Bacteria keep oxygen flowing

A combination of bacterial chemotaxis and gravitational falling creates vortices in water droplets, according to Idan Tuval (Universitat de Les Illes Balears, Spain), Raymond Goldstein (University of Arizona, Tucson, AZ), and colleagues. The vortices improve harvesting of oxygen from the surrounding atmosphere and mixing of the oxygen into the droplet.

By conventional thinking, bacterial swimming results in negligible flows, and thus diffusion effects dominate over stirring. But Goldstein and colleagues took advantage of the well-known phenomenon of bioconvection in which, in a drop lying on a surface, bacteria are moving cooperatively toward the more oxygenated upper layer. After concentrating in this layer they are subject to gravity and stochastically form downward plumes. Once started, these plumes feed themselves: the incompressibility of water means that falling bacteria draw in more bacteria behind themselves.The researchers discovered a new phenomenon near...