The swishing actions of tiny swimming organisms play a key role in distributing heat and nutrients throughout the world's oceans and lakes, but these mixing effects are more complicated than we first thought. That is according to two separate research groups, based in the US and the UK, that have examined the fluid disturbances that occur in the immediate vicinity of swimming algae.

Many microorganisms have evolved to be able to move through liquids for various biological processes, including foraging for food and reproduction, and this motion acts to stir the fluids. While scientists have examined these processes at relatively large scales, there is still a lack of quantitative data on the fluid dynamics behind these processes at the microscale.

Now, a group of physicists based at the University of Cambridge, UK, led by Knut Drescher, has succeeded in taking a closer look through an experiment involving two types of common algae. The first organism, Chlamydomonas reinhardtii, is a small alga that swims by paddling a pair of whip-like flagella. The second was Volvox carteri, a larger, spherical type of algae that propels itself with thousands of flagella covering its surface.

Flapping flagella

By suspending fluorescent polystyrene microspheres in the water surrounding the algae, Dresher's team was able to trace the time-averaged water flows using a tracking microscope. The experiments revealed that Volvox carteri interact with water in a similar way to sedimenting particles acted on by gravity. "People assumed that the effects of gravity would be minimal," says Ray Goldstein, a member of the Cambridge team. "The flow field arising from the gravitational force on the organism falls off very slowly with distance, so the mutual interaction of such organisms is much stronger than without gravity," he explained.

The researchers found that Chlamydomonas reinhardtii, on the other hand, trigger more complicated flow fields in the vicinity, set up by the combined action of the cell body and its two flagella.

In a separate study, based at the Haverford College in the US, a group led by Jeffrey Guasto created a different vantage point by confining Chlamydomonas reinhardtii in a thin film of water. The researchers focused on the two-dimensional motion of a single stroke of the algae's flagella, using tracer particles and a high-speed camera, a set-up that enabled them to study the impact of individual flagella movements in more detail.

While its time-averaged results agree with the Cambridge team, Guasto's group discovered that flow fields vary significantly over the course of one complete "breaststroke". This finding suggests that the shape and scale of the fluid flows triggered by this alga may be yet more complicated. "Researchers in this field have been content with the widely accepted – but simplified – hydrodynamic models of single swimming cells that ignore near-field and time-dependent effects of swimming," says Guasto.

Impact of the individual

The next stage in this research is to explore the dynamics across a range of scales to build a more complete picture of how individual swimmers can influence the impact of large groups of swimming organisms. "The locomotion of swimming organisms contributes to the distribution of nutrients, pollutants and heat. Only recently have researchers devoted considerable attention to these effects at the scale of unicellular microorganisms," says Guasto.

One avenue the Cambridge team intends to pursue is to consider how these latest findings fit in with their previous research that explored the interactions between individual Volvox alga, as reported on physicsworld.com last year.

Howard Stone, a fluid mechanics researcher at Princeton University, is impressed by the work of both groups. "I am confident that [both sets of] measurements will become standard references in the field," he says.

Both groups have recently published their findings in Physical Review Letters.