A new study led by the University of Southampton in the UK reveals why coral reefs thrive in waters that appear nutrient deficient, a phenomenon that has fascinated scientists since Charles Darwin.
This study shows that corals cultivate and feed on photosynthetic symbionts (microscopic algae that live inside their cells). This vegetarian diet makes corals available to large amounts of nutrients previously thought to be unavailable. In effect, they feed on some of the symbiotic algae to get the nutrients they need to survive.
Professor Jorg Wiedenmann, Director of the Coral Reef Institute at the University of Southampton, who led the study, commented: It inspired the discovery of several key processes that help explain this phenomenon. We can now add the missing piece of the puzzle and help solve the age-old mystery. ”
“When Charles Darwin set sail on the Beagle, he considered himself a geologist and was immediately interested in where and why coral reefs form while sailing through tropical waters. accurately predicted how crustal subsidence and steady ascending growth of corals would interact to form vast reef structures, but the biological mechanisms behind this vigorous growth have not been studied. It was left untouched.”
survive together
Stony corals are soft-bodied creatures that some people see as plants, but they are actually animals. These organisms consist of many individual polyps that live together in colonies and a secret limestone skeleton that forms the three-dimensional framework we know as ‘coral reefs’.
Coral reefs are important aquatic ecosystems that benefit many human societies. They provide habitat and feeding grounds for countless organisms and sustain approximately 25 percent of the world’s marine biodiversity. With it, they bring food and income to about 500 million people on the planet.
Coral animals rely on symbiosis, a mutually beneficial relationship with the microscopic algae that live within their cells. Photosynthetic algae produce large amounts of carbon-rich compounds such as sugars and transfer them to host corals for energy generation.
Symbiotic algae efficiently uptake dissolved mineral nutrients such as nitrates and phosphates from seawater. Even in nutrient-poor oceans, these compounds are found in significant amounts in the excreta of nearby sponges and other organisms. They can also be moved to coral reefs by ocean currents.
what scientists have discovered
In contrast to symbionts, coral hosts cannot directly absorb or utilize dissolved mineral nutrients, and it has been unclear how these nutrients promote coral growth. . However, the mechanisms by which these vital nutrients for growth are transferred to coral animals have been identified by scientists from the University of Southampton in collaboration with a team of collaborators from the UK’s Lancaster University, Tel Aviv University, Jerusalem University and others. Israel.
their findings are published in a journal Nature.
After conducting a series of long-term experiments at the University of Southampton’s Coral Reef Research Institute, scientists believe that corals actually digest some of the symbionts, taking in the nitrogen and phosphorus they absorb from the water. We proved that. If enough mineral nutrients are dissolved in the water, this mechanism allows corals to grow rapidly without receiving additional food. Fieldwork results in a remote coral reef atoll in the Indian Ocean corroborate the lab’s findings and indicate that this mechanism drives wild coral growth at the ecosystem level.
Dr Cecilia D’Angelo, Associate Professor of Coral Biology at the University of Southampton and one of the first authors, commented: could not be fed. Current knowledge could not explain how nutrients are exchanged by her two partners in a symbiotic relationship. So we thought we were missing a big piece of the big picture and started to systematically analyze the process. ”
Dr. Loreto Mardonez-Velozo, a researcher at the Coral Reef Institute who conducted the important experiments, adds: However, when the corals were kept in water with high concentrations of dissolved mineral nutrients, they appeared perfectly happy and grew rapidly. ”
The science behind the discoveries
Researchers used specially labeled compounds to track the transfer of nitrogen, an essential nutrient, between symbiotic partners. The chemical form of nitrogen used in the experiment is taken up intracellularly only by symbionts, but not by the coral host.
Bastian Hambach, manager of the Stable Isotope Mass Spectrometry Laboratory at the University of Southampton, explains: “We used isotopic labeling to ‘spike’ the nutrients that feed the corals with heavier-than-usual nitrogen atoms. These isotopes allowed us to track nutrient use by corals using ultrasensitive detection methods. ”
Professor Paul Wilson, a paleooceanographer at the University of Southampton, explains: “Using this technique, we were able to definitively demonstrate that the nitrogen atoms that sustain coral tissue growth originate from dissolved mineral nutrients fed to the symbionts in our experiments.”
Professor Jerg Wiedenmann of the University of Southampton added: seed Quantify how the symbiont population increased along with the host. Using a mathematical model of symbiont growth, we were able to show that corals harvested nutrients for growth by digesting the surplus part of the symbiont population. Our data suggest that most commensal corals can supplement their nutrition through such a ‘vegetarian diet’. ”
Scientists also analyzed corals growing around islands in the Indian Ocean, with and without seabirds, suggesting that corals may be farmed in the wild to feed on symbionts. Ta.
Professor Nick Graham, a marine ecologist at Lancaster University, explains: “The coral reefs surrounding these islands are heavily nourished by guano, the excrement of the seabirds nesting on the islands. Therefore, the associated reef receives less nutrients.We measured the growth of colonies of giant corals around islands with and without seabird densities and found that seabird nutrients were supplied. It was found that growth was more than twice as fast on reefs with
“We calculate that about half of the nitrogen molecules in coral fauna tissues on seabird-inhabited islands can be traced to uptake by symbionts and subsequent transfer to hosts.”
G.Global warming and the future
Excessive nutrient concentration caused by human activities can damage corals and is a growing threat to many reefs. However, some reefs may become nutrient depleted in the future, as global warming may cut them off from some of their natural supply routes.
Dr D’Angelo from the University of Southampton explains: Declining water productivity can mean less nutrients for symbionts and, in turn, less food for reef animals. ”
Scientists’ new finding suggests that reef animals may survive short-term starvation by feeding on symbionts, but in some areas they may not survive long-term nutrient depletion brought on by global warming. In response, it suggests that some reefs may be at risk of starvation.
Reference: “Shematypic corals cultivate and feed on photosynthetic symbionts,” Jörg Wiedenmann, Cecilia D’Angelo, M. Loreto Mardones, Shona Moore, Cassandra E. Benkwitt, Nicholas AJ Graham, Bastian Hambach, Paul A. Wilson, James Vanston, Gal Eiyar, Oru Ben Zvi, Yossi Roya, Amazia Genin, 23 August 2023, Nature.
DOI: 10.1038/s41586-023-06442-5