Corals have evolved over thousands of years to survive, and even thrive, in nutrient-poor waters. In healthy reefs, the water is often very clear, mainly because corals have found ways to make the most of the few resources around them. Any change in these conditions can throw coral health off balance.
Now, researchers at MIT and the Woods Hole Institute of Marine Science (WHOI), in collaboration with marine experts and marine biologists in Cuba, have identified microbes that live alongside -integrated biologics of some coral species that may help protect the coral against some nutritional imbalances.
The team found that these microbes can take up and scavenge nitrogen from a coral environment. At low concentrations, nitrogen can be an essential nutrient for corals, giving them energy for growth. But too much nitrogen, for example from melting nitrogen-rich fertilizers into the ocean, can warm mats of algae. The algae can lay out coral for resources, leaving stress on the reefs and glistening color.
By taking in too much nitrogen, the newly identified microbes can inhibit algal competition, thus serving as small protectors of the coral in which they live. While corals around the world are experiencing widespread stress and swelling from global warming, some species appear to have found ways to protect themselves from other sources of stress, related to nitrogen.
“One of the aspects of discovering these organisms in relation to corals is that there is a natural way for corals to be able to determine anthropogenic effects, at least in terms of what is available. of nitrogen, and that ‘s a great thing, “said Andrew Babbin, Doherty Associate Professor of Ocean Practice in MIT’ s Department of Earth, Atmospheric and Planetary Sciences.” This could be a very natural approach. reefs can defend themselves, at least to some extent. ”
Babbin and his colleagues have reported their findings in the Iris ISME.
Dead zone analogs
Babbin’s group studies how marine communities in an ocean circulate nitrogen, a key element for life. Nitrogen in the ocean can take various forms, such as ammonia, nitrite, and nitrate. Babbin has taken a particular interest in studying the formation, or uptake, of nitrogen cycles in anoxic environments – regions with low oxygen levels in the ocean, also known as “dead zones,” where they are not found. fish but rarely and where microbial life can thrive.
“Places without enough oxygen for fish are where bacteria start doing something different, which is interesting to us,” Babbin says. “For example, they can start eating nitrate, which in turn affects the productivity of a particular part of the water.”
Dead zones are not the onlyoxox regions of the ocean where bacteria exhibit nitrogen-fixing behavior. Low-oxygen environments are found at smaller scales, such as inside biofilms, the microbial-filled slime that covers sea surfaces from shipwrecked shells to coral reefs.
“We have biofilms inside us that allow various anaerobic processes to take place,” Babbin notes. “The same is true of corals, which can generate tons of mucus, which acts as this delay barrier for oxygen.”
Despite the fact that corals are close to the surface and within oxygen reach, Babbin questioned whether coral slime stimulates “anoxic pockets,” or thick regions of low oxygen, where eating nitrate thrives.
He inspired the idea to WHOI marine microbiologist Amy Apprill, and in 2017, the researchers went with a science team on a cruise to Cuba, where Apprill had designed a coral study in the national park under defense, Jardines de la Reina, or Queen’s Gardens.
“This protected area is one of the last refugees for healthy Caribbean corals,” Babbin says. “Our hope was to study one of these less influential areas to get a baseline for what kind of nitrogen cycle dynamics are associated with the corals themselves, which would allow us to understand what anthropogenic disturbance would do to that system. ”
Swabbing for scrubbers
In studying the reefs, the scientists took small samples from a coral species that was abundant in the area. On board the vessel, they stimulated each coral sample in its own seawater, along with nitrogen traces – a slightly heavier version of the molecules found naturally in seawater.
They took the samples back to Cambridge and analyzed them with a mass spectrometer to measure how the molecular balance of nitrogen changed over time. Depending on the type of molecule consumed or extracted in the sample, the researchers were able to estimate the degree to which nitrogen was reduced and necessarily reduced, or elevated through other metabolic processes.
In almost all coral samples, they observed higher disinfection rates than most other processes; something on the coral itself seems to have taken up the molecule.
The researchers shook the surface of each coral and extracted the slippery samples on Petri dishes, which they studied for specific bacteria known to metabolize nitrogen. This analysis revealed a number of nitrogen-fixing bacteria, which were present in most coral samples.
“Our results would mean that these organisms, which live in conjunction with the corals, have a way of cleaning up the very local environment,” Babbin says. coral species, such as this brain coral Diploria, which exhibit very fast nitrogen cycling and occur very hard, even through anthropogenic modification, but Acropora, which is in rough shape throughout the Caribbean , exhibiting very little nitrogen cycling. ”
It is not yet clear whether nitrogen-fixing microbes directly contribute to coral health. The team’s results are the first evidence of such a connection. Going forward, Babbin plans to study other parts of the ocean, such as the Minch, to see if other corals have similar microbes, and to what extent the bacteria help retaining the hosts. His view is that their function is similar to the microbes in our own systems.
“The more we look at human microorganisms, the more we understand the organisms that live with us directing our health,” Babbin says. true of coral reefs. It is the coral microbiome that defines the health of the coral system. And what we are trying to do is reveal exactly what metabolisms are part of this microbial network within the coral system. ”
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This research was supported, in part, by MIT Sea Grant, Simons Foundation, MIT Montrym, Ferry, and mTerra funds, and by Bruce Heflinger ’69, SM ’71, PhD ’80.
Written by Jennifer Chu, MIT Press Office
Additional background information
Paper: “Detection and quantification of anaerobic nitrogen metabolites among tropical Cuban stony corals”
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