Do the microorganisms that circulate in the atmosphere get there by chance or by contrivance?

Circumambulating his home near Oxford University, in the style of Charles Darwin treading his “thinking path,” the evolutionary biologist William D. “Bill” Hamilton often thought about life–all of it. He imagined evolutionary scenarios, extravagant and daring theories. Much as a novelist might imagine characters, he imbued each with personality and possibility and then helped them into the world to meet their fate.

Just such a theory took him to the Congo at the beginning of 2000. It had been suggested that HIV had jumped from chimps to humans via a tainted polio vaccine. Hamilton thought the idea had merit. But rather than simply argue the possibility, Hamilton and two students climbed trees in the rainforest to gather poop from the stick nests of chimpanzees. They hoped to extract R, NA and DNA to see which strains of HIV and related viruses plagued the chimps. It could be argued that any trip that involves climbing trees to collect poop has not started terribly well. But things got worse. One of the students stabbed himself on a pahn-leaf spike and suffered blood poisoning, requiring his evacuation. Then Hamilton, who had not taken any malaria prophylactics, was bitten by an infected mosquito. Soon feverish, he was sent home to England. On January 30, 2000, the day after his return, he lapsed into a coma.

The HIV hypothesis Hamihon risked his life for was potentially consequential for human history and health. But it wasn’t the most momentous idea he considered, nor the most unruly. Several of Hamilton’s wild stories have survived initial disbelief and dismissal to become foundation stones in the understanding of evolution. He imagined, for example, that microorganisms might live in clouds, and even make them.

Clouds have traditionally been beyond the purview of biologists–too rarefied to be the stuff of life. But Hamilton looked up (often when it was imprudent, while hurtling headlong on his bicycle), and imagined them as biomes teeming like a river with life. Into the clouds he cast until he felt a bite, the tug of something as big and wild as anything he’d reeled in before. He had already made revolutionary leaps from selfish genes to self-sacrificing kin and from parasites to the evolution of sex. He could, in his speculation, afford to be a little bit reckless.

In 1997, Hamilton shared his cloud hunches with Timothy M. Lenton, then a PhD student at the University of Norwich in East Anglia, and the two launched into a collaboration to explore the possibility that microbes both make and fly in clouds.

The theory began with overcrowded microbes in the sea. When their populations become too dense, individuals that can escape will have an advantage. But how can they do that? What if, Hamilton imagined, they could get up into clouds and ride to greener pastures? Yet single-celled creatures are seemingly at the mercy of the fates. Here came the novelty: maybe microbes produce chemicals that cause clouds to form, ride those clouds, and then produce a second kind of chemical to cause the clouds to rain or snow them back to the ground. The idea seemed better suited to a children’s book than to big science. And yet it was also somehow evidence of what a great mind, left to wander, can conjure–and what an eager graduate student can be roped into. Hamilton and Lenton published their story in 1998.

On its own, the idea that microbial life could be found in clouds was not entirely new. Earlier biologists had thrust an occasional vial or Petri dish out of a plane window or off a hot-air balloon or mountaintop (among them F.C. Meier, who disappeared in 1938 in his mid-forties while flying into dense clouds, searching for life). The vials those “aerobiologists” brought back down were chockfull of microscopic life. What was new was Hamilton and Lenton’s concept that microbes had, by natural selection, evolved adaptations for making clouds rather the way beavers have evolved adaptations for making ponds.

The paper was ahead of its time. The question it raised was too big for microbiologists, too biological for climatologists, and too airy for oceanographers. No one, not even Hamilton and Lenton, knew enough about all of the elements they discussed–clouds, bacteria, oceans, sea bubbles, and ice storms, to name a few. And then Bill Hamilton went to the Congo, caught malaria, and after five weeks in a coma, died. He was sixty-three.

As a child, Hamilton lost the tips of two fingers by playing with a bomb. As an adult, he got into a fight with a knife-wielding man in Brazil. While looking for ants in Rwanda, he was taken for a spy. He is said to have been hit quite a few times by cars while riding his bike at high speeds to his office at Oxford University. At his funeral, everyone recalled both Hamilton’s brilliance and his near-pathological disregard for his own safety. Together those two traits had flung him into the far corners of the world, and, it seemed implicit, to an untimely death. He was buried quietly at the edge of Wytham Wood near Oxford, though he had requested that his remains be left in the Amazon to be pulled to pieces by beetles and buried for their young.

