The colonies look like dried-up pond scum, and their green color comes from the chlorophyll inside a photosynthetic bacterium known as Chroococcidiopsis (pronounced crow-oh-*****-sid-ee-op-sis).

But not every rock is colonized. Warren-Rhodes wanted to understand why. She wanted to find out if, at the microbial scale, life is patchy.

The science of ecology has shown that patchiness, or clustering of organisms, is a common feature of living communities. But ecological studies have focused on plants and animals, life forms that are easy to see. Many scientists discount the notion that these macro-scale findings can be applied to microbial life. "The previous understanding," says Warren-Rhodes, "has been that because bacteria are light, and they can be moved by wind all over the place," they would colonize a landscape randomly. But Warren-Rhodes's work has led her to a different conclusion.

She began her search at Yungay, in Chile's Atacama Desert. She turned over several thousand rocks, but found only three - yes, three - that were colonized. It's unclear why Yungay is so utterly barren, but it may be because the vast majority of the rocks there are dark in color. Chroococcidiopsis favors translucent white rocks, such as quartz: the sunlight needed for photosynthesis can pass through them, but they block harmful UV radiation.

She then turned her attention to the deserts of northwest China, where white rocks are more prevalent. After turning over tens of thousands more rocks, she found that where rain is extremely scarce bacterial colonies favored not only white rocks, but also large rocks. In these hyperarid locations, almost no rocks smaller than 2 cm (about 1 inch) across were colonized, and the larger a rock was, the more likely it was to host a colony.

Warren-Rhodes speculates that large rocks concentrate moisture by directing individual drops of water into the surrounding soil. "We think of the tops of these rocks sticking out of the soil as water collectors." The leaves of plants perform a similar function.

Her most significant discovery, however, was that colonized rocks didn't appear to be distributed randomly, but rather to be clustered together. Many of her colleagues remained skeptical, suggesting that perhaps it was really the large white rocks that were clustered together, and that the bacterial clusters were merely echoing this rock pattern.

So Warren-Rhodes returned to the Atacama, this time to Aguas Calientes. Like Yungay, Aguas Calientes is hyperarid, but it has more white rocks - and more colonized rocks. She picked a 3-by-6-meter (10-by-20-foot) area, and turned over every single rock within it, a total of nearly one thousand rocks. Given that she had already examined a career total of tens of thousands, a mere thousand more might seem like child's play. But this time she not only checked the rocks for colonies, she also recorded their sizes, their colors, their precise locations, and, for colonized rocks, the side of the rock where the colony was present.

This detailed information enabled her to use statistical modeling techniques developed by plant ecologists to demonstrate that the bacterial colonies displayed a patchiness that was independent of the rock pattern. Even though the large white rocks are often clumped together, she says, "the organisms are even more clumped together."

Once an individual rock becomes colonized, she suspects, subsequent rainfall or fog events occasionally carry bacteria to nearby rocks, where new colonies get established. Warren-Rhodes also noticed that the soil around the colonized rocks was cemented, holding the rocks tightly in place, while the soil around nearby uncolonized rocks was loose. "I literally sometimes have to take a hammer to break the rock out of the soil, it's so cemented. But if you go a few feet over, when it's not colonized, they're very loose and you can pick them right up," she says.

The bacteria, it seemed, were engaged in a bit of environmental bio-engineering, releasing polysaccharides, a mucus-like substance that, Warren-Rhodes believes, promotes colony growth by preventing soil erosion and by acting as a sponge to concentrate and retain water. "It's all about conserving and capturing and retaining scarce water. The organisms themselves use mechanisms to create positive feedback, so that the more of them that are there, the more water they capture, the more they grow, the more water they capture, in this snowball effect," she says.

And there was one more thing: most of the colonies at Aguas Calientes appeared on the southeast sides of the colonized rocks. This could be due to micro-scale climatic effects, such as sunlight or prevailing winds that cause scarce moisture to evaporate more readily on one side of the rocks than the other. She plans to do additional work, wiring up rocks with temperature and humidity sensors, to tease out the environmental factors that affect moisture at the centimeter scale.

Long-term, Warren-Rhodes believes microbial ecology research like hers will prove useful in the search for life on Mars. Although she thinks it unlikely that environments like the ones she is currently studying exist on Mars - the martian surface is too harsh for even the robust Chroococcidiopsis to survive there - she hopes the microbial ecology techniques that she and her colleagues are developing will help focus the search for life in more-promising martian environments, such as the polar permafrost.

A random search of martian terrain, she argues, would be needlessly inefficient. "If you go with the assumption that life is random," she says, "then that's flying in the face of what life on Earth is telling you."

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