- Scientists in America believe that clouds have a dark
secret: they're created by viruses and bacteria as a means of global transport.
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- Do bugs control our weather? Can viruses travel thousands
of miles on the winds? Is there a whole ecosystem up in the clouds that
we have not discovered? The answer to all three questions could be yes,
according to scientists who are exploring the microbial metropolises in
the skies.
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- There is, they say, growing evidence that bacteria, fungal
spores and viruses may spend large amounts of time - even their entire
lives - in the air, riding clouds across the planet. And they don't just
inhabit the clouds - they may also be creating them. Certainly, many of
the clouds' newly discovered inhabitants are exquisitely designed to create
the maximum number of ice crystals, the basic building-blocks of clouds.
Some Darwinian biologists even argue that the bugs may have evolved for
that very job.
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- "The ecology of the atmosphere is one of the last
great frontiers of biological exploration on Earth," says Bruce Moffett
of the University of East London. Within the next year, he hopes to conduct
the first systematic bug-hunt in the clouds above Britain.
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- Until recently, nobody believed that bacteria and viruses
spent much more time in the air than it takes to sneeze on your neighbour.
Scientists assumed that if the material got caught up in the winds, it
would quickly be killed by ultraviolet radiation from the sun.
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- But Gene Shinn of the US Geological Survey in St Petersburg,
Florida, who has examined their airborne lifestyle in detail, says that
the bacteria seem to protect themselves from harmful rays by becoming attached
to dust particles. In dust clouds, the amount of UV radiation will be lower
than in "normal" situations. And one of Shinn's USGS colleagues,
Dale Griffin, suggests that bacteria might survive even longer if they
get into cracks in the particles. They can survive travelling long distances,
and spread disease on arrival.
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- Shinn has discovered that bacteria and fungi carried
aloft on dust storms coming out of the Sahel region of West Africa can
journey across the Atlantic in large numbers. So far, he has isolated more
than 130 species of African bacteria and fungal spores over the Caribbean.
Not only that, he says that they are probably responsible for a series
of dramatic epidemics among Caribbean coral reefs in recent years.
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- One example is an African soil fungus called Aspergillus
sydowii. It was first spotted in the Caribbean in 1983. That was a year
of intense African drought. Huge clouds of dust billowed into the upper
atmosphere and travelled west on the trade winds, forming a dense haze
over the waters of the Caribbean. Since those clouds brought A. sydowii,
says Shinn, the fungus has killed more than 90 percent of the region's
sea fans - a form of soft coral. "Much of the decline in coral reefs
in the Caribbean in recent years seems to be a result of pathogens transported
in dust from Africa," says Shinn.
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- Last year, Griffin dramatically raised the stakes when
he suggested that the epidemic of foot-and-mouth disease in Britain in
2001 may have arrived on winds from Africa. He noticed that the first case
of the disease was reported in February 2001, just a week after satellite
pictures had shown a huge dust storm carrying sand from the Sahara to Britain.
Saharan cattle are known to carry the same strain of the virus as turned
up on British farms. The evidence is purely circumstantial. But it is not
impossible, virologists admit. A previous outbreak of foot-and-mouth in
Britain was traced to the virus blowing across the English Channel from
France. So why not a longer journey?
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- Some researchers believe that bugs do more than hitch
a ride in clouds. They may make the clouds, too. It turns out that many
cloud-inhabiting bacteria are brilliantly designed for cloud-seeding: that
is, for triggering the formation of ice crystals around which water vapour
coalesces to create water droplets. They do this by producing a protein
that mimics the shape of an ice crystal's surface, which could help growth
to get started.
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- Many bacteria seem to be able to form ice crystals, but
the best equipped appears to be Pseudomonas syringae, which commonly grows
on plant matter, aiding the decomposition process. A single gram containing
a million bacteria could theoretically produce up to a million ice crystals.
It can trigger the formation of ice at temperatures of 13C, higher than
other "ice nucleators". This ability is so well known that the
bacteria are sometimes added to the water put into snow-making machines
at Alpine ski resorts. In the atmosphere, the bacteria create clouds.
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- All this raises some questions, which Moffett hopes to
answer. "We want to discover if it is true that microbes play an active
role in forming clouds and making rain," he says. "In other words,
whether there is an active, self-sustaining ecosystem up there."
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- One intriguing piece of evidence - barely noticed by
scientists at the time - came in research done 20 years ago by Russell
Schnell of the University of Colorado. Trying to find out why western Kenya
had so many hailstorms, he stumbled on the fact that most of the hailstones
there contained at their heart a scrap of P. syringae.
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- How do the bugs get into the atmosphere in the first
place? On land, one major route is in smoke from forest and bush fires.
Another is dust storms. Schnell reckoned the bacteria in his hail stones
were stirred up by the feet of pickers in the region's tea plantations.
On the oceans, tiny bacteria and plankton on the water's surface may gain
lift-off after getting caught in the air bubbles of whitecaps.
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- And why would bacteria have developed ice-making skills?
Or, to put it more correctly, what is the selective advantage in Darwinist
terms for bacteria to carry genes that trigger ice-making? This is the
million-dollar Darwinian question.
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- Most researchers believe that the skill first developed
on the ground, to make frost that decomposes leaf litter - thus providing
the bacteria with food. But why would bacteria living in the tropics retain
ice-creating skills when temperatures are generally too high for frost
formation? The answer could be in the clouds, where temperatures are cold
enough for ice formation. What evolutionary benefit might the bacteria
gain from this? One argument is that the resulting rain helps plants grow,
and makes more leaves for bacteria down below to eat.
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- But there may be another Darwinian purpose, says Tim
Linton of the Centre for Ecology and Hydrology in Edinburgh. Clouds are
an efficient transportation system for the bacteria to spread themselves
across the planet. Linton and the late William Hamilton, one of the world's
leading evolutionary theorists, have suggested that cloud formation allows
the bacteria to travel farther and to be "rained out" back on
to the ground.
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- Much of this is pretty speculative. But whatever the
possible motives for bacteria to make clouds, one practical spin-off of
their skills is that scientists might develop more efficient "organic"
methods of seeding clouds, using bacteria instead of chemicals. That is
one reason why agriculturalists and military strategists may be taking
notice when, probably later this year, Moffett takes a device rather like
a vacuum cleaner into the skies to capture and investigate the unknown
ecology in the clouds over England. Hopefully, this experiment will answer
many of the questions raised by the intriguing notion of ice-generating
genes and bacteria.
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- © 2003 Independent Digital (UK) Ltd
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- http://news.independent.co.uk/world/science_medical/story.jsp?story=435466
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