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The ecological handshake
Just as the same genes that allow Deinococcus to thrive on the ground may give it the ability to survive at high altitudes, the ice-nucleation gene may originally have given syringae and bacteria like it an advantage other than rainmaking. The nucleation gene appears to be unrelated to any of the more than a million genes that have been sequenced to date from various organisms. And the gene seems to have arisen only once in the course of evolution; after that, it passed from one species to another, changing little along the way. No one knows how long ago the gene emerged, but its appearance may have marked a pivotal moment in Earth's history. It may have provided a new way for life to modify the planet's environment.
Ice nucleation might have emerged as an ecological handshake between bacteria and the plants they lived on. Many wild plants (unlike most cultivated crops) are frost tolerant. They can survive as long as the freezing happens slowly, giving the plants time to activate their defenses. By causing frost to set in at higher temperatures — at 25ºF, say, rather than 15 — ice-nucleating bacteria would have caused freezing to happen more slowly, helping protect the plants they lived on.
Later on, the talent for forming ice may have found other uses. Syringae uses ice crystals to rip open the cells of plants that are not frost tolerant, so it can devour their nutrients. Microbes like syringae may also exploit ice nucleation to parachute down in raindrops or snowflakes, ensuring they do not remain stuck at high altitudes when swept up by storms.
Ice-nucleating bacteria might even influence the entire landscape. By triggering rain, Sands says, "they cause more plants." Just as humans farm wheat, syringae might cultivate leafy ecosystems that can sustain the bacteria once they reach the ground. Those ecosystems would then spawn more bacteria, some of which would return to the sky.
The realization that bacteria could have such profound impacts adds one more twist to the already convoluted connection between human activity, weather, and climate. Forests may make their own local rain by releasing bacteria and other organic compounds into the lower atmosphere. Deserts may trigger precipitation thousands of miles away when their dust and bacteria collide with water-rich masses of air. What effect, then, of deforestation or desertification?
Researchers have studied desert dust for decades, tracking its serpentine trajectory around the globe and trying to understand its environmental impact. Now it seems that dust might have been a decoy, hiding the bacteria that influence much of our planet's weather. "When I look at what physically forms the ice in clouds, I'd say (that in some of the clouds that we sampled) 80 per cent of it has some sort of biological signature," Prather says. "The dust by itself doesn't explain it."