This is the microbes' world—we just live in it. Throughout the history of Earth, microbes have radically reshaped life on the planet, from creating the very air we breath to wiping out almost all life on Earth. Don't underestimate the power of tiny, tiny microbes populating the Earth trillions of times over.
Here are some of the ways microbes have done what humans are doing now: geoengineering the climate.
How the Earth's Atmosphere Got Oxygen
Among the complications of traveling 3 billion years back in time is the fact that you would immediately suffocate. There wasn't much oxygen, if any, in Earth's atmosphere back then. But about 2.7 or 2.8 billion years ago, cyanobacteria—also known as blue-green algae—began to proliferate for reasons still unclear. Like their descendants today, these cyanobacteria could turn sunlight, water, and carbon dioxide into carbohydrates and oxygen. You might recognize this process as photosynthesis.
The oxygen in Earth's atmosphere increased rapidly—for a geologic time scale, anyway—reaching the 21 percent we breathe today. The Great Oxygenation Event had a profound impact on Earth's life. Oxygen is highly reactive, which means it messes up the metabolism of microbes unused to living in an oxygenated atmosphere. Those microbes, called anaerobes, once dominated the Earth but now live deep underground or underwater where oxygen is still scarce. On the flip side, oxygen's reactivity could also be harnessed for metabolism, making possible the very existence of energy-hungry, multicellular organisms like us. Complex animals could not exist if not for these cyanobacteria.
Geologists know about the Great Oxygenation Event because of the sudden appearance of iron oxides—basically forms of rust—over two billion years ago in the Earth's crust. Collectively, these tiny cyanobacteria made their mark on Earth, shaping both the planet's animate and inanimate forms.
Banded iron formations in red at Dales Gorge in Australia. Wikimedia Commons
The Worst Mass Extinction in the History of the Earth
250 million years ago, Earth went through the Great Dying. Temperatures rose, the oceans acidified, and ninety percent of all species were wiped off the face of the planet. One of the more prominent explanations for the Permian-Triassic Extinction is a burst of volcanic activity in the Siberian Traps of modern-day Russia. A recent study fingers an additional culprit: a bloom in methane-belching microbes called Methanosarcina.
According to this study, Methanosarcina simply acquired two genes from an unrelated bacterium about 250 million years ago. These genes let the microbes feed on a previously untapped food source: a carbon compound called acetate abundant in ocean sediments. Feed and grow they did, all the while releasing vast amounts of methane, a greenhouse gas, that warmed the atmosphere and acidified the oceans. Volcanoes could have still played in a role in spewing out nickel, which is necessary for the chemical reaction that lets microbes make methane gas. The abundance of nickel would have eased along the microbe's runaway growth—and decimation of the rest life on Earth.
Nitrogen-Fixing Microbes and Our Food
In 1910, the German chemist Fritz Haber invented a process to mimic what microbes had been already doing for millions of years: fix nitrogen from the atmosphere into ammonia. While all life on Earth requires nitrogen, the inert nitrogen gas that makes up 78 percent of the planet's atmosphere is useless to all but some nitrogen-fixing bacteria. The Haber process changed that. With a new source of nitrogen fertilizer, agriculture exploded and the human population more than quadrupled in that time. It's estimated that half of the nitrogen in all our bodies originated with the Haber process.
While Haber's invention enabled a human population boom in the past century, it's nitrogen-fixing microbes that sustained all life before it. (And, remember, the other half of nitrogen in our bodies still originated with these microbes.) The microbes that fix atmospheric nitrogen gas are called diazotrophs. They're a diverse group that inhabit nearly every ecosystem on the planet, from the soil to coral reefs to lichen. In a way, they're at the bottom of every food chain.
Root nodules of alfafa. Wikimedia Commons
The most famous of diazotrophs might be Rhizobia, bacteria that live inside the root nodules of legumes such as clovers, peanuts, and alfalfa. These plants feed sugars to the bacteria in exchange for nitrogen. The fixed nitrogen stays in the soil even after the plant dies, which is why farmers plant cover crops like clover and alfalfa in between seasons. As overuse of nitrogen fertilizers has wrought its own problems—like run-off that causes algae blooms—we might better appreciate role of these nitrogen-fixing bacteria.
What Microbes Mean For Climate Change
In July of 2012, 100 tons of iron filing were dumped off the coast of Canada in the world's largest geoengineering experiment—and an entirely unauthorized one at that. The American businessman Russ George was trying to prove a bizarre sounding idea to combat climate change: ocean fertilization. In theory, phytoplankton would capitalize on this sudden iron bonanza, growing like crazy and pulling in carbon dioxide from the atmosphere. While George's unauthorized experiment was soundly rebuked by the international community, the role of microbes in climate change is coming under increasing scrutiny.
Microbes can both absorb or release carbon, depending on their diets, so the direction of their influence is not so clear. But, in aggregate, they are huge players in the carbon cycle. Just the microbes that decompose dead plants in the soil, for example, release 55 billion tons of carbon dioxide a year, which is eight times what humans contribute through fossil fuels and deforestation.
And climate change is changing how these microbes function. In the cold Siberian tundra, for instance, there is normally not much microbial activity. In recent years, however, the tundra is releasing more carbon dioxide than it absorbs, which scientists believe is due to rising temperatures allowing more microbes to feed in the tundra and release carbon dioxide. The same could be happening in the oceans.
You might say microbes were the original "geoengineers" of the Earth, leaving a profound influence on the planet's climate and the lifeforms. As we begin to understand Earth's microbial world, geoengineers are also looking at how to harness the awesome power of these tiny microbes. Small changes in aggregate trillions of times over can entirely reshape life on Earth. [Nature News, Scientific American, Yale Environment 360]
Lead image: Stromatolites formed by cyanobacteria in Australia. Wikimedia Commons