In fact, even the desert's soil turns visibly green following the rare desert rain, as hidden filaments of photosynthesizing cyanobacteria suddenly hydrate. Lying a few millimeters deep, these primitive prokaryotes quickly glide upward, migrating en mass to the surface for an hour or so of light exposure until the dirt begins to dry. Then, just as suddenly, they return again to the subsurface, where they begin the long wait for the next rain.

The existence of such "cryptic" communities of microbes has long been known, and it has long been assumed that the organisms' behavior can be explained by common light-responsive behavior.

Now, a new finding by Arizona State University microbial ecologist Ferran Garcia-Pichel and Olivier Pringault of the Biological Oceanography Laboratory at the University of Bordeaux shows that phenomenon is actually more complicated, with significant implications for the behavior and ecology of other underground microbes. The research is reported in the September 27 issue of the journal Nature.

Observing several different species of soil crust-inhabiting cynobacteria, the team found that the bacteria's movements were affected by the presence or absence of water, not just light - the first time such behavior has ever been observed in bacteria.

According to Garcia-Pichel, the team was first intrigued by a "serendipitous" field observation. "What we discovered was that when one of these wetting events took place, the cyanobacteria came up to the surface of the soil

"But once the soil started drying out, the cyanobacteria returned to the subsurface though the light didn't change. Essentially nothing changed except the availability of water," he said.

Subsequently, the bacteria were moved to a laboratory setting and were tested under controlled lighting conditions, using microprobes to measure the relation of bacterial movement to water content in the soil surface. Test results showed clearly that the bacteria "tracked" the water.

"These migrations are really population migrations that occur in millimeter scale -- close to 100 percent of the population will come up to the surface," Garcia-Pichel noted. "Their tendency to track the water overwhelms their tendency to track the light. We've never seen this before."

Water, Garcia-Pichel hypothesizes, is critical to the bacteria not just for metabolism, but also for movement. "They go down because by tracking the water, they protect themselves. They will get dry eventually, and when they get dry they can't move. At the surface they would be more subject to hazardous conditions."

Garcia-Pichel points out that the finding may have large implications for investigating the ecology of the still poorly understood bacterial species that live deep beneath the earth's surface.

"Once traits like this are found, they're usually not restricted to one organism. We've seen this in a variety of cyanobacteria. If this really a widespread ability of bacteria, it also has implications on how we understand the bacterial communities in the deep subsurface. Bacterial communities may be following water in the subsurface over large distances," he said.

Similarly, there are implications for locating life in another extreme environment - Mars. Though cyanobacteria are among the most primitive living things, they have developed sophisticated skills for dealing with an environment where water is both scarce and transitory.

"Desert soils are one of the earthly ecosystems that may have some significance on Mars. If Mars had some water in the past, then these desiccation-resistant environments are probably going to be the last to have existed there. This is one of the most likely ecosystems to have left an imprint that we can find some evidence for," Garcia-Pichel said.

"'Follow the water' has become a productive shorthand for expressing the scientific directions of our exploration of Mars, and beyond," said Rose Grymes, Associate Director of the NASA Astrobiology Institute, of which Arizona State University is a member.

"This fascinating research contributes directly to our understanding of how living systems adapt to and impact the planetary environment, and how they leave their signature; even in places that appear highly inhospitable."

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