Franck Lartaud: “Cold-water corals are the forgotten ones”

How did you come to work on cold-water corals?
Franck Lartaud : : I first became interested in cold-water corals when I joined the OOB in 2010. At the time, the Parc naturel marin du Golfe du Lion was being created. At the time, very little was being said about these largely unexplored deep-water reefs. Everything had to be done: mapping, describing and understanding how they worked.   

I soon became fascinated by little-known corals like Desmophyllum pertusum (see illustration). They grow at the depths of several hundred meters, often in canyons and seamounts. These areas are ideal for marine currents, which transport the organic matter they need to survive.

How important are these corals for marine ecosystems?
F.L. : These cold-water corals form complex structures, entire reefs that become essential habitats for many species: fish, crustaceans, invertebrates...and others. They are ingenious species, shaping their environment and supporting a rich and functional biodiversity.
Tropical reefs are best known for their ecosystem services—tourism, fishing, coastal protection—but deep reefs also perform vital functions, including in deep marine food chains. Since these reefs are located in fishing zones, they are directly linked to the fishing economy, even if they are often neglected. The problem is that they grow very slowly. A few millimeters a year, in the best cases. It takes centuries to compensate for any disturbance.

What is the link between plastics and the reefs you study?
F.L. :While diving in underwater canyons, particularly off the coast of Banyuls, we began to observe more and more plastic waste: bags, nets, bottles, clinging to coral colonies, sometimes at depths of 500 or 1,000 meters. We're talking about several thousand pieces of waste per square kilometer in some areas! Macroplastics, like bags, can suffocate corals, blocking access to their food and reducing oxygenation. Microplastics, on the other hand, are caught and then discarded, but this requires a lot of energy from the corals. 
Measurements show that 90 percent of the captured particles are then rejected within 24 hours. But this is an enormous stress that weakens their organisms for a long time to come.

What actually happens in coral tissue?
F.L. : When a coral captures microplastics, it activates defense mechanisms: mucus production, tentacle contraction and active rejection. This disrupts its feeding behavior, slows its growth and makes it more vulnerable to infection. The effects can even be seen in the composition of its microbiota, with changes in bacterial flora.
The coral loses its equilibrium. And this imbalance opens the door to opportunistic pathogens, such as bacteria. It's not a massive contamination, but a profound weakening.

You also conduct laboratory experiments. Can you tell us about them?
F.L. : We have designed experimental aquariums that reproduce deep-sea conditions: such as temperature, darkness, current and salinity. We introduce fluorescent microplastics of various sizes to observe their behavior and effects.
We measure coral growth, capture behavior, energy reserves, and even the expression of certain genes linked to stress or immunity. The aim is to take a multi-scale approach, from cell metabolism to colony dynamics. We're also trying to simulate one-off exposures, such as a storm event or a massive particle release, to see how corals can adapt.
What we're trying to do is make the invisible visible.

Do you also work with mussels as bio-indicators?
F.L. : Absolutely, bivalves such as mussels and oysters are excellent biosensors. They filter water and accumulate particles, including microplastics. By analyzing them, we can get an accurate picture of local pollution, including particles too small to be detected by conventional tools.
With the start-up Plastic at Sea, we have launched a CIFRE thesis to develop these indicators, both at sea and in rivers, using freshwater mussels. It's a simple, inexpensive and particularly effective solution for tracking pollution trends over time and space.

Is it still possible to restore destroyed reefs?
F.L. : It's difficult, but possible. We're taking part in a European project to restore deep reefs. The idea is to identify degraded areas, test transplanting and assisted colonization and propose appropriate protection strategies.
But we're talking about reefs that take centuries to form. What we're doing today is for future generations. We have to accept this long time frame, while meeting the pressing expectations of managers.

You also work with fishermen. How is this cohabitation going?
F.L. : At first, relations were tense. The fishermen would say: “It's not our fault”. Today, thanks to dialogue and explanations of data, many are willing to work with us. We try to co-construct compromise zones, minimize conflicts of use, and sometimes even explore alternatives to trawling. We can't impose anything authoritatively. You have to build conservation with local players, not against them.

And on the political front? What progress has been made and what difficulties have their been?
F.L. : We pass on our data to managers, natural parks, the French Biodiversity Office and maritime authorities. This has led to the creation of strong protection zones, for example in certain Mediterranean canyons.
But cross-border zones, such as those between France and Spain, pose problems. There are legally grey areas, where no country can really act. And in these areas, protection is sometimes purely theoretical. 
What we're trying to do is make the invisible visible. To show that these ecosystems, however discreet, deserve to be protected. And that we can still act, experiment and demonstrate, even on a small scale. If we can show that a local restoration project works, it can become a model that can be reproduced elsewhere. That's how we make progress, by taking small steps.

Interview by Pauline Ponchaux