London: Scientists have discovered a significant number of bugs living in the middle and upper troposphere, the airy layer eight to 15 kilometres above the Earth`s surface.
The microbes could have a previously unrecognised impact on cloud formation, according to the research.
Long distance travel by the airborne organisms may also help spread infections around the world, researchers believe.
The bugs were discovered in air samples scooped up by a DC-8 aircraft flying over both land and sea across the US, Caribbean and western Atlantic.
Samples were collected at altitudes of eight to 15 kilometres before and after two major tropical hurricanes in 2010.
Scientists are still unsure whether the bacteria and fungi they found routinely inhabit the sky, living off carbon compounds, or are continually borne aloft by winds and air currents.
"We did not expect to find so many micro-organisms in the troposphere, which is considered a very difficult environment for life," lead researcher Dr Kostas Konstantinidis, from the Georgia Institute of Technology in the US, said.
"There seems to be quite a diversity of species, but not all bacteria make it into the upper atmosphere," Konstantinidis said.
DNA analysis revealed that bacteria made up 20 per cent of particles previously thought to consist of nothing but sea salt or dust.
Marine bacteria were mostly found over the ocean, while their terrestrial cousins tended to occupy the air above land.
There was strong evidence that hurricanes affected the distribution and dynamics of microbial populations.
Around 17 different families of bacteria were detected.
The bugs can help trigger cloud formation by supplying the particles around which ice crystals form.
"In the absence of dust or other materials that could provide a good nucleus for ice formation, just having a small number of these micro-organisms around could facilitate the formation of ice at these altitudes and attract surrounding moisture," said co-author Professor Athanasios Nenes, also from the Georgia Institute of Technology.
The scientists now want to look more closely at how the bugs survive so high above the ground.
"For these organisms, perhaps, the conditions may not be that harsh. I wouldn`t be surprised if there is active life and growth in clouds, but this is something we cannot say for sure now," said Konstantinidis.
The study was published in the journal Proceedings of the National Academy of Sciences.