The importance of micro-organisms for marine biodiversity

© Noan LeBescot / CNRS / Fondation Tara Expéditions

The ocean is home to a vast community invisible to the naked eye and yet vital to life on earth: micro-organisms. This treasure of underwater biodiversity forms the first link in the food chain and produces up to 50% of the oxygen we breathe.

Eric Karsenti, scientific director of the Tara Oceans expedition (2009-2013) and corresponding member of the Academy of Sciences, explains the importance of micro-organisms for marine biodiversity. 


La joie du co-directeur de Tara Oceans, Eric Karsenti
Eric Karsenti, scientific director of the Tara Oceans expedition © Vincent Hilaire / Tara Expeditions


How are micro-organisms defined?

Organisms a few microns in size, up to a maximum of 1 millimeter, are called micro-organisms. This represents thousands if not millions of different organisms and species, from viruses to bacteria to protists (rather complex unicellular organisms), and also some very small multicellular organisms.

These organisms exist on land, in rivers and lakes, but also in large quantities in the oceans. Some live in symbiosis with animals and plants. For example, we digest our food thanks to our “intestinal flora” which is largely composed of bacteria. Ruminants also digest the grass they eat with specific bacteria. The polyps that build coral reefs need a single-celled photosynthetic alga (zooxanthella) that lives inside their cytoplasm and provides them with nutrients using sunlight.


Example of a microorganism – © C.Sardet/CNRS/Tara Oceans Polar Circle


Bacteria and unicellular algae play a particularly important role in the general economy of our planet by forming the basis of the food chains, but also by recycling almost all the organic matter of the planet.

For example, photosynthetic bacteria and unicellular algae in the oceans produce close to 50% of the oxygen in our atmosphere. They are at the base of the marine food chain, which is organized in complex ecosystems composed of viruses, bacteria, protists and a myriad of tiny multicellular organisms such as copepods, small jellyfish and micro-crustaceans. These serve as food for krill, fish and whales.

Obviously, each organism feeds on other organisms within these chains, forming highly complex networks of interaction. When large organisms die, their matter breaks down and is recycled into carbon molecules by micro-organisms, which are also organized locally into ecosystems. The example of dead whales falling to the bottom of the oceans is particularly striking from this point of view: a very complex ecosystem of organisms develops around the body until it is completely degraded.


Why is the biodiversity of these micro-organisms so important?

The answer is both simple and complicated. It’s simple because this biodiversity comes from life’s history on earth. It’s complicated because we don’t exactly know how life started and therefore how the initial diversity occurred.


But it’s clear that all the diversity we see today is based on a single form of life chemistry that has been maintained and propagated since its appearance about 4 billion years ago. This life is based on what is called the “genetic code”, a series of letters representing complex molecules rich in energy which form a polymer: the famous molecule of DNA organized in a double helix. Each DNA molecule has the property of being able to modify itself by accumulating mutations or by mixing with other DNA molecules.



Margaux Carmichael at the long sampling station in the Eastern Siberian Sea - © A.Deniaud / Tara Expeditions Foundation


Since the sequence of letters in DNA largely determines the nature of a living organism, these processes have led, in the course of the 4 billion years that preceded our present era, to the formation of an incredible number of different organisms This is called evolution. The diversity of living organisms functioning so well today is due not only to changes in DNA molecules, but also to the gradual selection of organisms that contain this DNA. This selection process is complex: the organisms best suited to a given moment of evolution are not the only ones to survive. The evolution does not occur at the level of individuals but at the level of ecosystems. Indeed, for an organism to survive, it must be able to feed on the environment either by using chemical processes and/or light, or by eating other organisms. The one that is eaten can be considered the weakest. It must nevertheless continue to exist in the system in order for its predator to survive.

On the other hand, if a variant of the prey exists but is not the preferred prey of the predator, this variant may accumulate until it becomes a favorite prey for another slightly different predator. Biodiversity is therefore important in general for the proliferation and diversification of life on earth.



Exemple d’un micro-organisme : le Vellela © Patrick Chang / CNRS / Fondation Tara Expéditions


This principle is the same for organisms that feed on the environment and must adapt to its changes. Let us suppose that only one species of microscopic algae exists, perfectly adapted to an environment that prevails at time To at a given place. For example, a microscopic alga A, which needs a certain amount of light and an ambient temperature of 200 C. Suddenly the light decreases and the temperature rises. The alga A dies … and nothing else exists. Now let’s imagine that another alga B, very similar to alga A, coexists with it, although it does not function optimally under the conditions that A prefers. If B is able to survive perfectly under the new conditions which are unfavorable to A, then alga B will take over. Of course this is extremely sketchy, but it describes how ecosystems are working and biodiversity has increased over the past 4 billion years. This is what makes life incredibly resilient to environmental changes.

Biodiversity is the history of life on earth. Without the enormous biodiversity that has accumulated over billions of years bearing the imprint, in a way, of everything that has happened on our planet, life would be much less robust.

When a predator like humans begins to drastically reduce biodiversity, the system can evolve in an unpredictable way. It is clear that the human species, with almost 9 billion people and their enormous chemical transformation activity, is now having an important effect on the evolution of ecosystems composed of microorganisms (among others!) which recycle energy and organic matter on earth. It is therefore necessary to know this biodiversity on a global level in order to be able to follow its evolution and trends.


Expédition Tara Oceans (2009-2013) - Sortie de la revue Science
Expédition Tara Oceans (2009-2013) – Sortie de la revue Science © S.Bollet / Tara Expéditions.


This is why expeditions such as TARA OCEANS are so important right now. The aim of this expedition is to characterize the planktonic ecosystems of the oceans, including everything  from viruses and protists to fish larvae. To this end, the schooner TARA  is currently traveling around the world through the tropical and southern oceans. Scientists on board and on shore are identifying organisms collected every 200 nautical miles (360 km), and correlating the complexity of these ecosystems with their environmental parameters. Using sequence analysis of their genomes and quantitative imaging methods, researchers will be able to determine the composition of ecosystems in oceans with very different characteristics.

In addition to obtaining an initial map of the composition of marine planktonic ecosystems, these analyses will allow us to build dynamic models of organism distribution in the oceans. This will allow us to understand their evolution, and consequently our own, since life most likely began in the oceans.

Academic Perspectives on Biodiversity