Like commuters shuttling to and from the city every day, some bacteria living in the desert clamber up and down through the soil in response to water. Until now, scientists had never seen microorganisms make such migrations, which have implications for understanding life deep in the earth, as well as hunting for past life on Mars.

After observing the blooming of cyanobacteriaancient photosynthetic microbesin the field, Ferran Garcia-Pichel of Arizona State University and a colleague, who publish in todays Nature, took a sample of soil rich with these bugs from the badlands of Spain back to the lab. By comparing the intensity of different wavelengths of light reflected by the soil, they found that nearly 100 percent of these green bacteria rise millimeters upward to the surface, even in darkness, to reach water therethough intense light can make them slow or stop. When the moisture is gone, they retreat below. "Their tendency to track the water overwhelms their tendency to track the light. Weve never seen this before," Garcia-Pichel says. This descent seems to allow the cyanobacteria to survive erosion and drought.

If such behavior is widespread, then bacterial communities living deep beneath the earths surface could be following the movements of underground water across long distances, Garcia-Pichel says. And if there was once microbial life and water on the now arid Martian surface, then water-seeking communities might have emerged there as the water dried up. "This is one of the most likely ecosystems to have left an imprint that we can find some evidence for," he concludes.