IGM Colloquium: The fluid mechanics of plankton
Although we now know that microorganisms rule the oceans - controlling productivity and biogeochemical cycles - we largely ignore how they are affected by typical fluid flow conditions. For example, microbes are routinely exposed to turbulence, yet physicists have ignored microbes and biologists have ignored turbulence. Here I present microfluidic and millifluidic experiments, combined with mathematical models, to show that fluid flow can have profound effects on the biomechanics and the ecology of swimming microorganisms.
I illustrate this through a series of examples, and will focus in particular on 'gravitaxis', the tendency of many phytoplankton species to swim along the direction of gravity. I will show that, in the presence of flow, gravitaxis results in intense clustering of cells in layers and patches, akin to those often observed by oceanographers, which can have profound effects on plankton population dynamics. Intriguingly, plankton seem to 'know fluid mechanics' and I will present recent evidence that they are able to actively evade turbulence by sensing the simplest among the cues inherent in small-scale turbulent eddies… In addition to representing a new class of active particle problems that promises to keep the fluid mechanician busy for some time to come, these processes are environmentally important because they affect the ecological dynamics and biogeochemical consequences of some of the most important players in aquatic ecosystems.
Roman Stocker is a Professor in the Institute of Environmental Engineering at ETH Zurich. Roman's research focuses on environmental processes, particularly in the oceans: from harmful algal blooms, to coral bleaching, to oil spills, to the carbon cycle. Roman has pioneered the use of new technology - microfluidics - to zoom in on the smallest players behind these processes: the microbes. Roman’s work has contributed to reveal their importance, frequently appearing in high-profile scientific journals including Science, Nature and PNAS, and being featured in popular media including the BBC, CNN, and the New York Times. When he is not busy creating new technology to understand our environment, Roman is a keen traveler, enthusiastic scuba-diver, and proud father. More about him at http://stockerlab.ethz.ch/.