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Rain- Making Bacteria found Worldwide

The same bacteria that cause frost damage on plants can help clouds to produce rain and snow. Studies on freshly fallen snow suggest that ‘bio-precipitation’ might be much more commonthan was suspected. Before a cloud can produce rain or snow, rain drops or ice particles must form.

Act as nuclei

This requires the presence of aerosols: tiny particles that serve as the nuclei for condensation. Most such particles are of mineral origin, but airborne microbes – bacteria, fungi or tiny algae – can do the job just as well. Unlike mineral aerosols, living organisms can catalyze ice formation even at temperatures close to 0 degrees Celsius.

Now a team, led by Brent Christner, a microbiologist at Louisiana State University in Baton Rouge, has managed to catalogue these rainmaking microbes by looking at fresh snow collected at various mid-and high-latitude locations in North America, Europe and Antarctica. They filtered the snow sample to remove particles, put those particles into containers of pure water, and slowly lowered the temperature, watching closely to see when the water froze.

The higher the freezing temperature of any given sample, the greater the number of nuclei and the more likely they are to be biological in nature. To tease apart these two effects, the team treated the water samples with heat or chemicals to kill any bacteria inside, and again checked the freezing temperatures of the samples.

Mostly biological

In this way they found between 4 and 120 ice nucleators per litre of melted snow. Some 69 per cent to 100 percent of these particles were probably biological. The results were published in the journal Science. The researchers were surprised to find ‘rain-making’ bacteria in all samples; the snow from Antarctica had fewer than that from France and Montana, but it still had some.

“Biological particles do seem to play a very important part in generating snowfall and rain, especially at relatively warm cloud temperatures,” says Christner. Some scientists note that this freezing ability also means that the bacteria get out of clouds and back to Earth more quickly.

Human factor

Changes in land-use, forestry and agriculture, such as expanding monoculture, change the composition of microbes in the atmosphere. As biological components seem to have a large role in how rain forms, such changes may affect rainfall and climate in many places on Earth. “It is about time for atmospheric and climate scientists to start thinking about the implications,” says Christner.

(Source: ”The Hindu” Dated: 15 May, 2008)

ENVIS CENTRE Newsletter Vol.6, No 2 Jue 2008 Back 
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