Sulfur-oxidizing bacteria living in the still water at the bottom of salt marshes cooperate with one another to pull food in their direction faster than it would arrive through diffusion, researchers report in the Proceedings of the National Academy of Sciences.
“The only way that the bacteria can get their food is if it diffuses through their environment, but diffusion is very slow,” says Alexander Petroff, a physics fellow at the Center for Studies in Physics and Biology at The Rockefeller University. “What these bacteria have been able to do is that they’ve found a way around that problem.”
Through a mix of computation and experimentation, the authors sought to understand the underlying physics of the bacteria’s behavior.
The bacteria interact with their environment in a way that creates a convective flow. In oxygen-rich nutrient gradients, Thiovulum majus aggregate by attaching themselves to one another using mucus tethers. Then they beat their flagella to draw nutrient-laden water through the community. As the bacteria create a flow and the oxygen gradient moves, the bacteria effectively stir their environment.
But there’s more going on here, the authors found.
In these clusters, the bacteria form a surface called a veil that’s made as the cells weave together tethers. The convective flow that they create by pulling water toward them means that oxygen varies at different locations along the veil. Where oxygen levels are higher, cells nearby want to stick around while others break free to re-attach closer to the food source. In a well-observed phenomenon, the cells appear to hop randomly over the surface of the veil.
By mapping the movement of the bacteria over the veil onto a mathematical equation, the authors learned that the cellular movement process was one of diffusion.
“Interesting things are happening to a community of cells that you don’t see if you look at just one cell,” Petroff says. “If you have a bunch of things that are behaving in some way, if you look at them on the larger scale, you can see new behaviors.”