Bacteria live and grow in clouds, where they may trigger rainfall and alter climate. The discovery was made by a team of Austrian researchers, in clouds over the Alps.

‘We were astonished to find actively growing bacteria,’ says Birgitt Sattler of the University of Innsbruck. She says that although bacteria are known to be blown high into the atmosphere and dispersed around the Earth, no one knew if they could actually grow and divide in clouds. ‘The relatively clean and cold atmosphere of high altitudes was not regarded as a suitable place for bacterial growth.’

Yet bacteria do grow there, as Sattler found when the team looked at samples of a cloud that passed over a meteorological station on top of Mount Sonnblick, near Salzburg. Water droplets in the cloud were frozen onto Teflon plates then melted in the lab and monitored at the low temperatures found in clouds.

Each millilitre of meltwater contained around 1500 bacteria-some round, some rod-shaped and some shaped like long filaments. The researchers have not yet identified the bacteria, but they were definitely alive and well, using up traceable radioactive versions of thymidine (a component of DNA) and an amino acid called leucine to make new DNA and proteins.

Because the bacteria multiply over a time scale of several days-shorter than the typical lifetime of a cloud-they almost certainly reproduced inside the cloud. ‘Clouds should be considered as a microbial habitat,’ Sattler concludes. Her results will appear in a future issue of Geophysical Research Letters.

The next step, says Sattler, is to analyse the DNA of the bacteria to find out exactly which species are present. ‘We have so far just proved that there is life up there and that it can reproduce,’ she says. ‘Now we want to know who is up there.’

The bacteria must be able to survive sub-zero temperatures, intense ultraviolet radiation and limited nutrients. Identifying them will reveal whether they originate from plants, surface water or soils. Sattler also plans to find out what they are living on, and what they are producing.

Atmospheric chemist Daniel Jacob of Harvard University says that although the total amount of organic carbon compounds made by the bacteria is likely to be small, they could still have significant effects on climate. ‘Most hydrocarbons produced at the surface have a relatively short lifetime-they don’t affect the atmosphere beyond the lower troposphere,’ he says. ‘This could be a mechanism to deliver organic compounds to the upper atmosphere.’ He thinks the bacteria could be making oxygen-containing compounds called carbonyls, which react with light, forming free radicals and driving the production of ozone. ‘The origin of carbonyls in the atmosphere was something we couldn’t explain,’ he says.

Tim Lenton from the Centre of Ecology and Hydrology in Edinburgh is excited to hear that bacteria are active in the clouds, as it supports an idea he put forward in 1998. Lenton, along with the late Bill Hamilton of Oxford University, suggested that microorganisms alter climate by manipulating cloud formation and rainfall (New Scientist, 30 May 1998, p 28). This would help them disperse around the planet.

Lenton believes bacteria do this by seeding clouds with ice crystals, which grow into raindrops. Some species, such as Pseudomonas syringae, grow ice crystals around themselves by exuding chemicals that encourage supercooled water to freeze. These plant pathogens may have evolved this ability to promote frost damage on leaves, but colonies living in clouds could do the same. ‘If the bacteria are active they are more likely to be a cause of precipitation,’ says Lenton.

Author: Joanna Marchant

Notes for Editors


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