Discovery of one of the first friendships between different species | Science

Along paths far from the sea, such as those in certain towns in the provinces of León and Palencia (Spain), an attentive walker can detect an astonishing presence: archaic coral reefs in the middle of the mountains, among livestock and disused mines . These are the remains of another era, the Paleozoic, when tropical seas covered a large part of present-day Europe. A team led by geochemist Alfredo Martínez García made an unexpected discovery. Researchers analyzed fossil corals from the interior of Germany and North Africa and identified the oldest chemical trace of cooperation between strangers, on which much of life on Earth depends: the symbiosis between only animal visible from space – coral – and unicellular algae. . The discovery, a 385 million-year-old friendship, was published Wednesday in the journal Nature.

In the drama of evolution, symbiosis is the quiet force that redraws destinies and redefines possibilities, in the words of the American essayist Dorion Sagan. A German botanist, Albert Bernhard Frank, invented the term symbiosis in 1877, to designate the common life of two totally different species in a single organism: lichen, an inseparable couple formed by a fungus and an algae. Biologist Leopoldo García Sancho proclaims in a new book: The triumph of a strange friendship, that these little symbioses “make the world move”.

García Sancho, professor at the Faculty of Pharmacy at the Complutense University of Madrid, is not exaggerating. He recalls that in April 1836, a 22-year-old English naturalist named Charles Darwin arrived at the Cocos Atolls in the Indian Ocean aboard the ship HMS. Beagle. The young scientist felt the corals growing toward the light and tried to stay very close to the surface. García Sancho explains why: Corals are tiny sedentary animals with tentacles, but they get most of their nutrients from single-celled algae that live inside their bodies. It is an intracellular endosymbiosis, “the most intimate form of relationship between strangers”, according to the professor.

The algae that live inside corals, called zooxanthellae, need sunlight to carry out photosynthesis and convert carbon dioxide into sugars, which is why corals only live in crystal clear, clear waters. This perfect symbiosis produces “the miracle”, as García Sancho describes it. Reefs cover just 0.2% of the seabed, but they are home to a quarter of all marine species and provide food for 500 million people, according to the United Nations.

Martínez García has led his own group at the Max Planck Institute of Chemistry in Mainz (Germany) since 2015. He and his American colleague Daniel Sigman developed a revolutionary new technique more than ten years ago, capable of analyzing a few micrograms of material and determining the concentration of the two stable variants of nitrogen: nitrogen-14 (light) and nitrogen-15 (heavy). “This gives us information about the relationships between different organisms: who eats who,” explains Martínez García.

“When you eat, you metabolize light nitrogen more quickly and excrete it in your urine, so you are enriched with heavy nitrogen in proportion to your diet. This is very interesting, because it is a fairly fixed quantity between different organisms,” continues researcher Max Planck. Scientists measure this enrichment in parts per thousand. A herbivore would be enriched by about four parts per thousand relative to the plant it eats, while a carnivore would be enriched by about four parts per thousand relative to the herbivore it eats, and by about eight parts per thousand compared to the plant. , according to Martínez García. “By measuring the concentration of animal tissues, you can reconstruct complex food webs,” he explains.

His team analyzed fossil corals recently collected from the Sauerland, a mountainous area in the interior of Germany, and other historic specimens preserved at the Natural History Museum in Frankfurt, from Germany’s Eifel volcanic region , Tafilalet (Morocco) and Western Sahara. These are the remains of reefs from the Devonian, a period of the Paleozoic that began about 419 million years ago and ended about 359 million years ago, when the planet had two supercontinents: Gondwana and Laurasia.

Martínez García’s group also examined living corals. On the same reef, most contain single-celled algae, but others do not and feed on their tentacles. The team observed that corals that feed alone are enriched in heavy nitrogen by about four thousandths, compared to corals that get their nutrients from their algae. “If you have symbionts, you are one trophic level below, like a plant. From a nitrogen point of view, it’s like you’re doing photosynthesis yourself,” explains the geochemist.

This characteristic has led to the deduction that certain fossil corals already lived in symbiosis 385 million years ago, almost twice as long as previously known. This is the first evidence of symbiosis in corals, but other, older friendships between different species have been recorded. The fossil of a lichen discovered in Weng’an, southern China, is approximately 600 million years old.

The ancient friendship between corals and algae may explain why reefs grew to enormous sizes in the Paleozoic, despite the scarcity of nutrients in the environment. Today, Australia’s Great Barrier Reef is “the largest living structure on Earth and the only one visible from space,” according to the European Space Agency. García Sancho warns that these giants face a threat: bleaching, a phenomenon caused by the sudden increase in temperatures, which causes corals to expel their colorful algae, acquiring a pale hue.

“It is estimated that if the average temperature of surface waters increases by 1.5 degrees, much of this symbiosis will disappear with no possibility of recovery. It’s possible that some reefs will survive in places where they seem less susceptible to warming, like the Persian Gulf and the Red Sea, but they will be just remnants of a beautiful, fading world whose disappearance will take with it much of the diversity of our oceans,” warns García Sancho in The triumph of a strange friendship.

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