Bioconvection in the wild

Bioconvection is an intriguing example of collective biological activity in which microorganisms drive hydrodynamic processes that modify their host fluid environment. This phenomenon has been thoroughly documented in the laboratory. Here, we provide an example of its occurrence in a natural system. A community of motile and heavy, purple sulfur bacteria –Chromatium okenii– drive bioconvection in the permanently stratified Lake Cadagno (Switzerland). Measurements of the lake's physical, chemical and biological properties and a mathematical model of the bacteria's vertical migration provide compelling proof of concept. The lake exhibits a chemical gradient zone separating an oxygen-rich upper layer (top ~10 m) from anoxic and sulfide-rich deep waters. This environment offers an optimal niche for bacteria to accumulate at the uppermost anoxic zone, where also light and sulfide are available to perform anoxygenic photosynthesis – their primary metabolic process. The collective behavior driving bioconvection consists of bacteria vertically migrating upward towards light and stopping at the oxic-anoxic transition. This leads to a locally unstable density excess that fuels convection and mixing, demonstrating the ability of bacteria-inducing bioconvection to modify their surroundings.