Corals have evolved over millennia to live, even thrive, in waters with few nutrients. On healthy reefs, the water is often extremely clear, mainly because corals have found ways to make the best use of the few resources around them. Any change in these conditions can harm a coral’s health.
Now, researchers at MIT and the Woods Hole Oceanographic Institution (WHOI), in collaboration with oceanographers and marine biologists in Cuba, have identified microbes living in the slimy biofilm of certain species of coral that can help protect coral reefs. .
The team found that these germs could absorb and “purify” nitrogen from a coral environment. At low concentrations, nitrogen can be a key nutrient for corals, providing energy for their growth. But too much nitrogen, for example from leaching nitrogen-rich fertilizers into the ocean, can cause algae rugs to bloom. Algae can offset corals for pores, leaving reef tones and bleached in color.
By taking in excess nitrogen, newly identified microbes can prevent algae from competing, thus serving as tiny protectors for the resident corals. While corals around the world are experiencing extensive stress and whitening from global warming, some species seem to have found ways to protect themselves from other sources of nitrogen-related stress.
“One of the aspects of finding these organisms in conjunction with corals is, there is a natural way in which corals are able to fight anthropogenic influence, at least in terms of nitrogen availability, and that is very good,” he says. Andrew Babbin, Doherty Assistant Professor of Ocean Use at MIT, Department of Earth, Atmospheric and Planetary Sciences. “It could be a very natural way the reefs can be protected, at least to some extent.”
Babbin and his colleagues have reported their findings to ISME Magazine.
Depending on the dead zone
Babbin’s team studies how marine communities in the ocean cycle are nitrogen, a key element for life. Nitrogen in the ocean can take various forms, such as ammonia, nitrite and nitrate. Babbin was particularly interested in studying how nitrogen is circulated or absorbed in anoxic environments – areas of low oxygen in the ocean, also known as “dead zones”, where fish are rare and microbial life can thrive.
“Places without enough oxygen for fish are where bacteria start to do something different, something that is exciting for us,” says Babbin. “For example, they may start consuming nitrates, which then have an impact on how productive a particular part of the water can be.”
The dead zones are not the only anoxic areas of the ocean where bacteria exhibit nitrogen behavior. Low oxygen environments can be found on a smaller scale, such as in biofilm, the microbial-rich mud that covers seafronts from hullwrecks to coral reefs.
“We have biofilms within us that allow different anaerobic processes to occur,” notes Babbin. “The same goes for corals, which can produce a ton of mucus, which acts as this oxygen barrier.”
Although the corals are close to the surface and close to oxygen, Babbin wondered if the coral mud would be used to promote “anoxic pockets” or concentrated areas of low oxygen where bacteria that consume nitrates may thrive.
The idea came back to WHOI marine microbiologist Amy Apprill, and in 2017, researchers began with a scientific team on a Cuban cruise, where Apprill had planned a coral study in the protected national park, Jardines de la Reina or Queen’s Gardens.
“This protected area is one of the last resorts for healthy Caribbean corals,” says Babbin. “Our hope was to study one of these least affected areas to get a basis for the kind of nitrogen cycle dynamics associated with the corals themselves, which would allow us to understand what a man-made disturbance would do to it.” the system.”
Wiping for washers
Exploring the reefs, scientists took small samples of coral species that abounded in the area. On board, each coral sample was incubated in its own seawater, along with a nitrogen tracer – 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 consumed or produced in the sample, the researchers could estimate the rate at which nitrogen decreased and was essentially denitrified or increased through other metabolic processes.
In almost every coral sample, they found that denitrification rates were higher than most other procedures. something in the coral itself was likely to occupy the molecule.
The researchers wiped the surface of each coral and grew the slimy samples in Petri dishes, which they tested for specific bacteria known to metabolize nitrogen. This analysis revealed multiple nitrogen-cleaning bacteria, which lived in most coral specimens.
“Our results suggest that these coral-living organisms have a way of cleaning up the local environment itself,” says Babbin. “There are some types of corals, such as this Diploria brain coral, that show extremely fast nitrogen recycling and happen to be quite resilient, even through a man-made change, while Acropora, which is in a rough shape across the Caribbean, is very small nitrogen cycle. ”
Whether nitrogen-removing germs directly contribute to a coral’s health is still unclear. The results of the group are the first elements of such a connection. Going forward, Babbin plans to explore other parts of the ocean, such as the tropical Pacific, to see if there are similar germs on other corals and to what extent bacteria help retain their hosts. His guess is that their role is similar to that of germs in our systems.
“The more we look at the human germ, the more we realize that the organisms that work with us drive our health,” says Babbin. “The same is true for coral reefs. It is the coral microbe that determines the health of the coral system. And what we are trying to do is find out which metabolites are part of this microbial network in the coral system.”
This research was supported, in part, by the MIT Sea Grant, the Simons Foundation, the MIT Montrym, Ferry and mTerra chapters, and by Bruce Heflinger ’69, SM ’71, PhD ’80.
Written by Jennifer Chu, MIT News Office
Paper: “Detection and quantification of anaerobic nitrogen metabolites among oxygenated tropical Cuban corals”