Hydrodynamics and Collective Behavior of Tethered Microbes
January 27, 2015 - 10:00am
Microbial ecosystems in the top decimeters of sediment play an important role in determining the chemistry of the atmosphere and help support multicellular life. The metabolic rates of these microbes are strongly limited by the time it takes nutrients to diffuse from the surface. Here we combine experiments, mathematical models, and field work to understand how two microbes, the bacteria Thiovulum majus and the eukaryote Uronemella, respond collectively to overcome diffusion limitation. These microbes have independently evolved the ability attach to surfaces by means of a mucus tether. Once tethered, cells use their flagella or cilia to pump nutrient-rich water. Microbes also attach to members of their own species to form a centimeter-scale community called a ``veil''. In a veil, cells generate a macroscopic flow that mixes its environment 40 times more efficiently than do individuals. We show how this collective behavior arises from the individual behavior of cells. In the second part of the talk, we describe a new form of collective dynamics displayed by T. majus. Untethered bacteria self organize on a surface into rotating two-dimensional crystals of quickly spinning cells. These crystals show a number of rich phenomena including the formation of fixed points, limit cycles, and surface melting. Proceeding from a force balance on each cell, we show how this visually-striking behavior arises from the flow of water being created by each cell. These results provide mathematically tractable examples of how the large-scale behavior of microbial communities in the environment arises from the response of individual cells to nutrient limitation.