During the global bleaching crisis that began in 2015, more than 70 percent of corals worldwide experienced prolonged warm ocean temperatures hot enough to kill them. It was the longest and most severe in recorded history. Yet even in hard-hit regions, some corals toughed it out, lonely spots of color in a ghostly panorama.
There are many reasons why a reef might survive when its neighbors don’t, but scientists are now looking harder at one factor – the microbes that live on and within corals – that they think may play a role. Understanding every coral resiliency trait will be key to protecting reefs as global warming increases water temperatures and acidity.
“It is becoming increasingly apparent that the coral microbiome functions like the human gut microbiome,” said Andrea Grottoli, a researcher at Ohio State University who studies coral resilience.
Just as in humans, whose microbes help regulate metabolism and immune system function, Grottoli thinks that corals’ microbes may boost the animals’ ability to handle stress and disease, fighting off harmful bacteria and compensating for some of the stress of warming waters. In a study recently published in the journal PLOS ONE, Grottoli’s team of researchers grew two types of reef-building coral in aquarium tanks, subjecting some of them to a month of elevated temperatures and acidity at the levels expected by the end of the century.
One species in these hot tanks, a tough variety known as yellow scroll coral fared comparatively well – and its microbiome was relatively unchanged, too. The other species, a common branching staghorn coral, became unhealthy and so did the bacterial species living on it, decreasing in number and diversity.
For Grottoli, the results confirm that there is a relationship between a coral’s health and its microbes, although which influences which isn’t yet clear. What all those bacteria are doing is also a much murkier question. “There’s far more unknown than known,” she said. “We don’t know if it’s changes in the microbial community that are causing shifts in the physiology, or if it’s changes in the physiology that make the coral no longer a good host of the microbes.”
Corals are colonial organisms related to jellyfish and sea anemones. Their success depends on a mutually beneficial arrangement between the coral animal and single-celled algae that live within the coral’s cells, providing it with photosynthesized nutrients in exchange for shelter. More and more, however, scientists consider corals a three-party system, a closely coordinated relationship between animal, algae and the complex ecosystem of microbes that live on and around the coral tissue.
One group working to fill in some of the blanks is the Global Coral Microbiome Project, a global survey of reefs that aims to establish a baseline understanding of how coral microbiomes function. Led by Rebecca Vega Thurber at Oregon State University and Monica Medina of Pennsylvania State University, the project has collected samples from the islands of Hawaii and Madagascar to the South Pacific, Caribbean and bathwater-warm Red Sea. They’re hoping the exhaustive survey will reveal some set of microbiome traits that make certain species or populations more resilient or more vulnerable to stress or disease.
One common trend has emerged from all the research in Vega Thurber’s lab. She calls it an example of the Anna Karenina principle: Just like Tolstoy’s famous opening line about happy families, healthy microbiomes tend to look very similar, but every unhealthy microbiome is unhealthy in its own way.
“We’re trying to understand how the host coral regulates its microbiota,” said Becca Maher, a graduate fellow in Vega Thurber’s lab who is studying how pollution and predation combine with climate change to stress out corals’ microbiomes. “Some of our work shows that, maybe with stress, the host loses its ability to regulate its microbial players,” she said.
But it’s possible that microbes could quickly evolve to better serve their coral hosts, according to microbiologist Nicole Webster, a principal research scientist with the Australian Institute of Marine Science. “Just think about antibiotic resistance in the human gut,” she said. “Microbes go through many generations just in a single day, so they can evolve to their environment much faster than a long-lived organism like corals.”
That means bacteria may be the key to answering the question of the century for a crucial ocean ecosystem: How fast can coral reefs really adapt to a rapidly changing ocean climate? With average ocean surface temperatures predicted to rise over the next 30 years, there’s not much time for slow-growing coral to evolve higher heat tolerance.
Whether microbes can help corals adapt, Webster said, is a question that needs a lot more attention. Her work is tied up now in a years-long experiment at the Australian Institute of Marine Sciences, known as Evolution 21. The researchers built simulations of entire reef ecosystems in a series of tanks and set conditions to mirror the expected temperature and acidity of 2050 and 2100. By letting the experiment play out for years, they hope to get a glimpse into the ecosystems of future oceans, including its microbial life.
Other researchers are already studying possible interventions in coral’s microbial health, including Brazilian biologist Raquel Peixoto, who has been experimenting with a “probiotic” pill for reefs. The idea is that the pill would contain a sampling of the mix of bacteria seen in healthy corals and could be applied to ailing reefs to try to induce them to re-establish better bacterial communities. In the wake of oil spills, Peixoto found that probiotics helped corals recover faster. Meanwhile, Madeleine van Oppen of the Australian Institute of Marine Science and Ruth Gates of the University of Hawaii are trying to breed heat-tolerant corals – along with bacteria and algae to live in them – to transplant onto damaged or dying reefs.
These interventions can sound like science fiction, but increasingly scientists are investigating them as possible necessities in a future ocean that will continue to warm even if humans do get carbon dioxide emissions under control. Replanting reefs with heat-resistant corals or inoculating them with beneficial bacteria may be time-, labor- and money-intensive. Still, many scientists believe they have to try. “I don’t think it’s reasonable for us not to do it at all” said Grottoli, though probably only “on very small scales and in targeted ways, so reefs don’t entirely disappear.”
“I think it’s probably more efficient to get political action to reduce CO2 emissions than to try to replant the reefs of the world with resilient corals,” she said.
However, even just slowing down the rate of CO2 emissions would buy more time for natural processes like adaptation and acclimatization to happen, she said. If these recent studies of the coral microbiome have shown scientists anything, it’s that the relationship between coral, their genes and their symbiotic ride-along algae and bacteria is more responsive than once thought.
“There is potential for adaptation, acclimation, evolution,” said Grottoli. But she avoids the term “optimistic.” She prefers to be realistic, she said. Even in the best-case scenarios, “we’re just talking about degrees of reef loss – is it going to be bad, really bad or catastrophic?” Her hope: “It’s not guaranteed catastrophic.”