Bacterial Rainmakers Ubiquitous

By Will Parker on March 4, 2008 in News

Scientists from Montana State University and Louisiana State University have found evidence that globally distributed airborne bacteria play a pivotal role in the rain cycle. The research, published inScience, will have far reaching implications in fields such as ecology, microbiology, plant pathology and climatology.

The researchers examined precipitation from locations around the world, including California, the eastern U.S., Australia, South Africa, Morocco, France and Russia and found that the most active ice nuclei are actually biological in origin. Nuclei are the seeds around which ice is formed. Most rain begins with the formation of ice in clouds. While dust and soot can also serve as ice nuclei, only biological nuclei can cause freezing at warmer temperatures.

Montana scientist David Sands said the “bio-precipitation” cycle provides an effective method for the bacteria to spread and reproduce. “Bacteria form little groups on the surface of plants. Wind then sweeps the bacteria into the atmosphere, and ice crystals form around them. Water clumps on to the crystals, making them bigger and bigger. The ice crystals turn into rain and fall to the ground. When precipitation occurs, then, the bacteria have the opportunity to make it back down to the ground. If even one bacterium lands on a plant, it can multiply and form groups, thus causing the cycle to repeat itself. If the bacteria couldn’t cause ice to form, they couldn’t get back down to the ground. As long as it rains, the bacteria grow,” he explained.

The findings also indicate that a reduced amount of bacteria on crops could affect the climate. Because of the bio-precipitation cycle, overgrazing in a dry year could actually decrease rainfall, which could then make the next year even dryer. “Drought could be less of a problem once we understand all of this,” Sands said.

More studies must be done, though, because questions remain. For example, since the bacteria do not grow above 84 degrees, precipitation could be affected if the world’s weather continues to warm. The researchers are also examining the bacteria to find out if they vary by region.

What makes the bacterial-plant interplay more complicated is that most known ice-nucleating bacteria are actually plant pathogens. These pathogens can cause freezing injury in plants, resulting in devastating economic effects on agricultural crop yields. “Bacteria have probably been around for a million years,” Sands said. “They live on the surface of plants, and may occasionally cause plant disease. But their role in rain-making may be more important.”