Despite the lack of snow some ski seasons, resorts are not usually troubled to keep their tracks white during all the season. Part of the credit should be attributed to a peculiar group of bacteria that cause disease in plants.
Since its invention in the fifties and introduction in Spain in 1985, artificial snow cannons have become an indispensable tool for any ski resort that’s not willing to put its financial profit in the hands of natural precipitations. The process is quite simple, water is mixed with pressurized air and the resulting droplets are propelled into the air, where they become snow after being exposed to low temperatures. What you may not know is that in some resorts there is lack of the low temperatures required for this kind of process and rely on additives to ease the freezing process. In fact, water hardly freezes by itself and needs a starting point for the ice to spread:
At this point, our beloved phytopathogens come into play! Leaf surfaces usually harbor numerous bacteria that are sustained entirely on plant secretions leaking through stomata (specialized orifices that plants use for gas exchanges). However, while some of these bacteria interact with the plant to stimulate secretions others prefer to use brute force to enter the plant tissue through ice-made wounds. Generally, these pathogens are known as Ice nucleating active bacteria or Ina+ and the first species described was Pseudomonas syringae.
To understand how these bacteria work it’s important to know that plants are capable of lowering the freezing temperature by accumulating solutes in their tissues, easily establishing a freezing point of at least -5ºC. In fact, some plants adapted to cold environments like Heather (Calluna vulgaris) are capable of resisting up to -20ºC. Unfortunately, these adaptations don’t always work against pathogens like Pseudomonas syringae, which is able to catalyze the ice formation at maximums of -7 to -2 ºC.
It is believed that Ina+ bacteria are the second best ice nucleators, only after ice itself. As a matter of fact, it’s common to find these kind of bacteria at the core of hail or snowflakes.
Going back to ski resorts, it’s not wise to release pathogens such as Pseudomonas into the environment for the sole purpose of having snow all the season. Thus, Ina proteins are successfully produced in safe strains of Escherichia coli and afterwards only the membranes of this bacteria (bacterial ghosts) are used to produce artificial snow.
And so, it’s possible that in many occasions you have been skiing in company of millions of bacteria!
…or rather, their ghosts…
- Ice nucleation induced by pseudomonas syringae (1974). Maki LR, Galyan EL, Chang-Chien MM, Caldwell DR.
- Distribution of ice nucleation-active bacteria on plants in nature (1978). Lindow SE, Arny DC, Upper CD.
- Cloning and expression of bacterial ice nucleation genes in Escherichia coli (1985). Orser C, Staskawicz BJ, Panopoulos NJ, Dahlbeck D, Lindow SE.