Discovery of the oldest bacterial structures that filled the planet with oxygen Sciences

The first half of the history of life on Earth was written by bacteria. For millions of years they did this 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. One species about 1.75 billion years ago had something similar to vesicles called thylakoids, which allowed them to increase their ability to photosynthesize. These thylakoids are still found in cyanobacteria, algae and plants on the planet that convert sunlight into chemical energy.

Photosynthesis was a fascinating mechanism by which cyanobacteria, at some point in the early life of the planet, learned to convert energy from the sun into the chemical energy they needed. In this process, they took electrons from some compounds in their environment. Sometime more than 2.4 billion years ago, certain groups of cyanobacteria learned to perform a particular form of photosynthesis, oxygen. They obtained water (H₂O), an abundant fuel, and from it they obtained the hydrogen necessary to assimilate carbon from carbon dioxide in the atmosphere. During metabolism, they release excess waste, oxygen, which must have been consumed by oxidation of minerals in the rocks. But about 2.4 billion years ago, the so-called major oxidation occurred, due to which the Earth's atmosphere accumulated up to 1% O₂. It may not seem like much (current concentration is close to 21%), but the foundations have then been laid for an extraordinary diversity of organisms.

Cyanobacteria were responsible for this event. Some of them have been found in the fossil record before the great oxidation process, but what has only just been discovered is part of their geometry. At a site in Australia, they found tiny fossils of a microorganism called… Navifosa magenensisIt was thought to be a cyanobacteria, but it's not easy to identify such an insect, which is barely 25 microns in size (a micron is a thousandth of a millimeter) compressed in a 1,750-year fossilization process. As detailed in Work published in natureIts discoverers found thylakoids in a cell n. Magensis. These vesicles contain photosensitive elements that convert light into chemical energy. Cyanobacteria have been discovered that perform oxygenic photosynthesis, but not like ancient thylakoids.

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

Patricia Sanchez Barakaldo, microbiologist at the University of Bristol, UK

These now discovered thylakoids represent the first direct evidence of oxygenic photosynthesis using these basic units. As Emmanuelle Jafoux, a researcher at the University of Liège (Belgium) and senior author of the paper, says, the discovery “shows that cyanobacteria were actively producing oxygen 1.75 billion years ago, so in fact the deposits of the McDermott Formation [en la región de Australia donde las han encontrado] “They were not formed in a permanent or completely oxygen-free environment.” Before the Great Oxidation, there must not have been many nooks and crannies for oxygen-based life to resort to. But the scenario changed after the event. “We are now digging into the older fossil record to test the proposed hypothesis that the appearance of thylakoid membranes may have contributed to the increase in oxygen around the major oxidation process and the permanent oxygenation of the early Earth,” Javoux adds.

“There was oxygen before the major oxidation process, but they were bastions,” explains Patricia Sanchez Barakaldo, a microbiologist at the University of Bristol (UK). For Sánchez Barakaldo, who researches the bacterial origin of life, “cyanobacteria are important because the oxygen on the planet is the result of the activity of these biological organisms.” The Argentine scientist, who is also researching the origin of photosynthesis, recalls that “oxygen did not exist. Bacteria invented how to extract electrons from water by breaking it down, and it was this oxygen that accumulated.” “This is why it is important to determine when this type of photosynthesis appeared, which has amazed scientists and people as well because without oxygen, evolution would not have reached us,” he adds.

About 200 species of cyanobacteria have been described, of which only two do not contain thylakoids. In fact, the first cyanobacteria did not have these structures. By being present in their membranes, these microorganisms must have expanded their ability to photosynthesize, and thus generate oxygen. The new element was the creation of new ecological niches, and as Sánchez Barakaldo, who was not involved in this work, highlights, “there were organisms that perhaps started to learn to breathe this oxygen, and they were all single-celled.” Complex life emerges millions of years later, when O₂ accumulation accelerates, accelerated by at least two new similar events after the major oxidation. “Oxygen accumulates so much that it opens the way for animals to evolve. This is when the first eukaryotes appear. Of these first eukaryotes, still single-celled, later emerged those that ingested some cyanobacteria, leading to the greatest example of symbiosis Internal to the history of life. From these organisms containing cyanobacteria containing thylakoids within them, chloroplasts would arise that allow algae and plants to carry out the same process of photosynthesis.

“In cyanobacteria containing thylakoids, the membrane surface is greatly doubled, and thus the cell's ability to photosynthesize is doubled.”

Purificación Lopez, researcher at the University of Paris-Saclay, France

Researcher at the National Center for Scientific Research (France), Purificacion Lopez, who was not involved in this study, recalls where thylakoids are important: “They increase the surface area where the photosystems are located, where photosynthesis can take place.” There are other groups of cyanobacteria that do not have these structures and perform photosynthesis in the outer membrane. “In cyanobacteria that have thylakoids, the membrane surface is very much doubled, and therefore, the cell's ability to photosynthesize is doubled. What's relevant to this research is that they're seeing these fossilized thylakoids. “It's an amazing level of preservation for 1.7 billion years,” she said. Adds the Spanish microbiologist, professor at the University of California. University of Paris-Saclay.

In a sense, the great oxidation event carried out by cyanobacteria is similar to the K-Pg event, the mass extinction of animal life due to a meteorite impact 66 million years ago. If the K-Pg event created the conditions for mammalian diversification, then some small animals, in the process of major oxidation, laid the foundations for the arrival of multicellular organisms with complex life. Lopez likes the analogy, but rejects a fundamental aspect of it: “During the great oxidation of the atmosphere, no extinction occurred. Certainly new oxygenated environments were created, where there is already a diversity of oxygenic photosynthetic organisms and aerobic organisms that use oxygen. But other organisms “It hasn't gone away, it's still there in places where there's no oxygen, there are still deoxygenated photosynthetic organisms in lakes, in sediments, in microbial mats and there's still very important anaerobic biology, even in our gut, the microbiome.”

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