ITW with Eric Karsenti: a major breakthrough

© D.Sauveur /Tara Expéditions

Eric Karsenti, scientific director of Tara Oceans (CNRS/EMBL) explains the importance of the results published in Science magazine for understanding the planktonic world, and more generally, for knowing the oceans.

EMBL

The Tara Oceans expedition was the first worldwide collection of samples at sea. The published results are the culmination of 6 years of hard work.
This is the crowning of an outstanding collaborative work among all the involved scientists, as well as between the scientists and the Tara Expeditions team. They all worked together in order to successfully conceive and set up the sampling protocol aboard the boat. We would never have been able to achieve these results if the samples had been of poor quality. But they are of fabulous quality! And thanks to the team work, these data lead to first-class scientific articles.

Can you explain the first results that are currently the subject of five papers in a special issue of Science?
Three of the five papers describe the diversity of global plankton: eukaryotes*, viruses and bacteria. We have characterized 150 000 types of eukaryotic organisms, which is ten times more than what was known until now. This work has allowed us to characterize almost every eukaryotic organism living in temperate waters. As for viruses, we found there wasn’t a huge diversity, but that the diversity is more important locally than globally. Certain areas of the ocean seem to be “sources of virus diversity” probably linked to a great complexity of hosts. Eventually these viruses get scattered in the ocean. As for bacteria, of the 40 million genes identified, the majority is new.

Another number that should be pointed out: 39 marine viruses were known before this expedition; we discovered 5,437 marine viruses in the Tara Oceans samples!

Regarding the “Agulhas rings” – eddies generated south of Africa with diameters over 300 km and depths extending down to 4,000 m – the results highlight the connection between the structure of ecosystems and their environment. These rings capture a certain number of species from the Indian and Antarctic Oceans. The rings then move across the Atlantic Ocean for a year or two. We sampled a 9-month old ring. It appears that the structure of its ecosystem not only differs from the Atlantic Ocean it crossed, but also from the Indian and Antarctic Oceans where the ring originated. We can actually characterize how species change depending on the environment within the rings, under the effect of temperature, nitrate concentrations, etc.

Finally, we found that most planktonic organisms (80%) tend to coexist. This trend to “live together” corresponds to either symbiosis, parasitism or balanced host-predator relationships. The results of the last article suggest that these interactions between organisms play an equally important role – maybe an even more important one – in ecosystem diversity and complexity than the environmental conditions.

The Tara Oceans results represent the sequencing of about one billion genetic codes and, most of all, the largest ecosystem database ever created.

Do these results meet expectations?
Yes, they do, and even exceed them! I didn’t expect we would be able to publish all these results in a special issue of Science. It’s very rare. We actually did exactly what we said we would do, except this is just the beginning, there are more major publications to come. Now we must go further in our data analysis.

Talking about further analysis, will the collected data show a connection with climate change?
One of the applications of these data will be to build predictive models on ecosystem evolution as a function of parameters such as water temperature. Regarding bacteria, we have already shown a direct correlation between temperature and the structure of the bacterial ecosystem. For instance, for some combination of bacterial species, we are able to predict, to the degree, the temperature of the environment where they were sampled from.

We plan on going even further. We intend to extrapolate this relation to all organisms in order to achieve an interesting perspective about climate change and the future. It’s just the beginning. We haven’t finished sequencing yet! It may take another five to ten years of intensive work!

Interview by Maéva BARDY

* Eukaryotes : Uni- or multi-cellular beings whose genetic material is enclosed within the cell nucleus.

Related articles:

- Tara Oceans reveals a new world in the Ocean
- ITW of Etienne Bourgois and Romain Troublé : “There will be a before and after Tara Oceans”

Read the five publications:

- Structure and function of the global ocean microbiome
- Eukaryotic plankton diversity in the sunlit ocean
- Determinants of community structure in the global plankton
- Environmental characteristics of Agulhas rings affect inter-ocean plankton transport
- Patterns and ecological drivers of ocean viral