ENVIRONMENT

The race to create climate-resilient coral—before it's too late

In Australia, researchers are pioneering new methods to fortify coral reefs in a warming world.

Heat-evolved symbionts are distributed to coral fragments which had been previously chemically bleached. — These tiny algae cells have been cultivated for 10 years to withstand hot ocean temperatures. Scientists hope that by breeding corals to be more heat tolerant, they can help reefs survive heat waves.

ByAvery Schuyler Nunn

Photographs byGiacomo d’Orlando

July 2, 2024

In the heart of western Australia lies Coral Bay—a gem nestled within the expansive embrace of the Ningaloo Reef.

Historically, Ningaloo Reef has flourished, a kaleidoscope of coral and fish. Yet climate change’s caprice showed its hand in 2022, when weather conditions created a low oxygen event that choked the life from a section of this ecosystem.

About 70 percent of the bay’s floor had been covered by coral. By 2022, that number dropped to just over one percent.

During the same period, turf algae—small, fast-growing plants that can smother coral to death—substantially increased from covering 25 percent of the bay floor in 2021 to 79 percent in 2022.

Despite meticulous conservation efforts, coral reefs are at risk of vanishing as the planet continues warming. Their disappearance would be disastrous—these vibrant underwater gardens are sanctuaries of biodiversity, cradling a quarter of all marine species.

An unprecedented coral bleaching event is currently affecting the Great Barrier Reef. For the first time, the entire reef is affected, according to the Australian Institute of Marine Science (AIMS). Aerial surveys show that about 730 of the more than 1,000 reefs are experiencing bleaching, making it potentially the most extensive event on record.

For scientists like David Juszkiewicz, a coral conservationist and PhD student at Curtin University, the race is on to create innovative solutions that help corals adapt to a new world. While scientists say climate change must ultimately be curbed, they hope they can make corals stronger until the planet cools.

Researchers of the Australian Institute of Marine Science are waiting to collect hybrid coral eggs during a spawning event in their SeaSim lab. AIMS research on hybrid coral spawning is trying to help reefs adapt to climate change.

Hybrid coral eggs in a tank during a spawning event in the SeaSim lab. — When corals reproduce, they release millions of eggs into the water at once, a spawning event. The eggs here have been selectively bred to withstand stressful environmental conditions like heat waves and acidic waters.

A researcher holds a piece of coral during a coral spawning event in the SeaSim lab. — When corals reproduce, they release millions of eggs into the water at once, a spawning event. The eggs here have been selectively bred to withstand stressful environmental conditions like heat waves and acidic waters.

How corals react to stress

Corals, the architects of these underwater realms, are sustained by their symbiotic partners—zooxanthellae, microscopic algae nestled within their tissues. This partnership is a testament to nature’s ingenuity, transforming nutrient-poor waters into thriving ecosystems.

The zooxanthellae, through photosynthesis, provide the corals with essential nutrients, while the corals offer algae a protected environment and access to sunlight. This mutualistic relationship is the heartbeat of the reef, driving its productivity and supporting a myriad of marine life.

Yet, like any relationship, this bond is susceptible to strain. Climate change, with its rising temperatures and ocean acidification, is an ever-present threat. The symbiotic harmony falters under stress, leading to a phenomenon known as coral bleaching. In this state, corals expel their zooxanthellae, turning ghostly white and losing their primary source of sustenance. The reef’s vibrant colors fade, replaced by a stark, lifeless landscape.

Beyond the well-known coral-zooxanthellae partnership lies a complex web of microbial allies.

Bacteria, fungi, archaea, and viruses all play roles in coral health, residing in the coral’s surface mucus, tissues, and skeleton. These microbes provide benefits ranging from nutrient exchange to pathogen defense, enhancing the coral’s resilience. High microbial diversity within the coral’s microbiome offers a buffer against environmental stress like high heat, a safeguard woven into the fabric of the reef’s existence.

That's why scientists are exploring ways to manipulate the microbes that live within coral to enhance their survival, particularly during summer heatwaves.

Dr Matthew Nitschke, research scientist at AIMS, is about to release heat-evolved symbionts on juvenile corals in the experimental tank. — Matthew Nitschke, a research scientist at AIMS, releases heat-evolved symbionts on to juvenile corals growing in an experimental tank. Symbionts are the tiny cells of algae that live inside the coral tissue, providing corals with energy to grow.

