On roads very far from the sea, like those of some towns in Lion and Palenciaan attentive walker can detect an astonishing presence: archaic coral reefs in the middle of the mountain, between cows and old mines. They are the vestiges of another era, the Paleozoic, when tropical seas covered a good part of what is now Europe. The geochemist’s team Alfredo Martínez García has now made an unexpected discovery. Researchers have analyzed fossil corals from inland Germany and North Africa and have identified the oldest chemical trace of the cooperation between strangers on which much of life on Earth depends: the symbiosis between the only animal visible from spacecoral, and some single-cell algae. The discovery, a friendship of 385 million years, is published this Wednesday in the magazine Natureone of the showcases of the best world science.
In the drama of evolution, symbiosis is the calm force that redesigns destinies and redefines possibilities, in the words of the American essayist Dorion Sagan. A German botanist, Albert Bernhard Frank, coined the term symbiosis. in 1877to refer to the common life of two totally different species in a single organism: the lichen, an inseparable couple formed by a fungus and an algae. The biologist Leopoldo García Sancho proclaims in a new book, The triumph of a strange friendship (Ediciones Pirámide), that these small symbioses “move the world.”
García Sancho, professor at the Faculty of Pharmacy at the Complutense University of Madrid, does not exaggerate. The professor remembers 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 sensed that the corals were growing towards the light and trying to stay very close to the surface. García Sancho explains why. Corals are tiny sedentary animals with tentacles, but they obtain most of their nutrients thanks to single-celled algae that live inside their organism. It is an intracellular endosymbiosis, “the most intimate form of relationship between strangers,” according to the professor.
Those algae that live inside the animal, called zooxanthellae, need sunlight to carry out photosynthesis and transform carbon dioxide (CO₂) into sugars, which is why corals only live in crystal-clear, luminous waters. This perfect symbiosis produces “the miracle,” as García Sancho describes it. Reefs cover only 0.2% of the ocean floor, but are home to a quarter of all marine species, providing food to 500 million peopleaccording to the United Nations.
The geochemist Alfredo Martínez García, born in Castellón de la Plana 42 years ago, has led his own group at the Max Planck Institute of Chemistry since 2015, in Mainz (Germany). He and his American colleague Daniel Sigman More than a decade ago, they developed a new revolutionary technique, capable of analyzing a few micrograms of matter 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,” says Martínez García.
“When you eat, you metabolize light nitrogen faster and it is what you excrete in your urine, so in proportion you are enriched in heavy nitrogen compared to your food. It is very interesting, because it is a fairly fixed amount between different organisms,” continues the Max Planck researcher. Scientists measure this enrichment in parts per thousand. An herbivore would be enriched about four parts per thousand with respect to the plant it eats. And a carnivore would be enriched about four parts per thousand compared to the herbivore that eats and about eight parts per thousand compared to the plant, according to Martínez García. “By measuring the concentration of animal tissues you can reconstruct complicated food webs,” he celebrates.
His team has analyzed fossil corals recently collected in Sauerland, a mountainous area in the interior of Germany, and other historical specimens kept in the Natural History Museum in Frankfurtcoming from the German volcanic region of Eifel, Tafilálet (Morocco) and Western Sahara. They 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 Laurrusia.
Martínez García’s group has also examined current living corals. In the same reef, most have unicellular algae inside, but others do not have them and get food with their tentacles. The team has observed that corals that eat on their own are enriched in heavy nitrogen by around four per thousand, compared to corals that obtain nutrients from their algae. “If you have symbionts you are one trophic level below, like a plant. From the point of view of nitrogen, it is as if you were doing photosynthesis,” says the geochemist.
This characteristic has allowed us to infer that some fossil corals were already living in symbiosis 385 million years ago, almost twice as long as what was known until now. It is the first evidence of symbiosis in corals, but other, older friendships between different species are known. The fossil of a lichen Found in Weng’an, southern China, it is about 600 million years old.
The old friendship between corals and algae would explain why reefs reached enormous sizes in the Paleozoic, despite the lack 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. Biologist Leopoldo García Sancho warns that these giants face a threat: bleaching, a phenomenon caused by the abrupt increase in temperatures, which causes the corals to expel their colorful algae, acquiring a pale tone.
“It is estimated that, if the average surface water temperature increases by 1.5 degrees, a good part of this symbiosis will disappear with no chance of recovery. Some reefs may survive in places where they seem least sensitive to warming, such as the Persian Gulf and the Red Sea, but they will be just the remains of a magnificent, fading world whose disappearance will take much of the diversity of our oceans with it. “Warns García Sancho in The triumph of a strange friendship.
