A widely despised plant pathogen may play a vital role in the formation of rain and snow -- and had ecoterrorists not fought a genetically modified version of the bacteria, unforeseen weather shifts might have followed.

That's one interpretation of a sequence of events described to me by Brent Christner, a Louisiana State Microbiologist who studies the interactions of bacteria and clouds.

Most rain and snow begins as cloud-borne ice crystals, but contrary to popular belief, pure water vapors don't freeze at zero degrees Celsius. Their real freezing point is closer to -35 degrees. Nucleators -- tiny particles that give the water molecules a convenient place to coalesce -- allow freezing at warmer temperatures.

In in a study published this week in Science, Christner found that ice-forming bacteria, or 'biological ice nucleators,' are more common than anyone ever expected. Snow samples around the world contained high levels of bacteria, and these appear to be especially important to the precipitation cycles of moderate climes.

Of the different strains, the most common is Pseudomonas syringae, a microbe that causes frost to form on plant leaves. The leaves then wither and decompose, giving P. syringae a source of food. That now appears to be just one stage of their life cycle: another is spent in the sky, where they cause snowflakes and raindrops to form around them, and ride them down to a fresh crop.

This cycle and the role of P. syringae in precipitation is just now being studied, but scientists and farmers have long tried to eradicate the bug. That, said Christner, was the purpose of one of the first genetically modified organisms:

'There's been a lot of debate on the release of genetically modified organisms, and that was the bug that kicked off the GE controversy.

If you can take the organism that causes freeze damage, knock out the gene that causes ice nucleation, then you can scatter this organism on the plant and it competitively excludes ice nucleators and eliminates the problem of freeze damage.

Pseudomonas syringae was modified and released. Then the sample plots were destroyed by ecoterrorists. It's not clear whether scientific information came out of it ... and now we ask, is P. syringae a good guy? A bad guy? Do you really want to eliminate this from your crops? If it's acceptable to assume it has a meteorological role, do you take a 5 or 10 percent cut in your yield to actually keep these bugs around so they'lll continue their role? It's something to think about.'

Christner was careful not to justify the ecoterrorism, and I'd like to take a moment and do the same: this post is not in any way meant to validate or support the tactics -- including death threats [pdf*] -- used to stop the experiments. It's just a relevant piece of history ... and if you're interested in the interaction of bacteria and atmosphere, keep an eye on the main Wired page, where my article on Christner's study should soon appear.

Ubiquity of Biological Ice Nucleators in Snowfall [Science]
http://www.sciencemag.org/cgi/content/abstract/319/5867/1214

* http://www.asm.org/ASM/files/ccLibraryFiles/Filename/000000001830/znw01005000469.pdf