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Why is the Amazon not the lung of the planet?

The lungs in a human being are those organs that, after inhalation , provide oxygen to the blood for distribution throughout the body, and obtain carbon dioxide from it and then exhale it. If we assimilate the analogy to the biosphere, the “lungs of the planet” would be that part that provides oxygen to other living beings, and in return obtains carbon dioxide from the atmosphere. Of all the existing metabolic routes, there is one that does precisely that: acquire CO₂ and use it as a chemical substrate to obtain energy from sunlight, releasing gaseous oxygen. We call it photosynthesis.

Of course, the plants we all know are photosynthetic organisms, and as long as there is sunlight, they produce oxygen in exchange for retaining carbon. Of the plants, the most valued are the trees , perhaps because we assume that the larger, the more effective it will be. However, they are not as productive at their task as is commonly believed. Much of a tree’s biomass is made up of roots, trunk, branches, and bark, parts of the tree that do not photosynthesize. Comparatively, in photosynthetic terms, a ton of herbaceous plants, whose green stems also photosynthesize, are much more productive than a ton of trees.

However, what trees are good at is sequestering carbon . Most of a plant’s dry biomass is carbon taken from the air, and a ton of grasses will have acquired the same amount as a ton of trees. However, trees store this carbon in their tissues much longer, sometimes for centuries and even millennia.

The neutral balance of the Amazon

The Amazon rainforest is the place on the planet with the largest number of plants. At its different levels: trees, shrubs, lianas, herbs and epiphytes —plants that grow on top of other plants— water the horizon in a magnificent range of greens. Not without reason, tropical forests are responsible for a third of photosynthesis on land , and the Amazon forest is the largest. He only takes care of 16% of photosynthesis on land.

But a relevant feature is that it is a forest in its most mature state . This includes a certain amount of animals and microorganisms that populate its interior and its soil. Organisms that breathe, taking in oxygen and exhaling carbon dioxide. Also, during the night, the forest continues to breathe, but not photosynthesize. In the end, no matter how much photosynthesis the forest performs, the amount of net oxygen it contributes to the atmosphere is approximately zero.

The bacterium that dominates the world

But not all ecosystems are the same. Although up to 16% of the photosynthesis on land falls on the Amazon forest, this calculation does not include the photosynthesis that occurs in the oceans and other bodies of water. Furthermore, in the case of algae and cyanobacteria—photosynthetic bacteria—all of their biomass is photosynthetic, so comparatively, it is these organisms and not plants that win the prize for efficiency.

This might be irrelevant if algae and cyanobacteria were in the minority, if their biomass was insignificant compared to photosynthetic organisms on land. But it’s not like that. Up to 85% of the oxygen released into the atmosphere each year is produced by photosynthetic microorganisms . And among them, the cyanobacteria of the genus Prochlorococcus are the dominant… in the world . They are the smallest known photosynthetic organisms, and also the most abundant. Its population is estimated at three thousand quadrillion (a three followed by 27 zeros) and its biomass is about half that of all of humanity. And not only do they contain half of all the chlorophyll on the planet , but they are also the organisms that use it most efficiently. Prochlorococcus alone is responsible for 20% of the oxygen released into the atmosphere each year.

 

 

A merit of prehistory

However, neither are the algae or cyanobacteria that today inhabit our seas the ones that deserve the title of “lungs of the planet”. Although they are the ones that are producing the most oxygen, there is an impossible limit to exceed. And again, it is carbon dioxide that comes into play. In the reaction of photosynthesis, each molecule of carbon dioxide absorbed involves one molecule of oxygen released. The balance is strict , one enters, one leaves.

Our atmosphere is made up of approximately 21% oxygen, however, the amount of carbon dioxide , despite the constant and worrying increase in recent decades, is below 1%. That is the maximum oxygen that can be produced. It is the limit.

If all living things suddenly stopped breathing, and all carbon dioxide suddenly turned into oxygen, the end result would be a minimal, almost negligible increase in the amount of oxygen.

All that oxygen that abounds in the atmosphere is therefore not a modern product. Large forests are invaluable in sequestering carbon, which is essential to combat the climate change we are suffering —and in many other aspects. And the photosynthesis that happens on land and, above all, in the oceans helps keep those oxygen levels stable in the atmosphere. But that huge amount of oxygen that allows us to breathe is the legacy that prehistoric photosynthetic microorganisms have left us for billions of years. The true lung of the planet is, in fact, in the remote past of the history of life.

 

REFERENCES:

Beer, C., Reichstein, M., et al. 2010. Terrestrial Gross Carbon Dioxide Uptake: Global Distribution and Covariation with Climate. Science, 329(5993), 834-838. DOI: 10.1126/science.1184984

Chisholm, S. W. 2017. Prochlorococcus. Current Biology, 27(11), R447-R448. DOI: 10.1016/j.cub.2017.02.043

Ting, C. S., Rocap, G., et al. 2002. Cyanobacterial photosynthesis in the oceans: the origins and significance of divergent light-harvesting strategies. Trends in Microbiology, 10(3), 134-142. DOI: 10.1016/S0966-842X(02)02319-3

 

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