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Published Monday, January 25, 2021
Microbes keep coral reefs healthy,
study says
By the A.M. Costa Rica wire services
Corals have evolved over hundreds of years to live and even thrive in waters with few nutrients. In healthy reefs, the water is often exceptionally clear, mainly because corals have found ways to make optimal use of the few resources around them. Any change to these conditions can throw a coral's health off-balance.
Now, researchers at the Massachusetts Institute of Technology, MIT, and the Woods Hole Oceanographic Institution, WHOI, have identified microbes living within the slimy biofilms of some coral species that may help protect the coral against certain nutrient imbalances.
The team found these microbes can take up and "scrub out" nitrogen from a coral's surroundings. At low concentrations, nitrogen can be an essential nutrient for corals, providing energy for them to grow. But an overabundance of nitrogen, for instance from the leaching of nitrogen-rich fertilizers into the ocean, can trigger mats of algae to bloom. The algae can outcompete coral for resources, leaving the reefs stressed and bleached of color.
By taking up excess nitrogen, the newly identified microbes may prevent algal competition, thereby serving as tiny protectors of the coral they inhabit. While corals around the world are experiencing widespread stress and bleaching from global warming, it seems that some species have found ways to protect themselves from other, nitrogen-related sources of stress.
"One of the aspects of finding these organisms in association with corals is, there's a natural way that corals can combat anthropogenic influence, at least in terms of nitrogen availability, and that's a very good thing," Andrew Babbin, the Doherty Assistant Professor in Ocean Utilization in MIT's Department of Earth, Atmospheric and Planetary Sciences said. "This could be a very natural way that reefs can protect themselves, at least to some extent."
Babbin's group studies how marine communities in the ocean cycle nitrogen is a key element for life. Nitrogen in the ocean can take various forms, such as ammonia, nitrite, and nitrate. Babbin has been especially interested in studying how nitrogen cycles, or is taken up, in anoxic environments -- low-oxygen regions of the ocean, also known as "dead zones," where fish are rarely found and microbial life can thrive.
Dead zones are not the only anoxic regions of the ocean where bacteria exhibit nitrogen-feasting behavior. Low-oxygen environments can be found at smaller scales, such as within biofilms, the microbe-rich slime that covers marine surfaces from shipwrecked hulls to coral reefs.
Even though corals are close to the surface and within reach of oxygen, Babbin wondered whether coral slime would serve to promote "anoxic pockets" or concentrated regions of low oxygen, where nitrate-consuming bacteria might thrive.
He broached the idea to WHOI marine microbiologist Amy Apprill, and in 2017, the researchers set off with a science team on a cruise to Cuba, where Apprill had planned a study of corals in the protected national park, Jardines de la Reina, or Gardens of the Queen.
In exploring the reefs, the scientists took small samples from coral species that were abundant in the area. Onboard the ship, they incubated each coral specimen in its seawater, along with a tracer of nitrogen -- a slightly heavier version of the molecules found naturally in seawater.
They brought the samples back to Cambridge and analyzed them with a mass spectrometer to measure how the balance of nitrogen molecules changed over time. Depending on the type of molecule that was consumed or produced in the sample, the researchers could estimate the rate at which nitrogen was reduced and essentially denitrified, or increased through other metabolic processes.
In almost every coral sample, they observed how rates of denitrification were higher than most other processes; something on the coral itself was likely taking up the molecule.
The researchers swabbed the surface of each coral and grew the slimy specimens on Petri dishes, which they examined for specific bacteria that are known to metabolize nitrogen. This analysis revealed multiple nitrogen-scrubbing bacteria, which lived in most coral samples.
Whether nitrogen-scrubbing microbes directly contribute to a coral's health is still unclear. The team's results are the first evidence of such a connection. Going forward, Babbin plans to explore other parts of the ocean, such as the tropical Pacific, to see whether similar microbes exist on other corals, and to what extent the bacteria help to preserve their hosts. He guesses that their role is similar to the microbes in our systems.
"The more we look at the human microbiome, the more we realize the organisms that are living in association with us do drive our health," Babbin says. "The same thing is true of coral reefs. It's the coral microbiome that defines the health of the coral system. And what we're trying to do is reveal just what metabolisms are part of this microbial network within the coral system."
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How could the results of this study be used to protect the reefs? We would like to know your thoughts on this story. Send your comments to news@amcostarica.com
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