When Hamilton died, his ideas about clouds were discussed as an example of his beautiful mind, and promptly dismissed. Then a funny thing happened. Scientists began to examine parts of his cloud theory. First they tested, more rigorously, whether clouds really contained life. (They did.) Samples from clouds were scooped up, frozen, and later run through modern genetic analyses. Each and every sample seemed to be full of protein and DNA–and, implicitly, of life. An average cloud, if there is such a thing, contains tens of thousands of living cells in every milliliter of water. That’s many fewer cells than in a milliliter of swamp water, but many more than you might expect to find in wisps of air. Algae, bacteria, and lots of fungi not only ride in clouds, but actually live in them, taking up residence, surviving, and reproducing on the “foods”–organic acids and alcohols, sulfur, and nitrogen–that float above us on the wind. A recent study found three species of bacteria way up in the stratosphere, more than twelve miles above Earth and above all but the thinnest, most attenuated clouds. None of those species has ever been collected anywhere else, leaving open the possibility that some species live only and always in the clouds. In this new, post-Hamilton view, clouds are their own biological realms–as Thoreau called them, “drifting meadow[s] of the air.”

Yet the key to Hamilton and Lenton’s hypothesis was not simply that microbes occur in clouds, but that they have evolved specific traits–tiny chemical “wings”–to take them there. In forests and deserts, microorganisms might not need any special adaptations to arrive in clouds. Winds, dust storms, fires, and thunderstorms may be sufficient to blow them into the air. But for marine microbes to get airborne, they must escape the surface tension of water–no easy task. And yet clouds, as it turns out, are full of such minute sea creatures.

Hamilton and Lenton imagined how bacteria and other unicellular organisms might take advantage of rising air bubbles in wind-whipped whitecaps. It was already known that such air bubbles “scavenge” bacteria and other unicellular organisms as they rise to the water’s surface. A bubble can accumulate microbes in densities several hundredfold greater than in the surrounding water. Once at the surface, the microbes might be popped into the air by the bubbles’ bursts. Perhaps that would be enough to get microbes airborne, but Hamilton and Lenton envisioned something more specific.

They knew that marine microbes (particularly algae) produce an immediate precursor to dimethyl sulfide (DMS), a flammable, water-insoluble byproduct of bacterial metabolism. Moreover, they knew that the DMS generated by those microbes could initiate the process of cloud formation, and that therefore, many a cloud that rolls over your house, whether in the shape of a dog, a popsicle, or a piano, likely had its start in the products of microbes. Water droplets form around microbially produced DMS molecules, which thus catalyze condensation. That much is not in contention, at least not anymore. The question is whether an individual microbe’s genes benefit by producing DMS (a requirement for natural selection to act). Did a microbe that produced more DMS as it was flung into the air stand a better chance of survival? Would it be more likely to be drawn up into a new cloud, which would act as transportation to a patch of sea where its offspring would thrive and multiply? Hamilton and Lenton thought so. They imagined that DMS was, for microbes, like a kind of sail, extended when they needed to ride up into the air and catch the wind.

If the production of DMS is an adaptation of microbes that allows them to trigger cloud formation, it would be among the most magnificent and consequential adaptations of any lineage of life. Beavers may make ponds and wetlands, but microbes that build clouds alter Earth’s conditions vastly more. If microbes did not produce DMS, cloud cover would be reduced dramatically. We, among other species, would probably not be able to survive. But did the bacteria evolve specifically to do this, or is it just a chance byproduct?

The answer to that question is now testable, as Lenton outlined recently in an email to me: one could study the evolution of the genes for the production of DMS and whether those genes were found preferentially in microbes that travel in clouds. One could also look at whether the genes for DMS production are selectively “turned on” by microbes when they approach the water’s surface, where the DMS might most usefully propel them upward. We await an answer, but, I suspect, not for long. Lenton does not plan to do the work, but someone will–perhaps you.

After microbes get into clouds, how do they get down? With the exception of the three new bacterial species known so far only from the stratosphere, most of the microbes flying around above us appear to need to get down to Earth to prosper. Hamilton and Lenton proposed that microbes in clouds produce a second set of compounds, any of a group of proteins that cause ice to form around them. The frozen creatures begin to free fall to the Earth, and, if things go well, eventually melt, grow, and begin to divide. There was precedent for such a hypothesis, if a somewhat obscure one.

