The science schooner has spent two and a half years at sea studying coral reefs in the Pacific. These organisms, which cover less than 0.2% of the ocean's surface, are home to 30% of the known marine biodiversity.
Jean-François Ghiglione, research director at the CNRS and deputy director of the Oceanological Observatory at Banyuls, participated as a scientific diver in this mission. He is one of the coordinators of Tara Pacific's data analysis and is particularly interested in the role played by microorganisms in the health status of corals.
The objectives of the Tara Pacific mission
With 30 countries visited, 100,000 km covered and 35,000 samples collected, the Tara Pacific mission is by far the largest ever carried out on coral reefs because of its geographical size and the number of samples taken.
The goal of the Tara Pacific mission is to study the diversity of coral reefs at different scales (genetic, viral or bacterial) by traveling the Pacific from East to West and from South to North.
To analyze this diversity, scientists have considered corals as a whole. They are interested in the holobiont, that is to say, in all the organisms that make up the coral including those that cover and inhabit it. Among these organisms, there are algae called "zooxanthellae" that live in symbiosis with the coral and give it its color, but also the microbiota, composed of microorganisms (bacteria, viruses, fungi, etc.).
"We measure many environmental parameters and establish comparisons between these parameters and the coral diversity through variations observed on zooxanthellae and microbiota," explains Jean-François Ghiglione.
Inside the coral, the microbiota plays an important role in the corals’ adaptation to their living environments. By exploring this mibrobiota, researchers hope to discover the potential for resistance, adaptation and resilience of these ecosystems to climate change.
The first observations of Tara Pacific
Arriving at Upolu, an island in the Samoan archipelago known for its diving site in the South Pacific, researchers expected to find beautiful corals. Instead, they discovered "a coral cemetery". After circling the island and sampling 124 sites on 80 km of coast, they observed a coral mortality of more than 90% on most sites.
"It was the first time we saw so many dead corals and we saw the effect of global warming. It was a very emotional time. It was my first climate shock," says Jean-François Ghiglione.
The El Niño climate phenomenon, which brings warm currents to these areas, combined with the warming of the oceans, is one of the causes of this disaster. Accustomed to a certain thermal amplitude, the corals do not resist in waters that exceed these thresholds. The waters of the Samoan archipelago had already reached 35 ° C at the low season, when Tara passed. At this temperature, the corals die and the plants living in suspension in water (phytoplankton) develop completely bleached skeletons.
By observing that coral mortality was significantly lower in the protected sites, the researchers were able to establish another causal link to explain the coral mortality rate.
"On this island, sewage is not treated and overfishing is obvious. We have alerted the authorities to the situation because there is no regulation and the inhabitants are not aware that the coral is dying," says Jean-François Ghiglione.
Another major observation of the Tara Pacific mission is the demonstration of the role of the microbiome in the ability of corals to adapt to climate change. Studies have shown that some corals can resist in acidic environments thanks to their bacteria capable of regulating the pH. The question remains to what extent these microorganisms will allow corals to adapt to unprecedented changes.
With the end of Tara Pacific's offshore mission, analyzes are just beginning and Tara's discoveries of ocean biodiversity and coral reefs are likely to be numerous in the years to come.
The samples taken during the mission will involve the French Atomic Energy Commission’s Genoscope, designed for the genome sequencing work but the final results could take about ten years. By then, several billion DNA and RNA sequences will be analyzed in laboratories.
Sorbonne University’s involvement through its three marine stations?
"Sorbonne University is very involved,” says Jean-François Ghiglione. “Thanks to the impetus of Gaby Gorski, former director of the Oceanological Observatory of Villefranche-sur-Mer, the scientists from the three marine stations of Sorbonne University were among the pioneers in the various Tara missions."
In the Observatories of Villefranche-sur-Mer, Roscoff and Banyuls, researchers are working hand-in-hand on these programs. Sampling protocols required true coordination among all researchers to allow for large-scale comparative analysis of samples.
"The three stations have a very important and complementary role with different skills," says Jean-François Ghiglione.
Since September 2018, Tara researchers have organized themselves in the Tara Oceans Research Federation, which brings together about twenty laboratories in France and abroad, as well as the Tara Foundation and several supervisory bodies including the CNRS and Sorbonne University.
With these three marine stations on three maritime coasts and their recent organization into an "OSU marine stations", Sorbonne University plays a major role in this research federation, which brings together all the Tara missions and offers better coordination and a greater visibility to the research work. But Tara is still a human adventure for the researchers at Sorbonne University: "At first, it was mostly a group of friends, ready to try the Tara adventure and travel the oceans together. Today we publish special issues in the largest scientific journals, we have a lot of data and Tara has become a reference in the field of metagenomics," rejoices Jean-Francois Ghiglione.
Tara will leave next year to continue the expedition started in 2014 in the Mediterranean on the diversity of this region.
"Tara is an exceptional boat that has already made a big contribution to our knowledge of ocean biology. The idea is that it never stops and that it continues its mission throughout the world," says Jean-François Ghiglione.