7.10 Living snowflakes: bacteria important to snow formation
By Jonathan M. Gitlin |
Published: March 02, 2008 - 07:30PM CT
With winter still in effect,
the enthusiasm with which one greets fresh snowfall has tended to diminish.
Every snowflake may be unique and beautiful, but when millions of them are
standing between you and your car, beauty can be ignored. Now a team of
scientists has taken a closer look at the particulates that cause snow to form,
and it seems that most of them are... alive.
Ice formation in the
troposphere doesn't tend to happen on its own if the temperatures are higher
than -40˚C (coincidentally -40˚F), but biological particulates can catalyze ice
formation at higher temperatures by acting as a nucleus. A new study, published
in Science, examines samples
of fresh snowfall from sites in Montana, France, the Yukon, and Antarctica in
order to get a better idea as to the makeup of those ice nucleators.
Compared to remote sites,
snow samples from Montana and France contained higher concentrations of
particulates--more than 100 per liter. Antarctica, by contrast, averaged just
four parts per liter. Biological cells made up around one percent of these
particulates, with Pseudomonas syringae
being the most common bacteria. DNA-stained cells of P. syringae
(green dots) frozen within individual ice crystals. Image courtesy of Shawn
Doyle and Brent Christner, Louisiana State University
Exposing the particulates to
lysozymes—enzymes that disrupt cell walls—or 95˚C heat reduced their ability to
act as ice nucleators. Both of these treatments would affect living cells but
not inorganic particulates such as dust, sand or soot.
One of the authors of the
study, Prof. David Sands, postulates that forming rain or snow is part of the
bacterial life cycle; "We think if (the bacteria) couldn't cause ice to
form, they couldn't get back down to the ground," Sands said. "As
long as it rains, the bacteria grow."
Sands suggests that changing
bacterial populations may affect rainfall; for example, overgrazing during a
summer could reduce the bacterial population, resulting in lower raindrop
formation, although more work would need to be done to firm up this theory.
Regardless, it is an interesting glimpse into the world of the snowflake.
Science,
2008. DOI: 10.1126/science.1149757
