Preparation, maintenance and propagation of synchronous cultures of photoactive Chlamydomonas cells.

The systematic cultivation of species of photosynthetically active 'green' microorganisms in research labs started in the 1940s. Among these microorganisms, Chlamydomonas represents a genus of green biciliated microalgae, of which Chlamydomonas reinhardtii has become the main describing species. For decades C. reinhardtii has been used as an established model organism in biology, including research areas such as molecular biology of eukaryotes, photosynthesis, light receptors, cell metabolism, the dynamics of microtubule assembly and protein transport along cilia. More recently, the use of suspensions of light-responsive living microorganisms has seen a major expansion from the life sciences to the biophysics, statistical physics, fluid dynamics and bioengineering communities. Studies that substantially advance the state of the art in these research areas require the reliable preparation and maintenance of viable, monodisperse and synchronous cell cultures. Although some technical aspects are shared with standard procedures in cell biology and microbiology, Chlamydomonas and its relatives are photosensitive and, simultaneously, motile, meaning this microorganism requires tailored cultivation protocols that are specific to this species. Here we provide guidance on which Chlamydomonas wild-type and mutant strains are suitable for specific experiments and provide detailed step-by-step procedures to measure culture synchronicity, growth rate of the population, average cell size and motility features. The reliable preparation of cell cultures may facilitate future interdisciplinary research using living suspensions of photoactive microorganisms.