Living Snow

What do bacteria and snow, specifically snowflakes, have in common? Well it seems bacteria, the first form of live to appear on Earth may have an important role in the formation of snowflakes. Lets start first with bacteria. These little organisms make up much of the total mass of live on this planet. The average human body contains around one and a half kilos of them, approximately the weight of you brain. There are approximately ten times as many bacterial cells in the human body as there are cells, with large numbers of bacteria on the skin and in the gut. The vast majority of the bacteria in the body are rendered harmless by the immune system, and a few are beneficial. However, a few species of bacteria are pathogenic and cause infectious diseases, including such nasty ones as cholera, syphilis, anthrax, leprosy, and bubonic plague. Bacteria have been found living high in the atmosphere, floating around up to 22 miles high they seem immune to the UV radiation at that altitude. Indeed they seem to thrive and even reproduce up there. This is where the connection to snowflakes comes in.

Snow crystals form in the atmosphere when tiny supercooled cloud droplets freeze. These droplets are able to remain liquid at temperatures lower than −18 °C, because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice. Then the droplet freezes around this "nucleus". Experiments have shown that this nucleation of cloud droplets only occurs at temperatures lower than −35 °C. In warmer clouds an aerosol particle or "ice nucleus" must be present,or in contact with the droplet to act as a nucleus. Ice nuclei are very rare compared to that cloud condensation nuclei on which liquid droplets form. Dust and biological particles may be effective in this process. This is where bacteria come in to play. In 2008 researchers at Louisiana State University showed that micro-organisms were the most effective ice nucleators present in snow. It seems snowflakes are literally alive.

It seems then that bacteria play an even greater role than previously thought. The idea that snowflakes falling on a snowy day contain living bacteria is a strange, but in my opinion, a beautiful idea. One side effect though of this is for bacteria living high in the atmosphere is this process provides an effective way of moving themselves around and therefore spreading their growth. I will never look at snow in the same way again.