Tara’s Journal – Plankton: Lifting the Lid on a Mysterious World

© M.Ormestad/Tara Expéditions

Article from new Tara’s 10th journal

Nearly eighteen months since the conclusion of Tara Oceans, the most extensive plankton study ever undertaken across the world’s seas, five articles published in the scientific journal Science are revealing the secrets of the 35,000 samples collected during the expedition. At every level these results have changed forever our understanding of the marine ecosystem.

This small jellyfish was caught in the Mediterranean and is closely related to Turritopsis, a species reputed to be immortal. © Christian Sardet / CNRS / Tara Expeditions

May 22, 2015 will forever remain a key date in the history of Tara Expeditions. On that day a certain nervous excitement tinged with relief affected all the researchers connected with the Tara Oceans expedition.

Why? Because it was the official release of the results of the expedition undertaken by the schooner from 2009 to 2013. Five articles describing in detail the expedition’s findings were published in a special edition of the world-renowned scientific journal Science. The figures are mind-boggling. At the microbial level alone no less than forty million genes were sequenced and most of these were previously unknown to science (mostly viruses and bacteria).


“Thanks to the discoveries of Tara Oceans, the upper layer of the ocean has becomethe first major ecosystem to be described in its virtual entirety”

As for the eukaryotes—organisms whose DNA is contained within the nucleus, unlike bacteria—nearly a billion DNA barcodes drawn from the sequences indicated the existence of 150,000 different genetic types. And as several species might belong to the same genetic type, the number of planktonic eukaryote species could exceed one million despite only 11,000 having been described to date! Moreover, the biodiversity of these species is much greater than previously thought, and far exceeds that of bacteria. Better still, the analyses are reaching saturation point so that each new sample is producing fewer and fewer new genes. In other words, the teams working on Tara Oceans have succeeded in doing what they set out to do: collect virtually all the plankton species living on our planet.


A World of Infinite Interactions

This observation is not, in fact, confined to the field of oceanography. For the very first time a major ecosystem has revealed itself in its virtual entirety. If we take the example of a forest, we are familiar with all its principal organisms but what do we know about the viruses in the soil, the small parasites and the intestinal bacteria of the various animals that inhabit it?

Thanks to the discoveries of Tara Oceans and the results published in Science, the upper layer of the oceans has become the first major ecosystem to be described in its virtual entirety, from its viruses and unicellular organisms to its largest mammals. It is the first of its kind and gives, without a shadow of a doubt, a better understanding of how ecosystems work. It also gives us insights into evolution and ecology. Especially since the researchers aboard the Tara didn’t restrict themselves to listing the organisms living just under the surface of the ocean; instead they also tried to understand how they interact with each other through the use of complex computer models that were validated by the results drawn from the samples. Initial observations show that environmental factors (pressure, salinity, etc.) had less influence than expected on the spatial organization of species; but above all this pioneering mapping of interactions between plankton species reveals that parasitism is the most widespread mode of interaction, ahead of predation, symbiosis and even competition. Foremost what this means is that when a central species—one that is parasitized by many others—disappears, numerous other species are affected.

Tens of Millions of Genes Sequenced


In addition to the “plankton fishing”, each sampling operation carried out during Tara Oceans was an opportunity to measure a large number of the water’s physicochemical parameters: salinity, pressure, temperature and quantity of light. This data was aligned with the results of the vast gene sequencing of the plankton. Conclusion? It’s mainly temperature that determines the composition of species at any given point. This information might appear to be of capital importance in this period of climate change but it is just one of many discoveries unveiled on 22 May 2015 in the journal Science. Distribution of plankton across the oceans, mode of spreading viruses through currents, description of microbial communities in the ocean: listing the many scientific advances published is, truth be told, a difficult task. All these results, however important they may be, are merely the first step in this hitherto ignored field. We can safely say that plankton has many more secrets for us to discover. All the more so because tens of millions of sequenced genes resulting from the Tara Oceans expedition, amounting to 80% of all the marine genes deposited in data banks, are already online and available to scientists the world over.

In the coming years and decades the scientific community will surely continue to provide us with insights into the mysterious world of plankton. Major discoveries of the future that share a common starting point in the shape of a grey and orange schooner called Tara

Related articles:

- Editorial: The Ocean, our common project

- Tara Oceans reveals a new world in the Ocean

- COP21: Meeting with the first French ambassador of the Oceans

Download the whole Tara’s 10th journal here