Cool characters: Pseudomonas syringae bacterial cells trapped within an ice crystal lattice that was formed in the laboratory from a diluted culture.

Credit: Image courtesy of Shawn Doyle and Brent Christner, Louisiana State University

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CHICAGO: Biological organisms play a significant role in the formation of rain and snow, according to a study published in the journal Science.

The discovery of these organisms and their importance in the water cycle could help researchers improve climate forecasts and better understand the relationship between the biosphere and climate, the authors concluded.

They could also one day be used to bring rain to dry areas, said lead author Brent Christner of Louisiana State University.

Rain formers

Scientists have long known that the ice crystals in clouds which become rain or snow need to cling to some kind of particle, called ice nucleators, in order to form in temperatures above -40 ºC.

But they did not realise, until now, that the most active particles involved in this process are living ones, Christner said.

"Every snow and ice sample we've looked at, we found biological ice nucleators," he said in a telephone interview."Here's a component that has been completely ignored to date."

Biological ice nucleators were first discovered about 40 years ago by researchers trying to determine why some plants were damaged by frost and others were not.

They discovered that the plants which froze were covered in bacterial plant pathogens which are able to capture moisture in the air and turn it into ice at temperatures as warm as -2 ºC. Mineral ice nucleators like dust and soot can only capture and freeze the moisture at temperatures below minus about 10 ºC.

Global gradient

"It means that when cloud temperatures are in the range where inorganic matter are not active, these biological ice nucleators are active," Christner said.

Christner and his team have not yet determined how much precipitation is formed with these organisms. But they found a stronger presence in snow collected from Montana and France than that collected in northern Canada and Antarctica.

These particles can be released by the Sun's radiation or in the dust kicked up when farmers till the fields or even step on plants. They are then carried up into the atmosphere by wind. The fact that they were discovered in Antarctic snow samples shows that they can be carried great distances, Christner said.