Paddling for food

As unicellular organisms evolved into multicellular ones, particularly ones with differentiated cell functions, delivery of sufficient nutrients and removal of waste products took on important and often limiting roles. The group of spherical colonies known as volvocine green algae provides a useful system for studying those evolutionary transitions. The volvocines range from single-celled Chlamydomonas reinhardtii to the large Volvox genus, whose colonies can reach sizes of a millimeter and have thousands of cells differentiated into sterile, flagellated cells on the outside and reproductive germ cells on the inside. New research by Raymond Goldstein and collaborators at the University of Arizona and Brown University shows that the coordinated beating of the outer cells’ flagella is important not only for motility but also for generating advected flows that bring the needed supply of nutrients. Theoretically modeled flows were confirmed by experiments such as the one shown here. While a Volvox colony some...