One possible explanation is that the bacteria rely on the atmosphere—and rainfall—to disperse, much like plants rely on windblown pollen grains to colonize new habitats, Christner said.
For instance, an organism specialized to live on plants may become airborne, spur ice formation in clouds, and then travel back to Earth with that precipitation.
This may be an important and yet unrecognized component of a bacteria's normal life cycle, according to Christner, whose results were published in November 2008 in the journal Proceedings of the National Academy of Science.
The theory—called bioprecipitation—was pioneered by David Sands, a plant pathologist at Montana State University, in the 1980s. But little information existed on how the rainmaking bacteria moved through the atmosphere until Christner and his colleagues began their work in 2005.
Sands told National Geographic News that the critters may even employ creative means of transportation: For instance, they could "ride piggyback" on pollen or insects.
"We thought [the bacteria] were just plant pathogens [germs], but we found them in mountain lakes, in waterfalls, in Antarctica—they get around," Sands said.
Scientists still haven't identified most of the important ice nucleators in the atmosphere. For instance, a whole host of other microbes—as well as pollen grains, fungi, and other organisms—may be producing the ice nucleators detected, study author Christner added.
The vast majority of ice nucleators that are active at temperatures higher than 10 degrees Celsius (50 degrees Fahrenheit) have been found to be biological or bacterial.
Roy Rasmussen, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado, studies Earth's wintertime water cycle.
The theory that bacteria can shape the water cycle is "an interesting hypothesis, but one that is not verified, Rasmussen said in an email.
"The real issue is whether the concentration of these cells is high enough to impact precipitation formation in any significant way."
No Rain Dancing
Scientists had already suspected that cloud bacteria may be linked to plants and soils in a "feedback loop," a system of exchange between ecosystems.
In fact, these bacteria may have evolved with plants over millennia, building a dependent relationship, Sands said.
The concept also ties into Sands's ongoing study of the idea that drought cycles are connected to bacteria in clouds.
(Related: "Sun's Cycles Can Forecast Floods, Drought?" [December 10, 2008].)
For instance, if people overgraze lands, "these bacteria are without a home and can have serious consequences, possibly, for lack of rainfall," Sands said.
Simply put, a lack of vegetation may lead to a lack of bacteria, which could limit clouds' ability to shed rain.
But drought-affected farmers have alternatives, Sands said: They may be able to choose plant species that harbor more bacteria.
"It's better than rain dancing," he added.
Sands and colleagues have a network of researchers already observing bacteria in croplands in Syria, Uzbekistan, and New Zealand.
However, he said, "We're only halfway there. We haven't proven all these things yet."
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