Terrestrial microorganisms may have adapted to use clouds as a way station.

By Janet Raloff

Web edition: May 24, 2011

Scientists have been probing the role of microbes in cloud formation and precipitation, something discussed in a May 24 session at the American Society for Microbiology meeting, in New Orleans. But this bioprecipitation may not be an accident of chemistry so much as an evolutionary adaptation by certain bacteria and other nonsentient beings, argues Brent Christner of Louisiana State University in Baton Rouge.

He’s referring to the ability of certain one-celled organisms to foster the nucleation of ice crystals.

Even an airborne dust mote can serve as the surface on which water vapor condenses and freezes. But he notes that the most efficient ice nucleators — micro-particles that can catalyze freezing at the highest temperatures — are living organisms. Some bacteria, like Pseudomonas syringae, for instance, can serve as nuclei for ice formation at temperatures as warm as -2 degrees Celsius, more than 10 degrees warmer than the ice-forming limit for dust motes.

Christner now thinks it’s probably no accident that “the most active ice nucleators are biological.” As part of a survival strategy, he contends, many microbes may have evolved “to essentially piggyback on the hydrological cycle.”

The plant pathogen P. syringae “can probably be found on any plant in your back yard,” Christner says. But if winds fling this germ high enough into the air, it can be entrained by currents for up to a week or more. During that time, the fragile microbe faces a risk of deadly desiccation or irradiation by damaging solar ultraviolet rays. Safety, from its perspective, is a moist leaf back on the ground.

This bacterium can return to Earth, he says, by fostering the nucleation of moisture that will eventually rain out as liquid or frozen precipitation. (And don't worry about their getting cold along the way. Christner and other biologists have isolated live germs from precipitation — including the heart of a fallen hailstone). 

The LSU scientist's postulation of bioprecipitation as a survival tactic is certainly tantilizing. I can even picture the graphic novel story line he might offer students: wind-kidnapped microbes that turn on the synthesis of ice-nucleating proteins in hopes of skydiving back home.