Back at their lab Christner and Bryan examine two dozen petri dishes containing bacteria collected from flights that peaked between 10,000 and 80,000 feet. They have yet to sequence and ID the microbes’ DNA, but just looking at the petri dishes gives them some clues. Bright-colored colonies dot many of the dishes—reds, oranges, pinks, and yellows. “Those are natural sunscreens,” Christner says. Colored carotenoid pigments (similar to those in many plants, including the carrots that the compounds are named after) can neutralize damaging ultraviolet light.
Christner holds up a petri dish containing colonies mottled with beige, black, and white: an indication that these bacteria produce dehydration-resistant spores that could help them survive at extreme altitudes. They are probably some sort of Actinomyces (a group of bacteria that live in soils and include species that make streptomycin and other antibiotics), but the species could be new to science. “We don’t know,” Christner says, looking at a splotch. “That might make an antibiotic that nobody’s ever seen before.”
Searching the skies for high-living microbes may also lead to insights concerning some species that we already do know about. In the 1950s researchers funded by the U.S. military tried to sterilize canned meat by blasting it with radiation. When they opened the cans, they were surprised to find the meat rotten: It had been fermented by a bacterium, now called Deinococcus radiodurans, that is exceptionally resistant to radiation. The species carries a muscular set of enzymes that stitch its DNA strands back together as quickly as radiation splinters them apart. Deinococcus can survive 5,000 times as much radiation as human cells, but no natural environment on Earth comes close to those levels of irradiation. “So what the hell has an organism like this evolved tolerance for?” Christner asks.
John Battista, who studies the microbe at LSU, just upstairs from Christner, thinks Deinococcus’s DNA repair enzymes primarily help it survive dehydration in its native desert environment. But radiation tolerance could also allow the bacterium to join that 10-mile-high ecosystem. Windstorms in places like the Gobi Desert could easily pick up Deinococcus and propel it around the world. “If it managed to get into the upper atmosphere,” Battista says, “it has all the tools it needs to survive.” Deinococcus and other similarly hardy microbes may be lurking in the samples Christner’s team is culturing.
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 could be the real directors of much of our planet’s weather. “When I look at what physically forms the ice in clouds, I’d say 80 percent of it has some sort of biological signature,” Prather says. “The dust by itself doesn’t explain it.”
Four decades ago, scientists discovered that the bacterium Pseudomonas syringae triggers the formation of frost on plants. Since then, some researchers have proposed the ice-making bug and others like it might be creating ice crystals in clouds that result in precipitation. It’s not clear yet if airborne microbes really influence the weather, but that hasn’t stopped some optimistic scientists from studying the bacteria as a tool to increase rain and snow.
Harnessing bacteria to make precipitation could be big business. Dozens of states and countries run cloud-seeding programs using artificial ice-nucleating compounds. In California the effort is especially urgent. The Sierra Nevada snowpack, which provides about 65 percent of the state’s water, has been declining since 1950, and a quarter of the snow is projected to disappear by 2050.
Montana State University bacteriologist David Sands imagines using syringae to bring more rainfall to places like California. “I’m an agriculturalist,” he says. “I don’t like droughts.” Microbiologists have identified strains of syringae that form frost without damaging their hosts. It could be possible to plant large tracts of land with plants that harbor these strains. The microbes might then get into the air, form ice crystals in clouds overhead, and pull more rain from them.
No one knows if this is practical. Clouds would have to be hit at just the right time, when their humidity and temperature are ideal for ice formation. And the areas of land that have to be planted might also be prohibitively large. But Sands is plugging ahead, working with researchers in Syria and other countries to find varieties of wheat and barley that preferentially harbor the right strains of syringae. “We might be able to introduce new varieties with the bacteria on the seed,” he says. “We might be able to capture 10, 20, 30 percent more rainfall in some areas.”
Douglas Fox is a freelance science writer based in California. His work has appeared in The Best American Science and Nature Writing.