Bacteria In Atmosphere May Play Key Role In Hail, Rain, Snow

 After finding high concentrations of bacteria inside hailstones, researchers suggest there is mounting evidence that atmospheric microorganisms play a key role in how water molecules aggregate in hail, rain, snow and other weather events.

At the 111th General Meeting of the American Society for Microbiology (ASM) in New Orleans this week, Alexander Michaud of Montana State University in Bozeman presented his team's latest findings in the new and growing field of "bioprecipitation", where researchers investigate the extent to which bacteria and other microorganisms influence weather events.
In his presentation on Tuesday, Michaud spoke about how he and his group found a high concentration of bacteria in the center of hailstones. The center of the hailstone is the first part to develop, the "embryo":
"The embryo is a snapshot of what was involved with the event that initiated growth of the hailstone," explained Michaud.
He said water molecules need a nucleating particle to aggregate around and this is what leads to precipitation in the form of rain, snow and hail.
"There is growing evidence that these nuclei can be bacteria or other biological particles," Michaud added.
He and his team looked at hailstones bigger than 5 cm in diameter that fell in the University campus during a hailstorm in June 2010.
They analyzed the meltwater from four layers in each hailstone and found that the inner cores contained the highest number of live bacteria, as proved by them being able to grow on in cultures.
The term "bioprecipitation" was first coined in the early 1980s by David Sands, a professor and plant pathologist at Montana State University. It is now a growing field, where researchers investigate how ice clouds form, and how bacteria and other microorganisms contribute to this by providing ice nuclei (IN), particles that the ice crystals can form around.
Brent Christner of Louisiana State University, and colleagues are also working in this field. He too gave a presentation at the ASM general meeting, addressing some key questions about the nature and role of biological ice nucleators in the atmosphere.
He said that as temperatures in clouds reach higher than -40 degrees Celsius, ice does not form spontaneously:
"Aerosols in clouds play key roles in the processes leading to precipitation due to their ability to serve as sites for ice nucleation."
Christner explained that while many different types of particle can serve as ice nucleators, the most active naturally occurring ones are biological, capable of catalyzing ice formations at around -2 degrees Celsius.
The most well studied of these biological IN is the plant pathogen Pseudomonas syringae, which gardeners who have seen frost damage on their outdoor tomatoes will recognize as the bacterial specks on the fruits.
"Ice nucleating strains of P. syringae possess a gene that encodes a protein in their outer membrane that binds water molecules in an ordered arrangement, providing a very efficient nucleating template that enhances ice crystal formation," explained Christner.
From using computer models to simulate the conditions of aerosol clouds, researchers believe high concentrations of biological IN may influence many events in the Earth's atmosphere, such as the size and concentration of ice crystals inside clouds, cloud coverage, the amount of rain, snow, hail that falls to the ground, and even helping to insulate against solar radiation.
Given the amount of IN in the atmosphere and the warm temperatures at which they function, Christner said the evidence is mounting in favor of the view that "biological IN may play a role in the Earth's hydrological cycle and radiative balance".

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