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Saturday, March 1, 2008

Bacteria make it rain

Airborne Bacteria Make It Rain, Researchers Find

By Brandon Keim Email 02.29.08 | 4:45 PM
The green dots indicate Pseudomonas syringae bacteria suspended in ice. Like other so-called biological ice nucleators, P. syringae gives water vapor a place to meet, join and form ice crystals that later fall to Earth.
Brent Christner/Louisiana State University

The sky is not an ethereal, sterile realm. It's teeming with bacteria, and scientists say that the microbes play a powerful role in producing rain and snow.

While the idea that bacteria could prompt precipitation was previously known, a paper published this week in Science shows that they're more important than anyone expected.

Researchers led by Louisiana State University microbiologist Brent Christner analyzed snow samples from around the world, categorizing the content of their "nucleators" -- tiny particles that help water vapor coalesce and freeze.

All snow and most rain begins as ice. Though water is widely thought to have a freezing point of zero degrees Celsius, it's not so simple in the clouds, where pristine vapors only bind to form ice crystals at exceedingly cold temperatures. Nucleators let crystallization happen in the less extreme conditions that prevail in much of Earth's troposphere.

Christner found bacteria, technically known as "biological ice nucleators," in an atmospheric context. High levels of bacteria were present in nearly every sample.

"Atmospheric scientists haven't previously recognized that these particles are so widely distributed," he said.

The findings raise the question of how climate change and human activities will affect bacterial balances in the sky. More immediately, they're a starting point for research on bacterial contributions to cloud formation and precipitation.

In its latest report, the International Panel on Climate Change said that the impact of feedback loops involving clouds on global weather patterns are the "largest source of uncertainty" in current predictions of climate change.

Christner's findings won't overturn the IPCC's fundamental conclusions -- a high probability of dramatically rising global temperatures -- but they should spur research that will help scientists predict the changes in greater detail, said Princeton University climate scientist Leo Donner, who was not involved in the study.

Donner agreed that climate scientists had not appreciated the ubiquity of precipitation-causing bacteria in the atmosphere.

"One of the real uncertainties in the climate system is how cloud particles are nucleated," he said. "Climate models need information on nucleators. This is especially relevant for understanding how clouds change as atmospheric composition changes."

The fact that bacteria could cause snow and rain was discovered almost by accident in the 1970s by study co-author David Sands, a Montana State University plant pathologist, during his research on Pseudomonas syringae, a microbe that causes ice to form on leaves.

Unable to discover the source of repeatedly infected fields, Sands exasperatedly took to the skies. He did the scientific equivalent of dragging a cup through the clouds -- and lo and behold, there was P. syringae.

P. syringae is not the only biological ice nucleator, but it is the most common, and all varieties share a protein structure that provides a scaffold for free-floating water molecules. Once bound to the bacteria and to each other, the water vapors are able to freeze, and eventually fall back to Earth.

In a pure state, water vapors freeze at temperatures below -35 degrees Celsius. Nucleators allow this to happen in warmer conditions, and Christner's study found that bacteria are the most common warm-temperature nucleators of all.

Researchers never realized bacteria could be so widespread in the clouds, said Christner, because the technologies used to measure fine dust -- traditionally seen as the most important nucleator -- ignore microbe-sized particles.

"It's not that these atmospheric scientists are idiots -- they're not," he said. "But biological nucleators were not previously recognized as being that abundant or important. They're going to have to revise that."

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