In 1976, David C. Sands was hired at Montana State University during an outbreak on wheat of Pseudomonas syringae. That bacterium was known to produce a protein that raises the freezing temperature of water, and in doing so, causes frost damage to plants’ leaves at relatively warm temperatures. The frozen plant cells burst, and some strains of the bacteria then take advantage of the damage by consuming the cells’ contents. Sands had a mystery on his hands, though. He could not understand where the infections were coming from. Sands sterilized some wheat seeds and planted the sterilized seeds in experimental plots. Amazingly, the infection raged on. It was as though the bacteria dropped straight out of the sky–and so he decided to consider that possibility. He got in a plane with a petri dish and, at cruising altitude, hung it out the window. His hand must have nearly frozen off. But when he brought the petri dish back to the lab, P. syringae grew. The bacteria had been floating above his wheat fields in the clouds and, somehow, descending onto his sterilized plants.

Many of the details of how P. syringae raises the freezing temperature of water were eventually resolved by Sands and others. The structure of the special protein the pathogen produces mimics the structure of an ice crystal, and that causes water molecules in liquid or vapor form to congregate on it and freeze. Initially, it seemed as though the bacteria’s only use for this protein was in freezing plant cells. But those who paid attention noticed a strange pattern. The protein was present not only in P. syringae, or in plant pathogens more generally; it was also present in microbial lineages that did not infect plants at all. What other purpose might it serve?

Sands and others would soon reveal that P. syringae bacteria use the same protein that allows them to freeze plant cells to drop out of the sky, swaddled in the disguise of snowflakes. It seemed and seems possible that the proteins evolved because they help the microbes drop out of clouds, and then were secondarily co-opted to help some pathogenic varieties of the bacteria to eat plants. In 1976 Russell C. Schnell of the University of Colorado suggested that hailstones, at least in Kenya, very often had P. syringae at their core. Then in 2008, Brent C. Christner at Louisiana State University and colleagues, including Sands, collected snow from Antarctica, France, and Montana to see what, at the center of snowflakes, had initially enabled them to form. At each site a large proportion of the snow harbored evidence of ice-nucleating life. That finding was confirmed this year by a team of atmospheric scientists led by Kerri A. Pratt, a PhD student at the University of California, San Diego, who detected such life in situ, by analyzing ice crystals in clouds aboard a specially-equipped plane. All around us, as it rains or snows, one might, upon close inspection, find such collections of life–bacteria, algae, and fungi–falling toward us in great densities. It is not raining cats and dogs, but it is, more often than not, snowing bacteria.

Just as in the case of DMS, it is not fully resolved whether the cloud-riding microbesthat cause rain and snow to fall “mean” to do it (which is to say, whether the proteins they produce evolved as adaptations for initiating snow and rain), or whether such consequences are incidental. None of the many details that have accumulated since Hamilton and Lenton’s paper have ruled out their hypothesis. Meanwhile, around the seed of their speculation, the possibilities have grown more complex and wonderful than even Hamilton and Lenton might have imagined: a troposphere filled with life doing things we have yet to understand, things that affect the formation and dissolution of clouds and that ultimately, by stabilizing Earth’s climate, may have had a hand in the origin of terrestrial multicellular life.

Hamilton is deeply missed by those who knew and loved him. But many more people, thousands of us, miss his ideas, his awe-inspiring, sometimes wrong, often right, narrative of the living world. At Hamilton’s funeral, Luisa Bozzi, his partner of several years, offered over him: “You will live not only in a beetle, but in billions of spores of fungi and algae. Brought by the wind higher up into the troposphere, all of you will form the clouds, and wandering across the oceans, will fall down and fly up again and again.”

Every time you catch a snowflake on your tongue you have some chance at finding, buried inside it, if not Bill Hamilton, then the life he predicted. However such life tastes to you, to Hamilton it could only have tasted sweet. He was, like those snowflakes, one of a kind.

Robert R. Dunn “A head in the clouds: do the microorganisms that circulate in the atmosphere get there by chance—or by contrivance?“. Natural History. 12 Dec, 2011.

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