Arizona State University researchers have discovered for the first time that temperature determines where key soil microbes can thrive -- microbes that are critical to forming topsoil（表层土） crusts in arid lands. And of concern, the scientists predict that in as little as 50 years, global warming may push some of these microbes out of their present stronghold in colder U.S. deserts, with unknown consequences to soil fertility and erosion. The findings are featured as the cover story of the June 28 edition of the journal Science.

An international research team led by Ferran Garcia-Pichel, microbiologist and professor with ASU's School of Life Sciences, conducted continental-scale surveys of the microbial communities that live in soil crusts. The scientists collected crust samples from Oregon to New Mexico, and Utah to California and studied them by sequencing their microbial DNA.

While there are thousands of microbe species in just one pinch of crust, two cyanobacteria（蓝藻细菌） -- bacteria capable of photosynthesis -- were found to be the most common. Without cyanobacteria, the other microbes in the crust could not exist, as every other species depends on them for food and energy.

"We wanted to know which microbes are where in the crust and whether they displayed geographic distribution patterns at the continental scale," said Garcia-Pichel, also dean of natural sciences in ASU's College of Liberal Arts and Sciences. "To our surprise, where we thought a single cyanobacterium would dominate, we found that two had neatly split the territory between themselves. We used to think that one, called Microcoleus vaginatus, was the most important and dominant, but now we know that Microcoleus steenstrupii, the other one, is just as important, particularly in warmer climates," he added.

While the two look very much alike, M. vaginatus and M. steenstrupii are not even closely related. They have evolved to appear alike because their shape and behavior help them stabilize soil and form soil crusts.

Crusts are crucial to the ecological health of arid lands, as they protect the soil from erosion and contribute to land fertility by fixing carbon and nitrogen into the soil and by extracting other nutrients from trapped dust.

Temperature affects microbial communities After considering data about soil types and chemistry, rainfall, climate and temperature, researchers used a mathematical model that showed temperature best explained the geographic separation of the two microbes. While both are found throughout the studied area, M. vaginatus dominate the crusts in cooler deserts and M. steenstrupii are more prevalent in the southern deserts.

"But this was just a correlation," Garcia-Pichel explained. "To prove the role temperature plays, we tested cultivated forms of the microbes and confirmed that it does indeed make a difference -- temperature is what keeps them apart. The point now is that temperature is no longer stable because of global warming."