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Meet the snow pilots

Are bacteria manipulating the weather?

Snow can be a powerful thing. It can bring a nation’s transport sys¬tem to its knees and bring out the inner child from within the most dour and life-weary soul. It is a virgin canvas wait¬ing for the imprint of an angel. It is a fluffy mound, pregnant with sculptural potential and is a fort – complete with artillery.

Most of us have seen at least a dusting of snow in the last couple of days but what caused it to fall in the first place?

You might think that snow is just what happens to rain when it gets a bit chilly but things are a lot more complicated than that.
For a start, it hasn’t been cold enough for ice to form spontaneously.

We all know that water freezes at 0C but you might not know that at temperatures above -40C or so, it actually needs a kick-start to get those ice crystals forming.

This kick-start comes in the form of nucleators – tiny particles that act as a core around which water molecules can cluster and form crystals.Any small particle, such as dust or soot, works well but there is one nucleator that rules them all – bacteria. Some bacteria produce a special protein that is custom-built to encourage ice to form. The protein’s surface structure mimics that of an ice crystal, which encourages nearby water molecules to adopt the same structure. With the water molecules already arranged in an ice-like lattice, it is much easier for ice crystals to form – allowing water to freeze at relatively high temperatures.

One such bacterium is Pseudomonas syringae, which uses its ice-forming protein to create ice crystals in the plant cells it infects. The crystals cause its host’s cells to burst, allowing the bacterium easy access to its nutrients. In fact, P. syringae’s ice-forming talents are responsible for much of the frost damage that can wreck plants in the early spring.

But what have bacteria got to do with snow? In 2009, Louisiana State University looked at snow from all over the planet. They found that at the heart of almost every snowflake were cells containing microbial DNA. It seems that without bacteria there is no snow.

[Graphic: How bacteria might control the weather. Click to make bigger]

But what could bacteria be gaining from making it snow and what are they doing up in the clouds at all?

One theory is that they are using the clouds as a sort of giant conveyor belt that allows them to populate new areas. In fact, studies of clouds have shown that, on average, they carry with them tens of thousands of living cells in every millilitre of water.

But bacteria’s role in the planet’s weather mechanisms might be even more complex than just making it snow. Some studies suggest that they might actually be making clouds form in the first place. Many microbes, including bacteria, produce a chemical called dimethyl sulphide as a metabolic by-product, which acts as a nucleator for the formation of water droplets. In fact, dimethyl sulphide produced by phytoplankton in the oceans has been linked with cloud formation for some time.

But are bacteria ‘deliberately’ creating clouds to ferry themselves around the planet and then making it snow so they can get back to earth?
We don’t know. After all, it could be a coincidence that a biological by-product can form clouds and that an ice-forming protein that evolved to assist a bacterium to invade its plant host also makes it snow.

But what if it isn’t coincidence? What if these talents evolved to help bacteria spread around the planet to infect new hosts (like just malaria parasites use mosquitos)? Just imagine, one of Earth’s smallest life forms may have been manipulating an entire planet’s hydrological cycle all along.

Makes you think a little differently about all that snow, doesn’t it..?