I recently read an article on the role of bacteria in precipitation--that is, causing ice crystals to form in clouds, producing snow and/or rain. I can't find the article I read, but I found a couple others.
First, this article from Wired in 2008.
And this article from the BBC last year. It explains:
Now, Alexander Michaud of Montana State University has added to the idea, having collected hailstones on the university campus following a major hailstorm in 2010.
He analysed the hailstones' multi-layer structure, finding that while their outer layers had relatively few bacteria, the cores contained high concentrations.
"You have a high concentration of 'culturable' bacteria in the centres, on the order of thousands per millilitre of meltwater," he told the meeting.
The bacteria are known to gather together in "biofilms" on the plant surfaces and can form bacteria-rich aerosols in forest canopies - aerosols that can rise on updraughts, eventually stimulating precipitation in clouds at temperatures far higher than would be required if soot or dust served as the nucleation sites.
Dr Christner, also present at the meeting, said the result was another in favour of the bio-precipitation idea - that the bacteria's rise into clouds, stimulation of precipitation, and return to ground level may have evolved as a dispersal mechanism.
"It's an interesting idea that's been thrown around for decades but only recently has the data accumulated to support it," he told the meeting.
"As a microbiologist, this idea that... an organism could piggy-back on the water cycle I find just intriguing.
"We know that biology influences climate in some way, but directly in such a way as this is not only fascinating but also very important."
And, finally, an article from a Discovery Magazine blog, also from last year.