Earth might not have been the best place to set up home during the Archaean era, but life nevertheless managed to establish a foothold in the form of single-celled microbes that thrived in its iron-rich and oxygen-free ancient oceans. Although much of this period remains a mystery, micro-organisms residing in the still cool depths of Canada's Boreal Shield lakes could shed light on our early origin story that began more than 2.5 billion years ago.
To get a better understanding of what life was like back when the planet was still an infant, scientists often turn to analogs – places that share key characteristics with the Archaean ocean. Until recently, it was thought there were only four such sites – Lake Matano in Indonesia, Lake La Cruz in Spain, the sub-basin of Lake Kivu in East Africa and Lake Lavin in France – all in locations that are either remote or hard to reach.
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However thanks to a surprise discovery at two of the lakes in northwestern Ontario's IISD Experimental Lakes Area, this number could be bumped up to millions – thus expanding research site options for scientists dramatically – as these types of lakes are widespread across Canada's Boreal Shield ecozone and similar kinds can also be found in Finland, Norway, Sweden, and Russia.
Researchers from the University of Waterloo found Lakes 227 and 442 to be low in sulfur and high in iron, chemical conditions that are similar to characteristics ascribed to Archaean oceans. And just like those ancient bodies of water, they also have oxygen-free zones – during the summer, at least – that develop when the water stratifies naturally into distinct temperature layers. The surface water warms and becomes separate from the stagnant and cool bottom layer (also known as the hypolimnion). The latter becomes anoxic when organisms use up the oxygen, leaving the door open for a unique type of bacteria called photoferrotrophs, which live in oxygen-free zones and metabolize iron using light as an energy source, to take over. In the spring and fall, the layers mix again when the oxygenated water at the top gets denser and sinks.
But what happens to the photoferrotrophs when this happens? Previously, scientists had thought that these microbes, strains of which are believed to have kickstarted life in the ancient oceans, would not be able to survive outside an oxygen-free environment. However this new discovery shows that they're able to survive in an oxygenated environment as well, making them a lot more robust than thought, which could have potential implications for research on climate change and harmful algal blooms (also known as red tides). Given that iron plays a key role in the formation of algal blooms, these newly detected microbes could help scientists understand how to manage this deadly phenomenon.
We spoke to PhD student and study co-author Jackson Tsuji to find out more about this discovery.
How did you make this discovery – did the team already have a hunch about the lakes or was this something that happened by accident?
It was a huge surprise – very accidental. Our research was originally being done as part of a longer term monitoring [project] but as we took another look at the data, we realized that it might actually be quite helpful towards understanding some characteristics about the early earth.
The first thing that actually alerted us was the chemistry of the lakes. We realized the main chemical properties (i.e. they're high in iron, low in sulfate levels and they're oxygen free, at least for part of the summer) bear similarities to what's been predicted for the early Earth oceans.
What implications do the microbes at the bottom of these lakes have for our understanding of climate change?
In terms of the climate, methane is the main takeaway from our paper. It is around 30 times more potent of a greenhouse gas than carbon dioxide and there is a lot of it that gets generated at the bottom of lakes and oceans. Most of it is actually consumed by bacteria before it ever reaches the surface so we can thank microbes for not having crazy global warming problems. What we found in our lakes is that we have some unusual bacteria living in the oxygen-free zone that could be capable of consuming that methane. So that would mean there's a new kind of player, like a new kind of microbe, in the oxygen-free zone that could be helping to mitigate methane emissions. On a more basic level, to have an understanding of how microbes are actually helping with the climate, it helps to actually understand how different climate changes – such as how a warmer climate could lead to more oxygen-free zones worldwide – impact global climate.
Why is the resilience of these microbes so important to the debate surrounding the evolution of life in the Archaean ocean?
We're not the first people to come up with this idea to use modern lakes to simulate some of these ancient ecosystems on earth. There have already been other people who have been trying to do this but they are looking at lakes that are always oxygen-free at the bottom. This makes sense because ancient Earth is thought to have been permanently anoxic whereas the lakes we're studying actually mix twice per year. So they'll get fully oxygenated and [the prevailing belief is that this will] probably destroy any of these microbial communities that are dependent on having no oxygen in the water. So what's so interesting about these lakes is that the microbes are resilient after the oxygen mixes down. Within two months we see the same kind of microbial communities there again that we would have seen before the lake mixed.
What we're really providing is a new tool for scientists to take things they've never learned from fossils or rock records and test them in [these] modern living laboratories. For example, you can take a look and see some of the microbes that we think might have been living back in the early earth: can they actually live in these conditions that we find right in the natural environment?
What questions does this discovery raise for you?
There are a ton of follow-up questions because it's a very new idea. What I'll be doing on the microbiology side is to take the bacteria from these lakes and grow them in our laboratory to confirm what they do and learn some more specifics about the kind of environmental processes they can conduct. This will help us get a sense of the rates at which these processes are happening in the actual environment, which will tell us whether they are important in the global climate sense currently and then it can also inform some ideas about the early Earth.
The study has been published in Scientific Reports.
Source: University of Waterloo