25 May 2011 Last updated at 07:45 GMT
The layers representing the build-up of hailstones can be seen in cross-section
A study of hailstones has found large numbers of bacteria at their cores.
The find lends credence to the "bio-precipitation" idea, which suggests that bacteria are actively involved in stimulating precipitation.The bacteria have protein coatings that cause water to freeze at relatively warm temperatures.
Researchers at the American Society for Microbiology meeting suggest bacteria may have evolved to use the water cycle to facilitate their own dispersal.
The micro-organisms that can be found in precipitation such as snow have been studied since the 1960s.
One bacterium that has appeared in many contexts is Pseudomonas syringae, which expresses a protein on its surface that encourages an orderly arrangement of water molecules.
That in turn acts as a "nucleation" site, stimulating the formation of ice at temperatures far higher than those normally required.
So effective is P. syringae at the task that it is used in a commercially-available mixture for snow machines.
In nature, the ice that P. syringae stimulates can damage the walls of plant cells, allowing the bacterium to feed on the cells' interiors.
Only in recent years, however, has a wider role for the bacterium's strategy started to become more clear.
In 2008, Brent Christner of Louisiana State University reported finding significant numbers of bacteria in snow found around the world.
'Intriguing' 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.
P syringae bacteria are well-known "catalysts" for ice formation
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."
