At the bottom of a frozen Antarctic lake, a thin layer of cyanobacteria in microbial mats is generating a little oasis of oxygen, an international research team has found out. It is the first modern replica discovered of conditions on Earth two and a half billion years ago, before oxygen became common in the atmosphere. The discovery was reported recently in a paper in the journal Geology.
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.
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, but those might be the places that best preserve signatures of early oxygen accumulation.”
Sumner and collaborators including Ian Hawes of the University of Canterbury, New Zealand, have been studying life in these ice-covered lakes for several years. The microbes that survive in these remote and harsh environments are likely similar to the first forms of life to appear on Earth, and perhaps on other planets.
The discovery occurred “a little by accident,” Sumner said. Hawes and Tyler Mackey, a graduate student working with Sumner, were helping out another research team by diving in Lake Fryxell. The lakes of the Dry Valleys typically contain oxygen in their upper layers, but are usually anoxic further down, Sumner said. Lake Fryxell is unusual because it becomes anoxic at a depth where light can still penetrate.
During their dives below the oxygen zone, Hawes and Mackey noticed some bright green bacteria that looked like they could be photosynthesizing. They took measurements and found a thin layer of oxygen, just one or two millimetres thick, being generated by the bacteria. Something similar could have been happening billions of years ago, Sumner said.
“The thought is, that the lakes and rivers were anoxic, but there was light available, and little bits of oxygen could accumulate in the mats,” she said.
The researchers now want to know more about the chemical reactions between the “oxygen oasis” and the anoxic water immediately above it and sediments below. Is the oxygen absorbed? What reactions occur with minerals in the water? Understanding how this oxygen oasis reacts with the environment around it could help identify chemical signatures preserved in rocks. Researchers could then go looking for similar signatures in rocks from ancient lake beds to find traces of oxygen prior to the Great Oxidation Event.
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.
Source: Colin Barras, New Scientist and Andy Fell, Egghead – About research at UC Davis