Sand is not as passive as you might think. Down on the ocean floor, it’s filtering seawater and, thanks to a teeming set of microbes living on it, busily processing nutrients from the constant rain of sediment drifting downward. Microbiologists have studied the tiny inhabitants of sand by analyzing bulk DNA extracted from scoops of ocean floor, but while this can give you a general list of the bugs there, it doesn’t explain how they live with each other—which ones are neighbors, which ones are competitors, which grow together, what they are up to.
To start to get at that question, microbiologists recently used DNA to identify the bacteria living on 17 individual grains of sand from the seafloor near the German resort island of Helgoland and painstakingly examined hundreds of other grains with microscopes to see the creatures’ living arrangements. The results, which were published last month in The ISME Journal, provide a detailed picture of the ecology of each grain of sand, revealing the cities, villages, and hinterlands of these miniature worlds.
One notable observation is that bacteria don’t cover sand grains evenly. They are gathered together in the pockmarks and protected ravines of the surface, avoiding the exposed, open plains. What’s more, they tend to grow side by side, rather than on top of each other, arranged more like a tidy village than a series of high-rises. This may be a result of the constant swishing of water through the sand keeping layers from building up, says David Probandt, a microbiologist at the Max Planck Institute for Marine Microbiology who helped lead the work.