The oldest bacterial structures that filled the planet with oxygen have been discovered

The first half of the history of life on Earth was written by bacteria. And for millions of years they did so without the need for oxygen, which was absent from the atmosphere at that time. Now, the oldest structures that some of these microorganisms used to fill the planet with gaseous oxygen (O₂) have been discovered. A species from about 1.75 billion years ago already had something similar to vesicles called thylakoids that allowed them to increase their ability to photosynthesize. These thylakoids are still present in the planet’s cyanobacteria, algae and plants that convert sunlight into chemical energy.

Photosynthesis was a brilliant mechanism by which, at some point in the beginnings of life on the planet, cyanobacteria learned to convert the energy coming from the Sun into the chemical energy they needed. In the process, they took electrons from compounds present in their environment. At a time more than 2.4 billion years ago, certain groups of cyanobacteria learned to carry out oxygenic photosynthesis. They took in water (H₂O), which was an abundant fuel, from which they obtained the hydrogen necessary to assimilate carbon from CO₂ in the atmosphere. In their metabolism, they released the excess waste, oxygen, which must have been consumed by oxidizing the minerals in the rocks. But around 2.4 billion years ago, the so-called Great Oxidation Event occurred, due to which the Earth’s atmosphere accumulated up to 1% O₂. It may not seem like much (the current concentration is close to 21%), but the foundations were then laid for an extraordinary diversification of living beings.

Cyanobacteria were responsible for that event. Some have been found in the fossil record before the Great Oxidation Event, but what has just been discovered is part of their engineering. In a site in Australia, scientists found microfossils of a microorganism called Navifusa majensis, it was believed to be a cyanobacteria, but it is not easy to identify a bug like this, which barely measures 25 microns — a micron is one thousandth of a millimeter — compacted in a fossilization process 1.75 billion years ago. As detailed in a paper published in Nature, its discoverers have found thylakoids in the N cell. majensis. These vesicles contain photosensitive elements that convert light into chemical energy. Cyanobacteria that carried out oxygenic photosynthesis had already been discovered, but such ancient thylakoids had not.

“Cyanobacteria are important because the oxygen we have on the planet is the result of the activity of these biological organisms”

Patricia Sánchez Baracaldo, microbiologist with the University of Bristol, United Kingdom

The thylakoids that have now been discovered represent the first direct evidence of oxygenic photosynthesis with these basic units. As the researcher at the University of Liège (Belgium) and senior author of the research, Emmanuelle Javaux, says, the discovery “shows that cyanobacteria were actively producing oxygen 1.75 billion years ago, so in reality the sediments of the McDermott Formation [in the region of Australia where they have been found] they were not formed in a permanent or completely anoxic environment.” Before the Great Oxidation Event, there must not have been many corners, niches, where life based on oxygen had refuge. But the scenario changed after the event. “We are now digging into the even older fossil record to test the proposed hypothesis that the appearance of thylakoid membranes may have contributed to the increase in oxygen around the Great Oxidation Event and the permanent oxygenation of the early Earth,” Javaux adds.

Patricia Sánchez Baracaldo, a microbiologist at the University of Bristol (United Kingdom), explains that “there was oxygen before the Great Oxidation Event, but they were strongholds.” For Sánchez Baracaldo, who investigates the bacterial origin of life, “cyanobacteria are important because the oxygen we have on the planet is the result of the activity of these biological organisms.” The Colombian scientist, who also investigates the origin of photosynthesis, remembers that “oxygen did not exist, bacteria invented how to extract electrons from water by breaking it, and it was that oxygen that accumulated.” And she adds: “That is why it is important to determine when this type of photosynthesis appeared, which is something that has fascinated scientists and the general public because without oxygen, evolution would not have led to us.”

About 200 species of cyanobacteria have been described and only two do not have thylakoids. In fact, the first cyanobacteria did not have these structures. With these organelles in their membranes, the microorganisms must have expanded their photosynthetic capacity and, therefore, increased the generation of oxygen. The new element was creating new ecological niches and, as Sánchez Baracaldo, who did not participate in this work, points out, “there were unicellular organisms that probably began to learn to breathe that oxygen.” Complex life appears millions of years later, when the accumulation of O₂ was increasing, accelerated by at least two new similar events after the Great Oxidation Event. “Oxygen accumulated so much that it opens the possibility for animals to evolve. That is when the first eukaryotes appeared.” From these first eukaryotes, which were still unicellular organisms, those that must have engulfed some cyanobacteria would later emerge, initiating the greatest example of endosymbiosis in the history of life. From these organisms which had absorbed cyanobacteria with thylakoids inside, chloroplasts would arise that allow algae and plants to carry out the same photosynthesis today.

“In cyanobacteria with thylakoids, the membrane surface multiplies very significantly and so does the cell’s ability to photosynthesize”

Purificación López, researcher at University of Paris-Saclay, France

Purificación López, a researcher at the CNRS - Center National de la Recherche Scientifique (France), who was not involved in this study, recalls where the importance of thylakoids lies: “They increase the surface area where the photosystems are located, which is where photosynthesis can take place.” There are other groups of cyanobacteria that do not have these structures and carry out photosynthesis in the outer membrane. “In cyanobacteria with thylakoids, the membrane surface multiplies very significantly and, so does the cell’s ability to photosynthesize. What is important about this research is that they are seeing these fossilized thylakoids. It is an astonishing level of conservation of 1.75 billion years,” the Spanish microbiologist and professor at the University of Paris-Saclay adds.

In a sense, the Great Oxidation Event carried out by cyanobacteria has parallels with the K–Pg event, the mass extinction of animal life due to the impact of a meteorite 66 million years ago. If the K-Pg event created the conditions for the diversification of mammals, which were just small animals at the time, the Great Oxidation Event laid the foundations for the arrival of multicellular organisms and of complex life. López likes the analogy, but rejects its essential premise: “During the Great Oxidation of the atmosphere there was no extinction. New oxygenated habitats were surely created, where there is indeed a diversification of oxygenic photosynthetic organisms and aerobic organisms that use oxygen. But the others did not disappear, they continued to exist in places where there was no oxygen, and there are still anoxygenic photosynthetic organisms in lakes, in sediments, and in microbial mats, and there is still very significant anaerobic biology, even in our intestine, our microbiome.”

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