Abstract

Aqueous environments, with which we are familiar, such as puddles, rivers, ponds, and oceans, are teeming with microorganisms. Often propulsion is generated by the vigorous beating dynamics of organelles known as cilia and flagella. Like microscopic analogues of limbs, these often come in pairs. The twin anterior flagella of the tiny alga Chlamydomonas reinhardtii executes a bilateral breaststroke when actuated synchronously, but when this bilateral symmetry is broken, they are also capable of turning the cell. The latter phenomenon underlies tactic reorientation, which brings cells closer to sources of light or nutrients. The phylogenetic lineage to which Chlamydomonas belongs also includes the large spherical alga Volvox carteri: adult colonies possess dense coverings of biflagellate somatic cells whose flagellar beating exhibits remarkable spatiotemporal coherence, much like the cilia in our lungs or metachronal waves in a football stadium. From the few to the many, what are the strategies devised by these cilia and flagella to coordinate their beating in the absence of a brain or semblance of a nervous system?