The recent snow and ice, and deeper snow, and even more ice, across much of the U.S. over the past few weeks have finally inspired me to put together my first post for the new year. You’re probably wondering how on earth microbes have anything to do with the 3 feet of snow you had to dig your car out from under last week…

…but hear me out.

Blowing in the wind

I have two “believe it or not” statements for today: First, believe it or not, microbes are ubiquitous in the Earth’s atmosphere (Bowers et al. 2009, and others). “Ubiquitous” is a fantastic word that simply means “absolutely everywhere” and it’s especially true with microbes. As a soil microbiologist, I immediately think of soils and sediments all over the globe and the wide array of fungi and bacteria that keep the planet green (and purple and red and brown), and it makes sense because there are so many things to eat in soils. There’s a never-ending supply of nutrients from dead and decaying plants, worms, insects, other microbes, and even weathering rocks. But I also know that out in the open ocean microbes are abundant and provide the foundation for the food chain, not to mention nutrient cycling and overall marine ecosystem health. We’ve known these things for quite some time now (hence, my “microbe-centric” view of life).

What doesn’t always make sense to a terrestrial biologist is that microbes are also extremely abundant in the air around us, above and beyond our reach, floating in the breeze and being carried thousands of miles on trans-oceanic trade winds. It’s true, though, and for years we assumed that these microbes must be in a sort of hibernation mode, because there’s nothing to eat, harsh conditions often including extreme dryness, cold temperatures and powerful UV radiation from the sun. More recently, however, we’ve begun to understand that only a portion of these airborne microbes are hibernating, while others remain active, usually bound in soil particles or cloud droplets (Sattler et al. 2001). And as long as these little guys are metabolically active, they have the potential to make changes to their environment, even in the atmosphere.

Ice, Ice, Baby (sorry, I couldn’t help myself)

Which leads me to my second “believe it or not” statement for the day: many of those atmospheric microbes have been found to nucleate ice (Bauer et al. 2003). What I mean by “nucleate ice” is that they can serve as the starting point for ice crystals to begin to form. What makes this really cool (pardon the pun) is that ice-nucleating microbes have been found to make specific proteins on the surface of their cells which catalyze the formation of ice crystals at relatively high temperatures. This action not only allows the crystals to form outside the microbe, rather than inside where ice crystals would damage cellular membranes and kill the microbe, but the formation of these crystals also releases very small amounts of heat energy, keeping the microbe that much safer from freezing.

You might have heard about these guys (indirectly) before if you’ve ever heard of “cloud seeding.” There’s a commercially available freeze-dried preparation of ice-nucleating bacteria that many ski resorts will shoot up into the clouds to help encourage snowfall. A slightly less well-known practice is the application of “ice-minus” bacteria to reduce crop loss due to frost. In that case, growers have taken advantage of specific mutant bacteria which lack the genes for the ice-nucleating protein and spray these bacteria across the foliar surfaces so that ice won’t form as easily. The idea here is that ice-nucleating bacteria are very commonly found on plant surfaces, and can lead to frost damage. But those lacking the gene (called “ice-minus”) when applied to the plants, outcompete the natural bacteria, and reduce the formation of frost on plant surfaces.

Atmospheric Microbes = Snow

But these ice-nucleating bacteria exist all over the world, in the soil and in the air around us and may be affecting more than just the ski slopes and strawberries. A very interesting study by a group of scientists out at the University of Colorado in Boulder recently looked specifically at ice-nucleating bacteria and how microbial abundances in the atmosphere may alter atmospheric conditions (Bowers, et al 2009). In order to address this question, they took a number of air samples from the Storm Peak Laboratory at the top of Mt. Werner near Steamboat Springs, CO. Their air samples contained over 640 different bacterial species (via genetic sequence), but their data indicated they did not even begin to sample the full diversity of the airborne microbial community. Despite variable weather conditions during sampling, the total airborne microbial numbers remained stable and didn’t change throughout the sampling period. However, with increasing relative humidity, there was a significant increase in ice-nucleating bacteria. They found that the abundance of ice-nucleating bacteria was significantly greater in cloudy air samples, than in clear (or non-cloudy) air samples. They even suggested that some bacteria may be able to respond to favorable (humid and cloudy) conditions and adjust their concentrations of ice-nucleating proteins, consequently increasing the ice-nucleation potential of these species.

Take-home message…

So, what does all this have to do with the massive downfall of snow and ice this season? Well, as much as I love to blame global warming for more extreme weather events, we don’t have to connect a whole lot of dots to be able to believe that atmospheric microorganisms may be playing a role as well.

The more people we have on the planet, the greater population densities become, and the more disturbance we cause to land surfaces, the more soil, dust, particulate matter, bacteria and fungi rise into the atmosphere and interact with our weather patterns. Much the same way that cloud seeding works, it seems our activities down here are affecting the number of microbes and consequently cloud formation (bioprecipitation, if you will) up there.

____________________________________________________________________________________________________________

Bauer, H., Giebl, H., Hitzenberger, R., Kasper-Geibl, A., Reischl, G. Zibuschka, F., and H. Puxbaum. 2003. Airborne bacteria as cloud condensation nuclei. Journal of Geophysical Research, 108:4658.

Bowers, R., Lauber, C., Wiedinmyer, C., Hamady, M., Hallar, A., Fall, R., Knight, R., & Fierer, N. (2009). Characterization of Airborne Microbial Communities at a High-Elevation Site and Their Potential To Act as Atmospheric Ice Nuclei Applied and Environmental Microbiology, 75 (15): 5121-5130 DOI: 10.1128/AEM.00447-09

Griffin, D.W. 2004. Terrestrial microorganisms at an altitude of 20,000 m in Earth’s atmosphere. Aerobiologia, 20:135-140.

Sattler, B., Puxbaum, H., and R. Psenner. 2001. Bacterial growth in super-cooled cloud droplets. Geophysical Research Letters, 28:239-242.