The sky is alive with biological rainmakers

In 1937, Donald Barber thought space aliens had fallen from the sky and damaged his astronomical photographs. He discovered dark specks on his glass plates later identified by biologists as unknown bacteria obviously capable of thriving in caustic photographic chemicals. In Barber’s day the concept of ubiquitous airborne life seemed unreal. Today, scientists like Paul DeMott and Tom Hill understand that the air is alive with microscopic communities of bacteria, fungi and other microbes. These mini ecosystems, in combination with complex atmospheric weather systems, may determine just how and when the rain falls on our human parades…and farmlands.

DeMott, an atmospheric scientist at Colorado State University, and Hill, a biologist at the University of Wyoming, collaborate across scientific disciplines to study this microbe/weather interaction. They are pursuing a scientific mystery that has been quietly unfolding over the past several decades as the role of microbes in all ecosystems—including the very personal ecosystems of our own human bodies—has clarified.

Although bacteria, fungi and their diminutive relatives bedevil us sometimes with illnesses, they also permeate the living world and perform important services. Nine out of ten of the cells of our own body, for example, are microbes—mostly bacteria helping us break down food, making critical vitamins, and protecting us from dangerous invaders. They operate in all “higher” (multicellular) life forms in similar ways. Cats, dogs, humans, and petunias are, in a sense, cosmopolitan microbe metropolises.

DeMott and Hill and other scientists are now sketching the details of just how microbes make it easier for rain to fall on those metropolises. Water molecules, it turns out, need a tiny something — usually referred to as a condensation nucleus — on which to collect, accumulate, and bond together in the lattice-like crystals we call ice. Atmospheric scientists for many years felt that dust, soot, and mineral grains served as these watery platforms, but suspicions that living things were important contributors to the mix began to build in the 1960s and 1970s. Ice formed in some clouds that should have been too warm. Many of the ice nuclei (INs) in these clouds were organic—containing carbon, hydrogen, oxygen, and nitrogen (CHON) compounds.

One key experiment became an eye-opener: heat the INs from warm clouds and they lose the ability to form ice crystals at warmer temperatures. Scientists know today that an ancient bacterial gene creates a heat-sensitive protein on the surface of IN bacteria. Hill describes the gene this way: “It codes for a protein that, modeling suggests, has stretches of amino acids that form repeating switchbacks. The amino acids that comprise these pleated sheets form hydrogen bonds with water molecules above them, holding them in place in an array that mimics ice; this array acts as an ice crystal embryo that allows freezing to be triggered at relatively high temperatures.”

DeMott and Hill (and the recently deceased Gary Franc at Wyoming) collected ice nucleation active (INA) bacteria in the skies over Northern Colorado and Nebraska. In fact, last year they collected samples over Grant Family Farms in Wellington (a certified organic farm) to contrast INAs there with samples taken over a more traditional farm in Nebraska.

Vegetation, including agricultural crops, harbors loads of INA bacteria. On plants, these bacteria rip open leaf cells with spicules of frost at temperatures higher than 32 degrees. This allows the bacteria to feed on the lacerated cells, but makes gardeners unhappy. Bacteria with their INA genes removed became the first genetically engineered creatures in the 1970s and the active ingredient in FrostBan. When sprayed on tomatoes or other plants genetically modified (GM) ice minus strains ousted the native bacterial strains, preventing premature frost.

Somewhere between 20 and 80 percent of INs in warmer clouds are biogenic (microbes or non-living fragments). DeMott and Hill hope their research will help clarify where these biogenic INs come from. Air collected over farmlands contained relatively few local INA bacteria, except during harvest time when combines ejected them into the air. DeMott says that the number and kind of biogenic particles in clouds depends on location, the season, environmental conditions, and the temperature of the cloud. Based on measurements so far, it seems that most INAs originate on land, but sea spray may also produce a significant fraction. Weather systems can carry such particles great distances—although not quite as far as Barber thought back in 1937.

DeMott is unsure how, if at all, this research will impact agricultural practices “unless we can carefully quantify an important input of soil dust and plants to ice nuclei aerosol cycles, and document impacts on precipitation.” Hill points out, however, that the emission of INAs from plants could factor into how scientists place a “service value” on certain ecosystems. “For example,” he said, “mountain mahogany is, at first glance, an unremarkable and spiny shrub, but since finding that it supported roughly half a million INA bacteria per gram of leaf (not exceptional, but still more than I expected) I now see it differently.”

Some biologists even go so far as to contend that microbes actually control precipitation in a way that maximizes their distribution and biological success. In other words, they ride on watery taxis that they can trigger to dump them on suitable habitats. At the other extreme, says Demott, are scientists who are “skeptical that any particular contributing aerosol, inorganic or organic, is solely essential for creating rain.” The truth, more than likely, resides somewhere in between these extremes.

But when and where rain falls — especially in the often-arid west — is important to everyone. Microbes appear to be underappreciated players in the process—and nearly as mysterious in their high altitude aeries as space aliens. As DeMott says, “It is simply too fascinating and important of a topic to ignore.”

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