The abundance of animal life in the ocean has provided an enormous variety of services from the beginning of time. From food to adventure and leisure.
None of this would be possible without the unicellular organisms of phytoplankton, which float by the thousands in each drop of water in the upper layers of the sea.
The phytoplankton comprises two main groups: photosynthetic cyanobacteria and unicellular algae that move near the sunlit surface of the oceans.
They do it in the so-called euphotic zone, which can reach a depth of up to 200 meters in the tropics.
It seems logical to think that the great producers of oxygen are the meadows, the young forests, the crops and almost all the growing plants that surround us, which give off more oxygen than they consume. It’s not like that.
Plants with larger and more complex structures have a lower oxygen production balance. Those with a simple structure, green and with smaller trunks are those that have a higher net oxygen production.
Where are the plant populations that multiply continuously and do not stop growing?
The organisms responsible for our breathing can be found in the oceans; which, let’s not forget, cover 71% of the Earth’s surface.
Phytoplankton is at the base of the trophic chain of ocean ecosystems. Without the autotrophic microorganisms that compose it, seas and oceans would be lifeless deserts.
Thanks to their photosynthetic work, these microscopic creatures produce between 50 and 85% of the oxygen that is released every year to the atmosphere.
For a couple of decades, the images of Nimbus satellites of NASA and the US Meteorological Agency showed that oceanic productivity, evaluated based on chlorophyll concentrated on the sea surface, could be higher than the productivity of terrestrial ecosystems. This suggested that phytoplankton was the great oxygenator of the planet.
The hypothesis was confirmed in 2015 by the international project Tara Oceans, whose results concluded that phytoplankton generates at least half of the oxygen we breathe; about 270 billion tons per year, and transfers about 10 gigatons of carbon from the atmosphere to the depths of the ocean every year.
This is essential to maintaining life on Earth and mitigate the effects of climate change.
Phytoplankton has chlorophyll, the pigment that makes photosynthesis possible. In addition to this, it serves as food for zooplankton, which in turn feeds other marine animals.
Billions of microscopic plants that inhabit the sine of the oceans perform their cycle of renewal and death in just a few days.
In that infinite universe that is born and dies continuously, the phytoplankton is the pump that produces most of the O₂ we breathe. But, in addition to absorbing light and releasing O₂, chlorophyll allows these tiny plants to remove dissolved CO₂ to fix it, in the form of carbohydrates, to their biological structures.
Therein lies the crucial role of phytoplankton in the carbon cycle and, as a consequence, in its colossal ability to purify the air. Thanks to photosynthesis, phytoplankton consumes CO₂ on a scale equivalent to terrestrial ecosystems.
It is estimated that each year it incorporates between 45 and 50 million tons of inorganic carbon. Land plants incorporate about 52 million tons of carbon per year, but this returns to the atmosphere in the short or medium term.
When the phytoplankton dies, part of the carbon captured falls to the depths of the ocean.
All living organisms in the photic zone sink when they die, so there is a constant rain of organic matter into deeper waters.
Nutrients are returned to the upper layers of water, especially in places where there are strong updrafts due to the topography of the bottom and the patterns of ocean currents.
About 85% of the organic matter created each year by phytoplankton is recycled among organisms that live in the illuminated waters, while the remaining 15% is lost in the depths of the ocean.
There, where microorganisms have removed oxygen from water, the remains of organic matter remain buried under anaerobic conditions. This plant matter buried at the bottom of the ocean is the source of oil and gas.
Only a small fraction, around one thousandth of photosynthesis worldwide, escapes the processes described and is added to atmospheric oxygen.
But since the appearance of cyanobacteria, the first photosynthetic organisms, between 3,500 and 3,800 million years ago, the residual oxygen left by the small imbalance between growth and decomposition has accumulated to form the reservoir of breathable oxygen, whose volume represents 21% of the total atmosphere.
Therefore, although photosynthesis is ultimately responsible for respirable oxygen, only a small fraction of plant growth is added every year to atmospheric oxygen storage. Even if all the terrestrial organic matter burned at the same time, less than 1% of the oxygen available in the world would be consumed.
How is it possible that the phytoplankton mass does not run out if the biomass of organisms that prey on it is much higher?
The balance is offset by a high renewal rate. The high reproduction rate of phytoplankton makes their populations renew faster than they are consumed.
A whale shark that feeds on millions of these small photosynthetic cells is only able to give birth to one baby per year. On the other hand, a diatom is capable of generating a million descendants every day.