Why doesn’t the oxygen we breathe run out?

To the question: “Where does the oxygen we breathe come from?”, most of us would answer that from plants, bearing in mind the image of the Amazon rainforest or our mountains and mountains, associated with the importance of their conservation. . However, the correct answer includes, along with plants, tiny marine organisms that float by the thousands in each drop of water: cyanobacteria.

Marine cyanobacteria are responsible for more than 50% of the oxygen produced on Earth. They provide oxygen to the sea, allowing marine beings to breathe. If the cyanobacteria stopped fulfilling their function, the sea would be a graveyard. They gave us the primeval oxygen bag from which we still breathe.

How the oxygen we breathe was created

During the first half of our planet’s history there was no oxygen in the atmosphere. It was the primitive cyanobacteria that evolved oxygenic photosynthesis: a method of taking energy from sunlight to produce sugars from water and CO₂, which ultimately results in the release of oxygen.

This spectacular event known as the Great Oxidation Event or the oxygen revolution was decisive in our evolutionary history. The increase in oxygen concentration allowed the appearance of multicellular life forms, which increased in complexity until reaching the current biodiversity.

Today we continue to live on this reserve created over millions of years, which is maintained thanks to the fact that the balance with the other processes where oxygen is consumed is almost nil. Only one thousandth of the world’s photosynthetic activity escapes biological processes and is added to atmospheric oxygen.

The lack of oxygen that devastates marine life

At the surface of the oceans, marine cyanobacteria produce enormous amounts of oxygen. Enough for marine life. However, sometimes the system becomes unbalanced and the waters become uninhabitable for most aerobic organisms.

In them, the solubility of oxygen is lower, the water less dense and there are no currents for ventilation. These areas have multiplied in recent years, mainly due to ocean warming, which decreases the solubility of gases and due to the excess of nutrients, due to anthropogenic activity. This is what happens, for example, in the Mar Menor, which due to the discharge of large amounts of nutrients from agricultural activity (nitrates and phosphates) causes eutrophication and decreases the oxygen that fish need for life.

The consequences of these hypoxic zones on marine life are evident. Only those individuals that can migrate to other regions survive and the organisms that cannot move by themselves or move very slowly (algae, invertebrates, molluscs, corals, seagrasses, some echinoderms, etc.) die or will die.

If we ran out of oxygen in the oceans, there would be a huge loss of habitat and biodiversity.

The importance of marine cyanobacteria

Marine cyanobacteria form part, together with unicellular algae, of phytoplankton. These microorganisms float by the thousands in each drop of water in the upper layers of the ocean and constitute the first link in the food chain of these ecosystems. Without them, seas and oceans would be lifeless deserts. In addition, they contribute substantially to maintaining the cycles of carbon, oxygen and nitrogen in the biosphere.

These microorganisms complete their cycle of renewal and death in just a few days. They are the source that produces most of the world’s oxygen and in addition to absorbing light and releasing oxygen, they remove dissolved CO₂ to fix it, in the form of carbohydrates, to their biological structures. When phytoplankton die, some of the sequestered carbon falls into the deep ocean.

Cyanobacteria Synechococcus.
Wikimedia Commons / Masur

Marine Cyanobacteria: Synechococcus Y Prochlorococcus

Marine cyanobacteria are mostly made up of two large genera: Synechococcus Y Prochlorococcus. Until about 45 years ago, these microorganisms were completely unknown. Synechococcus was not discovered until the late 1970s and its closest relative, Prochlorococcusuntil 1986.

The oceanic distribution of these groups depends, among other factors, on the availability of nutrients and temperature. While Prochlorococcus abounds in nutrient-poor waters of subtropical and tropical zones, Synechococcus it thrives in waters with intermediate and moderately low levels of nutrients, colonizing a wide number of ecological niches. Recent studies have shown that interactions with predators are also an important factor in the distribution of these microorganisms.

Although cyanobacteria require nitrogen as an essential nutrient for growth, its availability is a limiting factor in the oceans. We can find this element in the form of ammonium, urea, nitrite, nitrate or amino acids, the first being the preferred source of these microorganisms.

Are both genders capable of coexisting?

Both organisms inhabit areas where nutrients are very scarce, and we might wonder if they can coexist, or the presence of one excludes the other as they are competitors for the same nutrients. The answer is yes, they coexist.

Though Prochlorococcus is more abundant Synechococcus marine is capable of successfully coexisting, even in oligotrophic zones of the oceans. So how do you get it? This answer is not yet known for sure, but one hypothesis is that Synechococcus prefer to use the nitrate in the medium and not compete for the ammonium.

For this reason, the assimilation of nitrate is of particular interest, because it is an abundant form of nitrogen in marine environments, although at the same time it is an expensive source for the cell, since it is completely oxidized and the cell needs to carry out two reactions of reduction to be able to use it: it has to pass the nitrate to nitrite and the nitrite to ammonium. In addition, almost all the marine lineages of Synechococcus possess the genes that encode the machinery to assimilate nitrate, unlike most Prochlorococcuswhich lack it.

Our work in the laboratory with Synechococcus It consists of the measurement of different parameters that indicate the state of the crops according to the availability of nitrogen. Some preliminary results of our group suggest the existence of a system that allows Synechococcus detect nanomolar concentrations of nitrate. Is it a specific system in its response to very low concentrations of nitrate? We continue working to answer this question that allows us to deepen our knowledge of the largest producers of oxygen on Earth.

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