Strange as it may seem, water doesn’t actually freeze at zero degrees. In fact, even at temperatures as cold as -10°C, water still needs help turning into ice. Living creatures of all stripes have learned to take advantage of this curious fact in different ways, though none have done so with quite as much style as bacteria.
It seems self-evident that water would freeze at “freezing” temperatures, but this is only because it’s usually full of impurities. Ice, like any other crystal, forms more easily around a nucleus. A crystal is a bit like a molecular jigsaw puzzle — all of the pieces have to fit together just right. Just as puzzles are easier to continue than to start, crystals grow more easily from an initial core — the nucleus. This acts as a template, limiting the possible arrangements for the surrounding molecules. By doing so, it provides a pattern around which the crystal can coalesce. Pure water — really pure water — lacks anything around which the water molecules could organize, so it has to be cooled to an incredible -48°C before the molecules will slow down enough to form a crystal — that is, to freeze!
Water is normally full of nucleators, microscopic particles like dust and soot that can act as an ice nucleus. These aren’t all equally effective, though; just as some parts of a jigsaw make better starting points than others (perhaps because they’re more visually distinctive or detailed), some nucleators are better at arranging the water molecules into ice crystals. Though mineral and organic dusts are very common, they’re not very good at helping ice crystals form at temperatures warmer than around -15°C.
In 1970, Russell Schnell was studying some very potent ice nucleators found in decaying plant matter and made the surprising discovery that they came from microbes. A few years later, Leroy Maki identified the bacteria Pseudomonas syringae as the source of these nucleators; at the same time, Deane Arny discovered that more frost formed on plants infected with P. syringae. The bacteria produce a special protein, InaZ, which can act as an ice nucleus at the relatively warm temperature of -2°C, probably because its repetitive shape is just right for coaxing water molecules into a crystalline arrangement.
Researchers think that causing frost damage on plants may give the bacteria better access to nutrients. Ice-nucleating proteins have since been found in a wide range of organisms, from gallflies and gastropods to frogs. In some cases, this may be an adaptation to protect the animal by forming ice where it won’t cause harm; in others, the ice nucleation is probably just a side effect of the protein’s shape.
In 1982, David Sands suggested the intriguing possibility that bacteria might cause rain and snow. Air, including clouds, is usually full of micro-organisms like bacteria and fungi, some of which produce ice-nucleators. Ice crystals which form in clouds will grow until they are big enough to fall as either rain or snow depending on whether they melt on the way down.
Bioprecipitation may sound like a far-fetched idea, but researchers have detected P. syringaein fresh rain, snow and ice from a wide range of locations including Louisiana, the French Alps and even Antarctica! Another team of researchers found that one-third of the ice crystals in clouds over Wyoming had formed around biological particles. Scientists have even been able to discover that the strains of P. syringae in rain falling over a soy bean field were different from those on the leaves, which means they probably came from somewhere else. These bacteria might be creating rain to help them travel long distances!
So does the fact that bacteria can cause rain have an impact on the global environment? Two different research groups added bacterial ice-nucleation to global climate models and found that it didn’t have a significant effect, though the researchers caution that they had to estimate a lot of parameters because of missing data. Regardless of their global impact, it seems clear that P. syringae has an effect on the local water cycle, which may even play a role in its life cycle. Whenever I’ve been asked for an example of other animals manipulating their environment the way we do, I’ve answered by pointing out the dams beavers build and the exquisite climate-control in ant nests. Next time someone asks, I’m going to tell them about these little critters, who control the weather to get around. I’m going to tell them how bacteria can make it rain.
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