Ancient oxygen source from crushed rocks
Around 2.3 billion years ago, the Earth experienced a massive change in the composition of its atmosphere: oxygen became an abundant element. This was partly owing to the evolution of microbes called cyanobacteria with the capability to photosynthesise, capturing the sun's energy to make food and releasing oxygen in the process. But if we try to look before this great oxidation event, it is much less clear how much oxygen was around, where it came from, and what was using it. This week though, researchers from Newcastle University revealed how crushing rocks, water and heat could be a recipe for releasing oxygen on the early Earth, which in turn could have affected how life evolved and flourished. Julia Ravey spoke to Jon Telling and Jordan Stone (destined to be a geologist with that name!) about the findings, with Jon explaining how oxygen may have been generated before photosynthesis…
Jon - One source is actually from water in the atmosphere. So when water goes up can interact with UV radiation, and that can actually start to split some of the water and in the process form things like oxygen. So it's thought that even before Cyanobacteria evolved, there would've been trace amounts of oxygen in the atmosphere, but very, very low concentrations - negligible compared to today. And then there's also been some theories which have suggested you maybe get oxygen from geological sources, from rocks as well. So maybe certain rock types, if they're crushed, could produce little bits of oxygen.
Julia - And if we're looking at this theory that we could have potentially got oxygen from rocks in some way, Jordan, how did you go about trying to recreate this environment in the lab?
Jordan - We started with borosilicate vials - glass little vials - and we wanted to make sure that we had no oxygen conditions like before the great oxidation event. So we were looking mostly at subsurface areas below the oceans. Rock types would be like basalt and peridotite. We crush those to simulate earthquake activity, which crushed those rocks and produce those reactive surfaces that we were looking for. So we added those to vials, all under completely nitrogen conditions, and then we'd add oxygen-free water, which we'd bubbled with nitrogen and then filled the air, the head space of the vials, with nitrogen as well and sealed those before we applied different temperature regimes.
Julia - And what did you find when you did this?
Jordan - First we found at lower temperatures, we were producing hydrogen up to about 80 degrees Celsius. And then the rocks did this weird thing where they shifted to hydrogen peroxide production, which is really exciting because hydrogen peroxide can naturally break down to form oxygen and water or microbes could do that themselves using enzymes, turning hydrogen peroxide into oxygen as well.
Julia - If there was some oxygen in this way, produced by rocks, do you know if this may have impacted the microbial life that was living on earth at the time?
Jon - Well, we don't know for sure, but we can certainly presume that it might have had an impact. Some people have gone back and tried to reconstruct what life was like from genetics, from existing life, seeing what's common to all life and sort of back-tracing it to the origin of life, the first cell that existed. And what they've found from these genetic studies is that the organisms liked it hot. There's genetic evidence that the organisms that first evolved were heat-loving ones that they used hydrogen gas, just like some of the experiments that we've been running, but also intriguingly, they contain genes for dealing with oxygen and hydrogen peroxide; things you wouldn't have thought were around very much at all on the early Earth. And maybe some of the results that we've got from these experiments could help explain that, that we have found this potential mechanism of generating more substantial local concentrations of oxygen, hydrogen peroxide, and oxygen, which could help explain that maybe early life was potentially using these compounds and how it might have actually affected the chemistry around early life; maybe in fractures under the sea floor. Maybe even contributed to the chemistry of prebiotic chemistry before life evolved.
Julia - And so Jordan, was there a matchup there between the temperatures where these microbes like to live and the temperatures that you saw in your experiments where you got this change in the rocks, in the chemistry?
Jordan - Yes, that was quite exciting. We found that lined up quite well with what we think the conditions were like where the earliest life forms existed. So we think that early life forms might have been hyperthermophiles. So they liked to live in hot temperatures. We found that we were only producing hydrogen peroxide at these high temperatures, which lines up really well with these organisms that we think survived there.