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Coral Microbiomes Offer Clues for Resilience and Conservation
Some coral species might be better equipped to adapt to a warmer, more acidic ocean. Finding out which ones, and why, could be the key to saving reefs around the world.
Different species of coral placed in tanks for a study that measured health indicators as the water’s acidity and temperature levels were raised to mimic climate change effects in the marine ecosystem. Credit: Andréa Grottoli
The prognosis for coral reefs has continued to spiral downward over the past year. A report published by the Intergovernmental Panel on Climate Change in October found that coral reefs could decline by between 70% and 90% if the global average temperature increases by 1.5°C above preindustrial levels. At 2°C, the chances of any reefs surviving become slim to none.
Some scientists, not willing to throw up their hands and call it quits on saving coral, are seeking out a more strategic plan of action by trying to identify species that can tolerate warmer, more acidic water and figure out why they survive. Driving conservation efforts toward these more tolerant species might secure the crucial ecosystem functions that reefs provide, like housing fish and protecting coastlines.
Andréa Grottoli, a biologist at Ohio State University, tested a hypothesis that a coral’s microbiome—all the bacteria that live in the slimy mucus of the reef—is somehow connected to its resilience. In a study published earlier this year, she found that microbiome stability and physiological health seem to go hand in hand.
“We don’t know if it’s the stable microbiome that facilitates a healthy host physiology, or if it’s the host physiology that facilitates a healthy microbiome. But they go together,” Grottoli said while presenting her work in December at AGU’s Fall Meeting 2018 in Washington, D. C. Grottoli was also recently awarded the Ocean Sciences Voyager Award by AGU for her contributions to ocean science and leadership in the field.
Identifying a Winner
Grottoli’s microbiome research builds on three previous papers she coauthored that looked at coral health under the dual stressors of warmer water and water with a lot more carbon dioxide in it. She said that when she first proposed the experiment, there were almost no other studies examining these two stress factors at the same time.
Back in 2011, she and her colleagues tested how four species responded to a ramp-up in temperature and acidity over 24 days, approximating expected ocean conditions at the end of this century. Taking a holistic approach, they assessed coral health using more than 10 criteria, examining both the host organism and the symbiotic algae that live in the coral.
The result of this earlier work was the discovery of a resilience champion. Turbinaria reniformis, commonly known as yellow scroll coral, tolerated the changing conditions well. In a photo comparing coral fragments from the treatment and control groups, it’s impossible to tell the difference. Acropora millepora, on the other hand, turned out to be much more sensitive. This branching coral became shrunken and pale and lost algae. The question then was, What was it about the yellow scroll coral that allowed it to adapt?
“We asked ourselves whether the coral microbial community may be playing a role,” Grottoli said. The corals’ microbiomes may be important to immune response, similar to the way the bacterial community in the human gut helps us stave off disease.
Coral fragments at the end of a 24-day experiment. A. millepora show significant changes, whereas T. reniformis look the same. Credit: Verena Shoepf
Grottoli didn’t have funding for another study, so she did a pilot investigation with just two species. She took fragments of coral from the 2011 experiment back out of the freezer and partnered with a colleague at Ohio State, a microbiologist named Michael Wilkins, to analyze the composition of their microbial communities.
She found that in the most sensitive species of the initial group, the diversity of the microbial communities decreased by 40% after the 24-day ramp-up. The microbes that remained turned out to be related to bacteria known to cause coral diseases.
By contrast, the yellow scroll coral, the most tolerant species, once again seemed to have everything going for it, with no significant change in its microbial community.
“We concluded that coral like Turbinaria would be good targets for conservation and restoration because they seem to be pretty robust on multiple fronts,” Grottoli said.
A Difficult Question for Conservation
One of the main challenges for coral conservation is that conventional methods aren’t keeping up with the pace of environmental change in a warming world.
“Maintaining coral species diversity is probably a bit of a losing battle,” Grottoli said. “I just don’t know if it’s practical with limited dollars.”
But ocean acidification and warming aren’t the only threats to reefs. They are endangered by development, pollution, and irresponsible fishing practices. Because biodiversity is looking like a losing battle, Grottoli thinks conservation and restoration efforts should strive to maintain the ecosystem function of reefs by targeting species that have proven to be resilient. It’s a compromise, but these reefs would continue to provide habitat for fish and protect coastlines from erosion and flooding.
“Unfortunately, the situation is so dire that we have to start making those kinds of trade-offs,” said Thomas Hourigan, chief scientist at the National Oceanic and Atmospheric Administration’s Deep Sea Coral Research and Technology Program. Hourigan said that his interest in coral reefs stemmed from their contribution to the world’s biodiversity. “It’s very saddening to me to think how much biodiversity is being lost, will be lost,” Hourigan said.
Grottoli is on a mission to find out whether there are coral species that can adapt to the rapid rate of change in the ocean. Her findings are promising but also limited by the short duration and small sample size of the experiment. She has since completed a much larger experiment in Hawaii looking at eight species of coral over the course of 22 months.
She hasn’t finished analyzing the results yet but said that her preliminary data do show some clear winners. “Because there are so many species from so many sites, we can make broader statements about resilience, adaptation, and acclimation,” she said.