If you’ve ever caught a snowflake on your tongue, you’ve likely swallowed a bacterium called Pseudomonas syringae. Don’t worry, it’s harmless. The rod-shaped, Gram-negative bacteria can be found in the exact center of trillions of snowflakes. Its presence there is directly tied to the formation of the snowflakes themselves.
Snowflakes don’t just materialize out of thin air. Every unique snowflake initially forms around what scientists call an ice nucleator. An ice nucleator is small enough to stay aloft in a cloud. It’s also solid, a tiny solid speck of matter onto which water vapor in a cloud can collect and freeze.
In higher, colder clouds, most ice nucleators are inorganic particles of dust, soot or ash.
But in lower clouds, where the majority of classically intricate snowflakes form, almost all ice nucleators are organic. They can be a bit of pollen, a fungal spore, or a microorganism. Surprisingly, many of latter are alive before, during, and after a snowflake develops around them.
Over the past two decades, scientists have discovered that the skies overhead are not an empty void but actually are absolutely teeming with microbial life. Microbes get swept up off the ground by wind and evaporation and become the ultimate jet-setters — traveling long distances in floating ecosystems. Clouds and airstreams transport googles of single-cell life forms around the globe.
Pseudomonas syringae is the most prolific snowflake-starting bacteria.
Like all bacteria, Pseudomonas syringae is a single-cell organism. What makes it an especially adept ice nucleator is a special protein that coats the outside of its body. The structure of this protein is such that it forces water molecules that come into contact with it to arrange like they do when water is in ice form. It happens that water molecules already arranged like ice become ice quicker, freezing and thereby starting a snowflake at slightly warmer temperatures than are otherwise required to start snowflakes. That’s how Pseudomonas syringae are able to transform water vapor into ice at temperatures higher than cloud freezing. The protein enables them to make more snowflakes than other ice nucleators.
Pseudomonas syringae are so good at starting snowflakes in fact, that we add dead ones to artificial snow machines all the time to make artificial snow. If you’ve ever skied on a trail covered in artificial snow, you’ve zipped over many Pseudomonas syringae.
So what happens once a snowflake starts forming around a Pseudomonas syringae? As more and more water molecules attach and freeze onto the microbe, the organism becomes entirely encased in ice. Patterned crystals extend in all directions from the bacterium, fanning out into an exquisite frozen parachute ten thousand times its size. When the snowflake has grown so large that it becomes heavier than the surrounding air, it begins somersaulting downward, hurtling the frozen bacterium toward earth.
It’s reasonable to speculate, since such large populations of bacteria regularly undergo this frigid ritual, that being frozen alive is not deadly but even somehow beneficial to them. Assuming Pseudomonas syringae are not a bunch of kamakazes, what good is being frozen alive?
From the microbe’s perspective, starting a snowflake represents an important aspect of its grand plan for world domination — or at least for expanding its species’ reach into new territories. Pseudomonas syringae bacteria prefer to feed on certain agricultural plants. (Which makes it not a favorite with farmers and agricultural interests.) A bacterium can be lifted from plants in a crop field by wind or evaporation up into the air, where it may travel hundreds or thousands of miles in any prevailing direction. It withstands cold temperatures and can even absorb airborne nutrients and reproduce up in the clouds.
But ultimately, life is easier on land, and that’s where a snowflake comes in handy. With the snowflake’s growth initiated by the bacterium, more and more water vapor molecules glom on in a symmetrical formation.
Photo Credit: Prof. Kenneth Libbrecht, CalTech
Once the snowflake grows heavier than the surrounding air, it falls, carrying its founding partner back to the ground. When the snowflake melts, the bacterium defrosts. Then it is free to snack on new food sources, and reproduce to create new bacterial colonies that will spread its genes.
Bacterial ice nucleators including Pseudomonas syringae don’t know they’re being so clever of course. But they don’t need to know because, as one of the earliest life forms on the planet, bacteria have been shaped longer than any other species by the optimizing genius of Evolution.