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In which they experimentally confirm their hypothesis explaining the appearance on the Earth of oxygen of non-plant origin.

Almost all living things use oxygen for breathing. Without delving into the physics and chemistry of cellular respiration processes, let's say that the choice of evolution fell on oxygen because of its high oxidation capacity, that is, to easily attach an extra electron. An electron enters the electric transport chain from NADH or FADH 2 travels along it, and everything ends with the synthesis of the ATP molecule - the material equivalent of stored energy and the addition of an electron to oxygen. This whole reaction becomes possible because such electron transfer is energetically beneficial, and this is partly due to the properties of oxygen.

When life on Earth was born, there was practically no oxygen in the atmosphere, just as there is no oxygen on Venus or Mars today. Ancient bacteria were forced to use other oxidants, often energetically less favorable, but available. NO 3 -, NO 2 -, Fe 3+, fumarate and dimethyl sulfoxide, used by some types of bacteria, have a higher redox potential and are less beneficial as oxidants. Many bacteria using one of these oxidants are also capable of breathing oxygen. In the presence of oxygen, they breathe it (this is more profitable), and when there is no oxygen, they breathe with their other oxidizing agent (it must be somehow). Sulfur-containing oxidants (S, SO 4 -) have a lower redox potential. This, however, renders the oxygen toxic to the microorganisms concerned, and they die in an atmosphere containing oxygen. In more highly organized life forms, anaerobic respiration is rare and almost never serves as the main source of energy.

Could highly evolved life forms use other than oxygen as an oxidizing agent? Oxygen as an oxidizing agent is energetically more favorable than most other substrates (the lower the redox potential of the oxidizing agent, the more energy is released when an electron passes through the electric transport chain). This means that organisms breathing oxygen had a more efficient metabolism and were better adapted. From an energy point of view, sulfur-containing substrates are also quite beneficial. The problem, however, is that those with this type of respiration die in the presence of oxygen. It is still not entirely clear why exactly this is happening. That is, if oxygen did not appear in the Earth's atmosphere, over time, the owners of sulfate respiration could evolve further. But oxygen appeared, and they had to go to "reservations" where oxygen is not supplied.

The question is, where did the oxygen come from. Today, the Earth's atmosphere contains approximately 20% oxygen. In such huge quantities, it is secreted by photosynthetic plants, mainly trees and algae. But photosynthetic plants themselves now mostly breathe oxygen. In order for the mutations that allow oxygen to breathe to be fixed in the course of evolution, it must be beneficial, which means there must be oxygen. Oxygen in large quantities on Earth appeared thanks to cyanobacteria. These are nitrogen-fixing bacteria that can photosynthesize. That is, massively oxygen appeared on Earth as a by-product of photosynthesis. This event is called the "Oxygen catastrophe", apparently, for the scale of the consequences.

But the question of whether there was oxygen before that remains open. Over the past 40 years, they have begun to say more and more confidently that there was oxygen before the Oxygen Catastrophe, and now the possibility of its existence has been confirmed experimentally.

Until now, only one method of the formation of molecular oxygen under the conditions of that time was known. It consists of two stages: the dissociation of carbon dioxide under the influence of solar ultraviolet radiation into carbon monoxide and atomic oxygen and the reaction of two oxygen atoms, requiring a third participant: the atoms combine into a molecule, and the carrier (M) carries away excess energy.

CO 2 + hν (UV) → CO + O

O + O + M → O 2 + M

However, the calculations, and then the experiment carried out by the authors of the article under discussion, showed that oxygen can be formed from carbon dioxide under the influence of ultraviolet radiation in one step:

CO 2 + hν (UV) → C + O 2

The experiment used a laser with a wavelength of 200 nm, light with this wavelength is usually absorbed by the atmosphere, so the reaction had to take place in its upper layers. Such a reaction can even now, when the content of carbon dioxide in the atmosphere is increasing, occur in the upper layers of the Earth's atmosphere, and maybe in the atmospheres of other planets.

For centuries, scientists have been debating about the real source of oxygen on Earth. According to preliminary data, the first half of its life, the planet Earth was generally without oxygen. Most scientists put forward the theory that 2.4 billion years ago oxygen on Earth was negligible. Oxygen gradually filled our atmosphere.

How did oxygen appear on Earth? It is believed that the main source of oxygen on Earth is cyanobacteria. It is a photosynthetic microbe that generates oxygen. And thanks to cyanobacteria, there was a sharp jump in the oxygen content in the atmosphere. But when and how these microbes appeared is not yet fully known. Also, it is not yet fully understood how exactly the process of filling the Earth's atmosphere with oxygen took place. It is known that this was a combination of a sharp global cooling, the emergence of new species, and the emergence of new mineral rocks. As Dominique Papinet (specialist at the Carnegie Institute, Washington) said, the doctrine is not yet able to clearly define what was the cause and what was the effect. Much happened almost simultaneously and for this reason there are so many different inconsistencies and contradictions. To further clarify the geological side of this issue, Dominique Papinet studies in detail the process of the formation of iron, as well as sedimentary rocks that form at the very bottom of the ancient seas.

His research focuses on specific minerals. These minerals are contained precisely in the formations of iron, and they may well be associated with the emergence of the life of ancient microbes and their death. Iron minerals, which are found fairly at the bottom of the seas, are the largest source of iron ore. And it's not just a material for making steel. According to geologists, it is in it that the rich history of the origin of life on planet Earth is hidden.

And the origin of this source is still a big mystery. Scientists have found that for its formation, the help of special microelements is needed, but, however, it is not yet known which ones. These marine organisms are simple, unicellular, but unfortunately they did not leave any information behind. And researchers cannot now find out exactly what they were, and what they were.

It is believed that the builder of such iron minerals was precisely cyanobacteria. The oxygen that came out of it oxidized iron in the seas and oceans long before the great oxygen explosion took place. But one thing remains unclear. Cyanobacteria appeared on planet Earth long before the accumulation of oxygen. It turns out that hundreds of millions of years passed before our atmosphere was filled with oxygen?

Perhaps the answer lies in the complex interweaving of biology and geology. The oxygen that cyanobacteria breathed out could be destroyed by methane. And when these two gases interact, water and carbon dioxide are formed. Scientists noted that oxygen cannot accumulate in any way in an environment rich in methane. Methanogens produced methane and blocked all pathways for the accumulation of oxygen on the planet and also heated the Earth as a result of the greenhouse effect. And after the planet Earth was filled with oxygen, the number of these organisms decreased.

Today we will talk in more detail about where oxygen comes from.

Photosynthesis

As you know, oxygen is produced by green plants during photosynthesis. Photosynthesis occurs precisely in the green parts of the plant, where chlorophyll is the most pigment. In order for photosynthesis to take place, two elements are required: solar energy and water. Using the energy of the sun, the plant absorbs carbon dioxide from the air, and under the influence of the energy of the sun, this gas reacts with water, which the plant absorbs by roots from the earth. The products of photosynthesis are carbohydrates, which the plants themselves feed on, and the oxygen we need so much. Plants have been found to emit approximately 6 tonnes of oxygen per tonne of matter consumed for respiration.

You can make up the following formula for photosynthesis: water + carbon dioxide + solar energy = carbohydrates + oxygen.

However, it is wrong to think that only terrestrial plants emit oxygen. In fact, the lion's share of oxygen (over 80%) is emitted by algae in the seas and oceans. These blue-green algae or phytoplankton supply oxygen to the Earth's atmosphere through the water column. That is why it is more correct to call the oceans and seas "the lungs of our planet."

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