Manipulating a coral's microbes

Experimental evolution, where microbial cultures are selected under elevated temperatures, has shown potential as a solution. These heat-tolerant strains, once reintroduced into corals, could arm reefs with a fighting chance against thermal stress. The researcher's primary mission? That in the future, everyone can borrow and deploy this type of management action.

“We can grow those essential coral somewhere else, under high temperatures. Then through natural selection, we can find the few cells that have the genetic material that allows them to cope with high temperatures,” Matthew Nitschke, a marine biologist at the Australian Institute of Marine Science, says.

But it’s not just about innovation, it’s about how realistically these innovations can be shared across the globe, says Cedric Robillot, executive director of the reef restoration and adaptation program at the Great Barrier Reef Foundation.

Young corals are released on a damaged reefs. — In the Whitsundays Islands, the Great Barrier Reef Foundation captures excess coral eggs and sperm from healthy reefs and rears millions of baby corals in specially designed floating pools. These young corals are then released on damaged reefs, helping repopulate them.

“The Great Barrier Reef in itself is the size of Italy,” says Robillot, “How can we do that at a large scale, that will actually have an impact?”

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Researchers are also experimenting with probiotic treatments, using beneficial bacteria to shield corals from diseases like the devastating stony coral tissue loss disease. Lab trials have shown promise, and efforts are underway to test these treatments in the wild.

Additionally, microbiome transplantation, where heat-resistant symbionts are introduced to vulnerable corals, could make reefs more resilient.

“Reef systems possess an innate resilience due to their interconnectedness and complexity,” says Robillot. “By reinforcing these natural strengths, we can significantly enhance their capacity to withstand and recover from stress.”

David Juszkiewicz, PhD Candidate, Curtin University, School of Molecular and Life Sciences, and Lindsey Kraemer, master’s student from James Cook University, are sampling and picture a massive Porites in Ningaloo Reef. — David Juszkiewicz, a PhD Candidate at Curtin University and Lindsey Kraemer, a masters student from James Cook University, are sampling and taking pictures of a massive Porites coral in Ningaloo Reef. Each sample will be genetically sequenced for their coral genome and to understand their unique relationship with their microbes. Pairing photographic images with the samples provides valuable information for future scientists.

David Juszkiewicz, PhD candidate of Molecular and Life Science at Curtin University, imaging the living corallites of a massive Porites bommie along the Ningaloo Reef. — David Juszkiewicz takes a picture of a massive Porites coral growing at the edge of the Ningaloo Reef. These corals are shaped like massive boulders and can live for 600 years.

An Acropora sample on a Petri dish. — David Juszkiewicz takes a picture of a massive Porites coral growing at the edge of the Ningaloo Reef. These corals are shaped like massive boulders and can live for 600 years.

Harnessing nature’s inherent resilience

Monitoring coral health is akin to taking the pulse of the ocean. Coral bleaching is an obvious sign of distress, but subtler chemical signatures can provide early warnings of poor coral health. Scientists are developing technologies to detect these signals early so that they can intervene before visible bleaching occurs.

Juszkietiz notes that even a slight increase in temperature by 1-2°C can stress corals. These increasingly frequent events contribute to dramatic changes in coral reefs, raising the risk of silent extinctions and the disappearance of species before they can be documented and studied.

As a last ditch effort to record species before they’re lost, he locates and photographs massive Porites corals, which form large, boulder-like colonies. After logging essential information about their size, color, shape, and habitat, he then collects small samples from the colonies, which he analyzes in the lab. One sample is bleached to study the coral's skeleton, another is preserved in ethanol for DNA analysis, and a third is fixed in a formaldehyde solution to determine the coral's sex and spawning readiness—helping to identify new species and understand the range of existing ones.

“There is so much to discover,” Juszkiewicz says. “And our reefs are changing dramatically. Based on the current trajectory, we will only have reefs around for a little while longer.”

That’s what makes the fight to save coral reefs a race against time. “We have a really narrow window of time [before the damage is irreversible]—10 years or so.”

Climate models reveal a stark reality: “The first is that even if we stop emissions tomorrow, there will still be a certain amount of warming locked into ocean ecosystems. So, there’s going to be a period of time where reefs are still suffering stress.”

Researchers emphasize that while scientific research can lead to innovative solutions, it is ultimately a band-aid for a global problem. True coral protection requires a drastic reduction in greenhouse gas emissions.

“I have a glimmer of hope—you have to,” says Juszkiewicz. “ If we want to save our reefs, we need to put the brakes on this rapidly changing climate.”