behaviour can also be triggered by direct mechanical contact, and discuss possible implications of these findings for an early evolution of excitable signal transduction in a primitive species.