Beneath the blue ice of Antarctica’s Lake Fryxell are microbial mats that, in summer, produce pockets of oxygen on the lake bed (Image: Joe Mastroianni, NSF)
Where did life take its first, oxygen-rich breath? An important clue has been discovered at the bottom of an Antarctic lake, in an environment that gives us a sense of the conditions on Earth billions of years ago.
Life evolved some 3.8 billion years ago – a time known as the Archaean – when there was no oxygen in the atmosphere. In fact, the gas only began to accumulate about 2.4 billion years ago, maybe even later. This period, known as the great oxidation event, is linked to the evolution of cyanobacteria, which generate oxygen through photosynthesis.
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But there’s a good chance that cyanobacteria evolved before the great oxidation event. In that case, small groups of these microbes would have created oxygen “oases” on the early Earth.
“People generally thought of oxygen oases as large and in the oceans,” says Dawn Sumner at the University of California-Davis. But her team’s discovery suggests an alternative.
Water at the bottom of Lake Fryxell in McMurdo Dry Valleys, a generally ice-free region of Antarctica, contains little or no free oxygen. In the summer, though, microbial mats on the lake bed photosynthesise and generate pockets of free oxygen just a couple of millimetres thick.
Similar oxygen pockets might have formed around microbial mats far back in life’s history. So perhaps we should be looking for signs of the earliest oxygen oases in ancient rocks on land rather than in the sea, says Sumner.
“Most geologists have not focused on lake deposits,” says Sumner. “But those might be the places that best preserve signatures of early oxygen accumulation.”
“This is a lovely data set from a fascinating setting,” says Timothy Lyons at the University of California-Riverside.
But it doesn’t exclude the possibility that there were also large, oceanic oxygen oases during the Archaean, he says, especially given previous evidence of substantial amount of the gas present at the time, not just tiny millimetre-thick pockets.
Either way, the research is valuable, says Lyons. “The ancient oceans were very different than what we see in the modern ocean,” he says. “Lakes today can provide our best analogues for those ancient marine conditions.”
Lakes like Lake Fryxell might be useful to study for other reasons, says Sumner. Oxygen is vital for our survival today, but when the gas first appeared it would have been toxic to the already-existing life.
So, the lakes today could be sites of adaptive evolution for organisms to develop tolerance for oxygen, says Sumner – something she plans to explore in the future.
Journal reference: Geology, DOI: 10.1130/g36966.1
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