Oxygen. Without it, organisms consisting of more than one cell would not have arisen on Earth. No plants, no animals, no complex life. 2.4 billion years ago, what is more accurately called dioxygen (the O₂ we breathe) spread into the atmosphere. This episode is called the Great Oxygenation Event.
What caused this? Cyanobacteria, so named because of their bluish hue, are believed to have played a large role in releasing the molecule we need through photosynthesis. But what led to this primordial event and to which we owe our existence? Did cyanobacteria already exist with internal structures designed for this task? “It’s vague,” admits Emmanuel Javot.
However, together with several of his colleagues, a biologist from the University of Liege (Belgium) today offers us a big leap in time in the search for our origins. In a paper published Wednesday, January 3, in the journal Nature, the team presents a 1.75-billion-year-old cyanobacterium fossil equipped with thylakoids, complex membranes that are the matrix of oxygenic photosynthesis, of which it is the oldest direct trace.
“Grand Breakthrough”
“We push back the age of these thylakoid fossils by 1.2 billion years. This makes it possible to give a minimum age to cyanobacteria that have such membranes,” the researcher rejoices. Now it’s a fact: they appeared more than 1.75 billion years ago. Since this still doesn’t tell us whether they were present before the Great Oxygenation Event, a team from the University of Liège is looking for older fossils.
However, for Eric Maréchal, the work represents a “major advance”: “These authors have developed methods that make it possible to search for thylakoids inside fossil cells in other rocks, which could enable future discoveries,” enthuses the CNRS research director.
“Finding structures of this type is time-consuming and difficult,” says Emmanuel Javot. The microfossil in question was discovered in clay rocks collected in Australia. “The cells were preserved, compacted in mud that became rock, namely shale. There was a slight transformation with the weight of the sediments, but not too much, which allowed the exceptional preservation of these membranes in fossil cells,” the scientist continues.
Disturbed atmosphere until today
Where to look now? On Earth, rocks have been heavily altered by plate tectonics, and ancient traces of life have often been damaged or even erased. “For example, we are going to target very ancient places where there was no formation of mountain ranges,” explains Emmanuel Javot. Areas that can be found in Scotland, South Africa, China, Russia or India…
That the oxygen we breathe was produced very early by the famous membranes is an idea that is being seriously studied. In the past, “protocyanobacteria produced so much O₂ that the atmosphere was still changing,” notes Eric Maréchal, who recalls that the molecule accounts for about 20%.
To explain the scale of the Great Oxygenation Event, the director of the Laboratory of Cellular and Plant Physiology in Grenoble (Isere) last year formulated the “multiplier element” hypothesis, citing these thylakoids as the most likely option. “To date, we do not have fossils to evaluate this possibility or, in any case, to date the appearance of thylakoids within archaic protocyanobacteria. ” That’s all.
Source: Le Parisien
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