A Population That Pollutes Itself Into Extinction (and It’s Not Us)
Posted April 30, 2018 4:23 p.m. EDT
For “simple” single-celled organisms, microbes can often seem as civic-minded as an Amish barn-raising crew.
They cooperate to mince unwieldy complex carbohydrates into bite-size pieces for all to enjoy. They share the artisanally crafted molecules needed to extract essential iron from their surroundings.
They jointly construct a kind of slimy tarp, a biofilm, on a convenient surface like a pond rock or your teeth, under which the microbes then safely hide. Through teamwork and constant chemical communication, microorganisms shape the world on which we macro-organisms preen.
Yet scientists have discovered that just as a big holiday sale can quickly crush the wisdom of a human crowd underfoot, so can a microbial village be infected with mass stupidity, to devastating effect.
Jeff Gore, a biophysicist at the Massachusetts Institute of Technology, and his colleagues Christoph Ratzke and Jonas Denk, report that when a sample of Paenibacillus sp., a soil bacterium, is fed a diet of glucose and nutrients in the laboratory and allowed to grow at will, the microbes end up polluting their local environment so quickly and completely that the entire population soon kills itself off.
In essence, the researchers said, the microbes commit “ecological suicide.” The study was published in April in Nature Ecology and Evolution.
“The effect is so dramatic,” Gore said. “You have an exponential growth of the population followed by exponential death.” Within 24 hours of the onset of the death spiral, he said, “no viable cells were left in the entire culture.”
It’s not that the microbes had exhausted their resources: Food remained to feed on. But in gorging heedlessly on the glucose bounty, each bacterium had secreted a steady flow of acidic waste into the culture medium, until the ambient pH level had plunged lethally low.
“It’s a strong pH change,” Gore said. “The cells don’t realize what they’re doing in time to stop doing it.”
Nor was Paenibacillus the only microbe found capable of committing eco-suicide. Testing a series of bacterial strains isolated from soil, the researchers found that about a quarter of them, if given the chance, would alter the acidity of their surroundings to the point of mass extinction.
In each case, the researchers showed they could block the die-offs by either adding a buffer to the medium to sop up the acid or by slowing the reckless pace of microbial growth through modest applications of antibiotics.
“This is a very important discovery,” said Jo Handelsman, who studies microbial diversity at the University of Wisconsin-Madison, where she directs the Wisconsin Institute of Discovery. “I didn’t think bacteria were so self-destructive, but this is a very simple phenomenon. The pH changes, and the bugs all die. How did we miss it all these years?”
By the standard dogma, she said, inhibitory processes would intervene before an entire bacterial population is wiped out, with at least a scattering of microbes going quiescent, forming spores or otherwise waiting out the catastrophe until prevailing conditions improved.
The new research suggests that extinction is more easily set in motion than previously thought, and that once it gets started, the responsible parties may be helpless to make it stop.
Gore emphasized that the study was not a recapitulation of how microbes behave in the wild. They looked at individual isolates of bacteria, monocultures pampered with an assured source of glucose and freed of the need to contend with other microbes.
In the real world of, say, a forest floor, “soil is a tremendously complex environment, with high bacterial densities and diversity and competing interests at play,” he said. Some bacteria are acidifying their neighborhood, while others turn it alkaline and still others expel politely neutral waste. That diversity, Gore suggested, likely keeps microbial populations from annihilating themselves under ordinary circumstances.
The paper nonetheless illustrates the perpetual struggle found at every stratum of the natural world and its human hyperprojection: between cooperation and selfishness, the tribe and the individual — between the creation of a public good, like a siderophore agent for scavenging iron, and a public hazard, like acid waste.
Nature is at best a fickle conservationist, and many animals can prove quite destructive to their surroundings. Flocks of parrots can strip stands of fruiting trees of all their seeds, risking the long-term viability of the very food source on which the birds depend. Elephants are endangered in much of Africa, but in some South African reserves like Makilali they are merry marauders, yanking down trees and denuding all visible vegetation at a dizzying rate.
“It’s no surprise,” said Simon Levin, a theoretical ecologist at Princeton University. “Organisms engage in actions that provide short-term benefits that ultimately can be damaging to the societies of which they are part.”
And humans, well, where do we start? Digging up lead, forging it into water pipelines and adding it to cosmetics, paint, gasoline and other everyday goods before realizing that lead is toxic in tiny doses, especially to the brain.
Dumping sewage into lakes and rivers and shoveling plastic waste into the ocean, where it swirls and accretes into a ghastly new continent.
Burning ancient sources of carbon at a fever pitch and hoping somebody figures out what to do about our battered climate before Osaka and Miami disappear.
Gore calls himself “an optimist” and points to positive trends in earth management: cheaper solar power, cleaner air and water in some places, tigers rebounding in others. “There are some bright spots,” he said.
Still, humans are a monoculture, a single macrobial strain, with none but ourselves to keep the petri dish clean.