Here and there along the steep face of the reef, clumps of coral have turned stark white. This bleaching means the coral has begun to eject the micro-algae that normally live within its tissues and provide up to 90 percent of the nutrients that coral needs to live. And that has scientists worried, because similar things are happening in tropical waters around the world. Coral reefs are one of the planet’s keystone habitats, as rich in species as the rain forest. But they’re even more vulnerable to climate change and the warm, acidic ocean conditions it is creating.
Yet scientists may be coming up with a way to protect the fragile reefs for the warmer world of the future
Ruth Gates, director of the University of Hawaii’s Hawaii Institute for Marine Biology, calls the process human-assisted evolution. Last spring, she and Madeleine van Oppen of the Australian Institute for Marine Sciences received a $4 million grant from the family foundation of Microsoft billionaire Paul Allen for a plan to develop strains of coral that will be able to withstand changing ocean conditions.
Gates emphasizes that now is the time for scientists to act, while there is still enough diversity on the reef. “As a biologist who’s been looking at reefs for 30 years,” she says, “I’m spectacularly realistic about what I see, and it’s not pretty — and if we don’t do anything about it, it’s going to intensify.”
The secret lives of coral
Like their close relatives, sea anemones and jellyfish, corals begin their life as free-swimming larvae. Eventually, though, they settle permanently on a rock or a patch of dead coral and transform into polyps, the basic units of coral. Almost immediately, the polyps begin to secrete the hard exoskeleton that we think of as coral reef. Collectively, corals are nature’s most prodigious architects. The Great Barrier Reef, where van Oppen does her research, is large enough to be seen from space.
In a sense, each coral polyp is an individual, with a mouth and tentacles and its own community of symbionts. But the concept of individuality in coral is a complex one. Although larvae are the result of sexual reproduction, corals also reproduce clonally. Polyps will divide over and over again so that all the polyps in a colony, or a head of coral, may be genetically identical. Each may feed and spawn independently, but they’re also all connected by tissue and by a kind of nervous system called a nerve net. If you touch one end of a colony, the tentacles on the other end will retract.
Then there’s the relationship between coral and its symbiotic microorganisms. Because their lives are so intertwined, scientists generally think of all these organisms as a single entity they call a holobiont. If human-assisted evolution is going save coral, it will have to work on the entire holobiont.
Despite the provocative label, human-assisted evolution relies largely upon old-fashioned selective breeding. Gates points out that, even during a dramatic warming event, like last summer’s in Hawaii, when mean sea temperatures in Kaneohe Bay were several degrees above normal, not all the coral on a reef bleaches. Some individuals are clearly more tolerant of these kinds of stresses. Gates is collecting small samples of those individuals and bringing them into her lab to crossbreed them. By selecting the most robust offspring, she hopes to produce more-resilient strains of coral.
Creating ‘super corals’
That’s just the first step. Ultimately, the plan is to return these corals back onto the damaged reefs they came from so they can interbreed with the wild coral. But before that happens, Gates and van Oppen believe they can exploit the complex biology of these organisms to create “super corals.”
There are two main thrusts to their plan. The first involves a concept called epigenetics, the science of how genes are turned on and off.
Healthy coral gets its brown or purple color from Symbiodinium, micro-algae that live in the coral's tissue. Each of the red dots in this photo is a single Symbiodinium. This color, confocal microscopic image show coral polyps before bleaching. (Katie Barrott/Gates Lab)
“There are two ways you can actually change genetic information.” Gates says. “You can, over time, insert new genes, or mix genes among generations. This changes the actual structure of the DNA. Or you can change the way in which the existing genes are used. In other words, you can regulate the genetic material you already have in a different way. We’re focusing on the latter approach. We’re trying to turn on, if you will, genetic pathways that allow corals to sustain exposure to stress better.”
To try to turn on these genetic pathways, the Gates lab selects the most resilient individuals from the crossbreeding program and exposes them to conditions that mimic the higher ocean temperatures and acidity expected in the future.
Gates likens this acclimatization process to the conditioning of athletes.
