Microorganisms Flock in a Turbulent Flow*

The understanding of collective, cooperative motion of organisms is one of the most important problems in areas spanning physical biology, soft matter, and statistical physics. Most of the work in this field has focussed on showing how flocking emerges in a collection of organisms without taking into account the typically noisy, random and spatio-temporally complex environment experienced by these individuals. We now show, for the first time, that self-organised, collective behaviour can emerge, contrary to naıve expectations, in a group of microorganisms moving in a turbulent flow. By combining ideas from the Vicsek model and turbulent transport, we show that non-trivial correlations between the flow and individual dynamics are crucial for the microorganisms to flock in, for example, a turbulent marine environment. Crucially, we show – as in nature – that the degree of flocking depends sensitively on the shape and size of the organism.