The role of bacteria and algae in the balance of life in the sea (and on land)

Is it true that half of the oxygen we breathe is produced in the sea?

The cycle of life inevitably involves creation and destruction. Organisms are born, grow, reproduce (with some luck), and die. In nature, death means an increase in the survival probabilities of someone else. Plankton are no exception to these basic rules of life. In fact, the members of the plankton take these rules to such an extreme that many times their communities (especially those in nutrient-poor waters) are maintained by recycling excretions and the remains of corpses. This recycling allows life to continue, although not eternally; a small allochthonous input of nutrients is always needed from time to time to reinvigorate the ecosystem.

Those in charge of doing the tremendous task of recycling in the water column are the bacteria. Tiny, but numerous, bacteria have colonized all the ecosystems of the planet, both aquatic and terrestrial. Without them the corpses and excrement would pile up. Taking into account the years we have lived in the planet, without the activity of bacteria we could already walk, or even climb, on a sea of ​​corpses. Bacteria carry out complex reactions in which organic matter is broken down into inorganic matter that can be used by the primary producers, the phytoplankton. They also contribute to the cycle of many chemical elements, such as sulfur, iron, etc.

Photomicrograph of bacteria and protists taken with an epifluorescence microscope

The algae and photosynthetic bacteria that compose the phytoplankton are responsible for assimilating the inorganic salts produced by the action of bacteria (especially nitrates and other forms of nitrogen, phosphates, and silicates) and incorporating them into living matter. They also capture CO2 (+ H2O) and turn it into organic matter with the help of the sun’s energy. In this process, called photosynthesis, they also release oxygen; a by-product that has a bad habit of rusting things, but that is essential for our lives. Regarding oxygen, it is quite common to read on outreach websites or the press, and even in some scientific publications, that marine algae contribute to approximately half of the oxygen we breathe. Although this is true on a geological scale of millions of years, it is not correct on a day-to-day basis. It is true that phytoplankton, throughout the life of the planet, has contributed to accumulating perhaps more than half of the oxygen in the atmosphere, and that without its photosynthetic activity, animals (at least as the we know) had not colonized the earth’s surface. It is also true that phytoplankton contribute approximately 50% of primary production (photosynthesis), and that they are responsible for more or less half of the oxygen produced on the planet each day. However, this oxygen produced in seas and oceans is mostly consumed within them by the different micro- and macro-organisms that live there. In other words, only a very small percentage reaches the atmosphere. But for now, we shouldn’t worry, it doesn’t seem like the oxygen in the atmosphere is going to run out in the immediate future.

Marine algae

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