BOZEMAN (MONTANA): Walking across the
campus of Montana State University here, David Sands, a plant pathologist, says
the blanket of snow draped over the mountains around town contains a surprise.
The cause of most of it, he said, is a living organism, a bacterium, called
pseudomonas syringae.
In the last few years, Sands and other
researchers have accumulated evidence that the well-known group of bacteria,
long known to live on agricultural crops, are far more widespread and may be
part of a little-studied weather ecosystem. The principle is well accepted, but
how widespread the phenomenon is remains a matter of debate.
The
accepted precipitation model is that soot, dust and other inert things form the
nuclei for raindrops and snowflakes. Scientists have found these bacteria in
abundance on the leaves of a wide range of wild and domestic plants, including
trees and grasses, everywhere they have looked, including Montana, Morocco,
France, the Yukon and in long-buried ice of Antarctica. The bacteria have been
found in clouds and in streams and irrigation ditches. In one study of several
mountaintops here, 70% of the snow crystals examined had formed around a
bacterial nucleus.
Some of the bacteria promote freezing as a means
of attacking plants. They make proteins that will trigger freezing at higher
temperatures than usual and the resulting water ice damages the plant, giving
the bacteria access to the nutrients they need.
This ability to
promote freezing of water at higher-than-normal freezing temperatures has led
Sands and other scientists to believe the bacteria are part of an unstudied
system. After the bacteria infect plants and multiply, he says, they may be
swept as aerosols into the sky, where it seems they prompt the formation of ice
crystals (which melt as they fall to earth, causing rain) at higher temperatures
than do dust or mineral particles that also function as the nuclei of ice
crystals.
If Sands is correct about the importance of bacteria, there
would be implications for destruction of vegetation through overgrazing or
logging, which might decrease the presence of bacteria and contribute to
droughts. On the other hand, because the bacteria flourish on some plants and
are sparse on others, planting the right vegetation could enhance
rain.
The research continues. In England, experts are flying into
clouds to take samples of cloud water, and analyzing the DNA of microbes in it.
Researchers at Virginia Tech have sequenced the DNA of 126 strains of the
bacteria to create a database that could allow scientists to trace the bacteria
to their geographic origin.
|
