Biomixing in Lago di Cadagno

Swimming organisms can enhance mixing in their natural environments by creating whirls in their wake and by dragging along water. However, these mixing mechanisms are inefficient for microorganisms, because swimming-induced variations in velocity, temperature and other water constituents are evened out before they can be advected. In Lake Cadagno, the bacterium Chromatium okenii induces water movement not by propulsion, but by locally changing the fluid density, which drives convection and efficiently homogenises fluids. Such observations of bioconvection are known from other microorganisms, but have so far been limited to laboratory settings. In Lake Cadagno, we report the first natural observation of bioconvection by combining microstructure profiles of temperature and conductivity with measurements of cell properties of C. okenii.