Algae shows how to move in fluids

by Mike Miller

Mathematicians are unravelling a mystery that has baffled naturalists for more than a century. The puzzle concerns the way that populations of swimming microorganisms spontaneously form intricate and often beautiful patterns, a phenomenon called bioconvection.

Algae or bacteria, if left to stand in a fluid, can aggregate into patterns such as discrete columns, dense honeycombs, or a maze of intertwining projections, rather like the surface of the brain. To understand how these organisms move and aggregate is of more than aesthetic interest. Around half the world’s biomass consists of microorganisms that live in water and play a central role in the food chain and the carbon cycle.

At the University of Leeds, Dr Nick Hill and Professor Tim Pedley are developing mathematical models to describe the movement of the organisms. By taking into account factors such as gravity, the velocity of the swimming organisms and the viscous forces due to the surrounding fluid, the researchers can derive complex equations that explain how the patterns form.

The ultimate aim of the research is to develop computer programs that can predict what patterns will form under a given set of circumstances.

The pattern-forming microorganisms all have two things in common: they swim by thrashing tiny threads called flagella and they are denser than water. One bioconvection system that the Leeds team is studying is where the microbes tend to swim upwards, gradually forming a dense layer at the surface. Eventually the layer becomes unstable and starts to collapse at various locations. As blobs of organisms fall downwards, they displace the fluid surrounding them, creating a slow-motion swirling. This in turn draws in nearby organisms, displacing more fluid. Ultimately, this complex fluid motion is reflected in the bioconvection pattern.

According to Dr Hill, the ability to understand the movement of particles in a fluid could have wider implications in a variety of fields, from industry to medicine