WE HAVE all heard tales of frogs, toads and fish raining from the heavens: these are rare events triggered by freak weather. But there are land and water-based life forms that seem to be present in the atmosphere just about all the time. These include algae, fungi and bacteria. What are they doing up there?
The late, great evolutionary biologist William Hamilton thought about this from the microbes鈥� point of view. In 1998, he argued that they were probably using the atmosphere as a dispersal medium (). He had previously shown that dispersal is third on the list of priorities for an organism, after survival and reproduction, because the benefits of colonising a new area are so great.
For lightweight bacteria, then, the problem is not getting up into the atmosphere but getting down. Hamilton proposed that some bacteria do this by acting as ice-nucleation centres, which are essential in the creation of raindrops and snowflakes. Since 1998, replicating bacteria have been found in clouds and this week we hear that snow from different continents is full of ice-nucleating proteins from bacteria (see 鈥淏acteria make snow and rain fall鈥�).
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There is one part of Hamilton鈥檚 idea left to be tested. He suggested that microbes may play a key role in making our weather, and even form part of the global biofeedback that seems to keep the planet healthy 鈥� what is called the Gaia hypothesis. This is highly controversial yet leads to the most intriguing questions. Could a better understanding of this role improve forecasting, and could we use that knowledge to 鈥渕ake deserts bloom鈥�? We await further forays into aerobiology with anticipation.