“That’s exactly what we’re doing with corals,” she says. “We’re bringing them in and exposing them to the conditions of the future, to experiences that we hope they will ‘remember,’ and that that memory will be held in the way they regulate their genetic material.” The idea is that, by exposing the coral to stress, they will turn on formerly idle genes that are beneficial in the new conditions.
There are some early signs of success. Some of the acclimatized corals that were returned to their reefs appeared to exhibit higher resiliency. Even during last year’s dramatic bleaching event, when between 40 and 70 percent of the coral in Kaneohe Bay were affected, none of these corals showed signs of bleaching.
But it’s not enough to induce these changes in an individual. For assisted evolution to work, Gates says, the changes have to be heritable. This is, whatever characteristics the corals develop that make them more resilient have to be inherited by their offspring. That’s the essence of epigenetics.
So, can acclimatizing individual corals to the conditions of the future create traits that can be passed on to the next generation?
“Our preliminary work suggests that the answer to that is ‘Yes,’ ” Gates says.
In fact, in a paper this year in the Journal of Experimental Biology, she and her former student Hollie Putnam demonstrated just this kind of intergenerational exchange of “memory.” They found that, when they submitted a reef-building coral called Pocillopora to higher temperatures and more acidic conditions, the coral’s larvae “exhibited size differences and metabolic acclimatization” that improved their resiliency. In other words, even though the adults suffered from being exposed to future climate conditions, the offspring of those that survived were better able to tolerate higher temperatures and acidity than the offspring of adults that weren’t exposed to those conditions.
The second focus of research in the Gates lab looks at the relationship between corals and the microorganisms that live on and in them.
Until recently, scientists thought the symbiotic community within all corals was composed of a single micro-alga of the genus Symbiodinium. But modern genetic tools have revealed that there are hundreds of species of these micro-algae, as well as thousands of species of symbiotic bacteria and other microorganisms. Importantly, some of these microorganisms are more tolerant of the warm, acidic conditions of the future than others are. Gates plans to exploit these differences.
For example, it may be possible to improve a coral’s community of micro-algae. As larvae, most corals have none of their own symbionts. Instead, they recruit the Symbiodinium and other microorganisms they need to survive early in their life cycle. This biology may allow researchers to choose which micro-algae the coral will host.
“You can insert a symbiont [micro-alga] that’s really tolerant of stress and continues to provide the coral with nutrition through that stress,” Gates says. In other words, scientists may be able to improve the resilience of coral by improving the resilience of its assemblage of microorganisms.
Gates and van Oppen also plan to take advantage of the fact that simple microorganisms, such as Symbiodinium, reproduce and evolve very quickly. Using common lab techniques, they may be able to guide that evolution by inducing random mutations into their DNA, then breeding and selecting the new strains for characteristics that will increase the resilience of their host corals.
The key to all this — and maybe the reason there has been so little criticism in the scientific community — is that none of what Gates and van Oppen are doing is genetic engineering in the technical sense. They’re not gene-splicing or creating Franken-corals. On the epigenetics side, they’re simply turning on genes that are already present in the coral’s own DNA. Similarly, the symbiotic micro-algae that they’re working with already exist in the corals on the reef. No new genetic material is being introduced. And in the future, when the coral is returned to the wild, it will go back to its home reef.
“We’ve been very strong in saying we’re not genetically modifying anything,” Gates says. “All we’re doing is accelerating or assisting evolution.”
But Gates isn’t cavalier about the risks. For example, she acknowledges that, however unlikely, it’s possible that these new “super corals” could become invasive or have other unintended consequences. These are issues that environmental managers, policymakers, scientists and the general public will have to discuss, Gates says. But for now, the more urgent goal is for scientists to develop what she calls a “biological tool box” to address the threat that climate change poses for coral reefs. The question of whether to use those tools comes later.
“We can often pick holes in potential solutions and have a very nuanced argument about why we shouldn’t do anything,” Gates says. “Or, we can turn the argument on its head and ask, ‘What is the risk of doing nothing?’ The risk of doing nothing is the obliteration of coral reefs worldwide.”
Hollier is a Hawaii-based science writer.