Bacteria often leave their hosts feeling under the weather. And even when the hosts are high-altitude parcels of air, microbes can be a source of inclement conditions, a Montana research team finds. Cloudborne bacteria might even pose climate threats by boosting the production of a greenhouse gas, another team proposes.

These data add to a growing body of evidence that biological organisms are affecting clouds, notes Anthony Prenni of Colorado State University in Fort Collins, an atmospheric scientist who did not participate in the new studies. Right now, he cautions, “We still don’t know on a global scale how important these processes are.” But research into microbial impacts on weather and climate is really heating up, he adds, so “within a few years, I think we’re going to have a much better handle on it.”

Alexander Michaud’s new research was triggered by a June storm that pummeled Montana State University’s campus in Bozeman last year with golf-ball–sized and larger hailstones. The microbial ecologist normally studies subglacial aquatic environments in Antarctica. But after saving 27 of the hailstones, he says, “I suddenly realized, no one had really ever thought about studying hailstones — in a layered sense — for biology.”

So his team dissected the icy balls, along with hundreds of smaller ones collected during a July hail storm south of campus. Michaud now reports finding germs throughout, with the highest concentrations by far — some 1,000 cells per milliliter of meltwater — in the hailstones’ cores.

Since at least the 1980s, scientists have argued that some share of clouds, and their precipitation, likely traces to microbes. Their reasoning: Strong winds can loft germs many kilometers into the sky. And since the 1970s, agricultural scientists have recognized that certain compounds made by microbes serve as efficient water magnets around which ice crystals can form at relatively high temperatures — occasionally leading to frost devastation of crops.

In 2008, Brent Christner of Louisiana State University in Baton Rouge and his colleagues reported isolating ice-nucleating bacteria from rain and snow. A year later, Prenni’s group found microbes associated with at least a third of the cloud ice-crystals they sampled at an altitude of 8 km.

“But finding ice-nucleating bacteria in snow or hail is very different from saying they were responsible for the ice,” says Noah Fierer of the University of Colorado at Boulder. “I say that,” he admits, “even though as a microbiologist, I’d love to believe that bacteria control weather.”

Pure water molecules won’t freeze in air at temperatures above about minus 40 degrees Celsius [minus 40 F], Christner notes. Add tiny motes of mineral dust or clay, and water droplets may coalesce around them — or nucleate — at perhaps minus 15 C [5 F]. But certain bacteria can catalyze ice nucleation at even minus 2 C [28 F], he reported at the meeting in New Orleans.

Through chemical techniques, Michaud’s group determined that the ice nucleation in their hail occurred around minus 11.5 C [11.3 F] for the June hailstones and at roughly minus 8.5 C [16.7 F] for the July stones.

Michaud’s data on the role of microbes in precipitation “is pretty strong evidence,” Prenni says.

Also at the meeting, Pierre Amato of Clermont University in Clermont-Ferrand, France, reported biological activity in materials sampled from a cloud at an altitude of 1,500 meters. The air hosted many organic pollutants, including formaldehyde, acetate and oxalate. Sunlight can break these down to carbon dioxide, a greenhouse gas, something Amato’s group confirmed in the lab. But sunlight didn’t fully degrade some organics unless microbes were also present.

Moreover, certain cloudborne bacteria — the French team identified at least 17 types — degraded organic pollutants to carbon dioxide at least as efficiently as the sun did. Amato’s team reported these findings online Feb. 9 in Atmospheric Chemistry and Physics Discussions.

This microbial transformation of pollutants to carbon dioxide occurs even in darkness. Amato has calculated the total nighttime microbial production of carbon dioxide in clouds and pegs it “on the order of 1 million tons per year.” Though not a huge sum — equal to the carbon dioxide from perhaps 180,000 cars per year — he cautions that this amount could increase based on airborne pollutant levels, temperatures and microbial populations.

Image: Three adjacent ice crystals (borders resemble forked road) contain green-stained Pseudomonas syringae bacteria isolated from precipitation. This plant pathogen, one of the most efficient bacteria at nucleating ice, is commonly found in clouds. (Brent Christner/LSU)