Some lakes in Canada look like the oceans on our planet that existed some 2.5 billion years ago, when microbial life thrived in an oxygen-free environment. And what's living in those lakes today could help scientists understand what early life forms on Earth were like, too, an important piece of the puzzle in understanding how all of us came to be.
A new study in Scientific Reports describes lakes in northern Canada's Boreal Shield that are similar to oceans during the Archean Eon period, when microbial life on Earth was still in a primordial stage, and could exist essentially without oxygen.
These microbes will be useful in studying the formation of harmful algal blooms, like the one that temporarily shut down a Lake Erie water treatment plant in 2014. Microbes are believed to metabolize iron compounds with the help of sunlight, and iron plays an important role in algal bloom formation.
Co-author Jackson Tsuji, a PhD student at the University of Waterloo, told me the team made this discovery during a longer-term study of the microbiology of Canada's northern lakes.
"It was one of those eureka moments," he said in a phone interview. "There's always excitement whenever we have a new discovery, but it always leaves a lot of questions for follow-up work."
Many Boreal Shield lakes are low in sulphur and high in iron, like the oxygen-free oceans where Earth's earliest life forms originated. The team found that, over five or six months in the summer, an oxygen-free layer develops at the bottom of the lakes they studied (Lakes 227 and 442 at IISD Experimental Lakes Area of northwestern Ontario). That's because water sits stagnant and doesn't mix with the oxygen at top, while microbes are using all the oxygen at bottom.
"All kinds of organisms will use up the oxygen at the bottom of the lake—microorganisms, zooplankton, maybe even fish if present," he said in a follow-up email. "Once the oxygen is gone, it gives the opportunity for different types of microorganisms to thrive." Some of them, like the one that could be involved in iron cycling, can't survive when oxygen is present.
In the fall, the water at the top cools down, gets more dense, and sinks to the bottom, mixing in. Tsuji said that some lakes develop an oxygen-free zone in the winter too.
Previously, researchers thought that if water layers with oxygen mix with other layers without, the microbial area would be disrupted, making it difficult to study. Due to this belief, until now there were only four "analogue" lakes with similarly primordial conditions, which scientists used as proxies to study early life—in Indonesia, France, Spain, and the Democratic Republic of the Congo. They were all pretty remote and difficult to access.
Two months after the oxygenated and oxygen-starved layers mixed in the Canadian lakes, Tsuji and the team found that the organisms in the oxygen-free community look similar to what they had looked like before the mixing.
"Having these lakes in our own backyard is incredibly helpful if we want to see what ancient life on earth could have looked like," Tsuji said. "We don't have to fly around the world to collect these samples and preserve them, we just have to drive up north." Tsuji said that millions of other lakes in the Boreal Shield region now have the potential to be similarly studied, and compared.
Many mysteries remain about microbial life before humans walked the Earth, like why these algal-blooms form and how iron cycling organisms are feeding them. If all goes as Tsuji hopes, scientists will have many more places to look for signs of early life.
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