Photosynthetic structures have been observed in fossilized cells that are 1.75 billion years old. A record.
These structures, called thylakoids, were discovered in microfossils of cyanobacteria (Navifusa majensis) discovered in the McDermott Formation in Australia by paleobiologist Catherine Demoulin and her colleagues at the University of Liège in Belgium.
A thylakoid resembles a flattened spherical vesicle. It's like a space inside a cell surrounded by a membrane, explains biologist David Morse of the Plant Biology Research Institute at the University of Montreal, who was not involved in the study.
The entire photosynthesis machinery is located in these flattened spheres, the scientist adds.
Sightseeing features
- Oxygenous photosynthesis, in which sunlight catalyzes the conversion of water and carbon dioxide into glucose and oxygen, is unique to cyanobacteria and related organelles of eukaryotes.
- Eukaryotes are all single- or multicellular organisms whose cells have a cell nucleus and organelles bounded by membranes.
- Oxygen-containing photosynthesis played a fundamental role in the emergence of life on Earth. It caused a major ecological upheaval by transforming a methane-rich atmosphere into today's atmosphere, which consists essentially of nitrogen (78.08%) and oxygen (20.95%).
Normally bacteria do not have an internal membrane. But cyanobacteria are a major exception to the rule! notes the scientist, who considers this work convincing.
In the microscopic images we can clearly see a cell with several lines in it. We do not observe these types of structures anywhere other than in cells with thylakoids.
The role of thylakoid-equipped cyanobacteria is considered by several biologists to be crucial in the intense oxygenation of Earth's atmosphere 2.4 billion years ago, which profoundly disrupted the development of early life on the planet.
However, this hypothesis has not yet been confirmed in the fossil record by the discovery of physical evidence that would confirm this chronology.
There is a discrepancy between the start of the increase in oxygen concentrations on Earth and the appearance of organisms with thylakoids in the fossil record, says David Morse of the Plant Biology Research Institute.
Closer to great oxygenation
In fact, the oldest known thylakoid fossils were 550 million years old. The fossils we have identified therefore make it possible to push back the fossil record by 1.2 billion years, explains paleobiologist Emmanuelle Javaux.
It is therefore still not possible to establish a cause-and-effect relationship between thylakoids and high oxygen enrichment, notes David Morse.
Since the oldest fossils containing thylakoids are 550 million years old and the increase in oxygen began 2.4 billion years ago, we can't prove anything.
But recent discoveries bring us closer to proving that cyanobacteria and thylakoids are involved in oxygen enrichment in Earth's atmosphere.
But the chronology of the origin of oxygenic photosynthesis and the type of cyanobacteria involved in the large-scale oxygenation remain controversial, points out David Morse.
Could another source be responsible for the oxygen increase, such as proto-cyanobacteria unknown to science?
According to Mr. Morse, the discovery, described in the study published in the journal Nature (New Window), allows us to glimpse the possibility of discovering thylakoids in even older fossil cyanobacteria.
And thus prove once and for all the role of thylakoids in the massive accumulation of oxygen on Earth. We're getting closer to the magic number of 2.4 billion years, notes David Morse.
Emmanuelle Javaux reminds us that microscopic life represents the most diverse and abundant life form on Earth since the origin of life. Studying its fossil record using new approaches allows us to understand how life has evolved for at least 3.5 billion years. Some of this research even shows us how to search for traces of life beyond Earth, she concludes.