High-fidelity phototaxis in biflagellate algae

The single-cell alga {\it Chlamydomonas reinhardtii} is a motile biflagellate that can swim towards light for its photosynthetic requirements, a behavior referred to as phototaxis. The cell responds upon light stimulation through its rudimentary eye -- the eyespot -- by changing the beating amplitude of its two flagella accordingly -- a process called the photoresponse. All this occurs in a coordinated fashion as {\it Chlamydomonas} spins about its body axis while swimming, thus experiencing oscillating intensities of light. We use high-speed video microscopy to measure the flagellar dynamics of the photoresponse on immobilized cells and interpret the results with a mathematical model of adaptation similar to that used previously for {\it Volvox}. These results are incorporated into a model of phototactic steering to yield trajectories that are compared to those obtained by three-dimensional tracking. Implications of these results for the evolution of multicellularity in the Volvocales are discussed.