In splash of colours, hope for coral reefs

In 2003, researchers declared Coral Castle dead. On the floor of a remote island lagoon halfway between Hawaii and Fiji, the giant reef had been devastated by unusually warm water. Its remains looked like a pile of drab dinner plates tossed into the sea. Research dives in 2009 and 2012 had shown little improvement in the coral. Then in 2015, a team of marine biologists was stunned and overjoyed to find Coral Castle, genus Acropora, once again teeming with life. But the rebound came with a big question: could the enormous and presumably still fragile coral survive what would be the hottest year on record?

Recently, the Massachusetts-based research team finished a new exploration of the reefs in the secluded Phoenix Islands, a tiny Pacific archipelago, and was thrilled by what they saw. When they splashed out of an inflatable dinghy to examine Coral Castle closely, they were greeted with a vista of bright greens and purples — unmistakable signs of life. “Everything looked just magnificent,” said Jan Witting, the expedition’s chief scientist and a researcher at the Woods Hole Oceanographic Institution, USA.

Wreaking havoc
Global climate change is wreaking havoc on corals worldwide. Coral bleaching has caused extensive damage to regions extending from the Great Barrier Reef to the Caribbean and nearly everywhere in between. “Threats to tropical coral reefs worldwide have escalated to a level that imperils the survival of these complex, diverse and beautiful ecosystems,” Janice M Lough, an Australian researcher, wrote in a February opinion piece in Nature.

Coral can be severely damaged by rising water temperatures, which cause acidification, as well as by pollution and human activity like tourism, fishing and shipping — prompting some governments to restrict such activities. If Coral Castle can continue to revive after years of apparent lifelessness, even as water temperatures rise, there might be hope for other reefs with similar damage, said another team member, Randi Rotjan, a research scientist who led and tracked the Phoenix Islands expedition from her base at the New England Aquarium in Boston. No one actually knows what drives resilience or even what a coral looks like as it is rebounding. Our understanding of coral is roughly akin to a doctor’s knowing only what a patient looks like in perfect health and after death, Randi said.

Coral Castle’s revival might be an isolated situation, a fluke in a faraway place. But Randi and her team are on a quest to find out why this coral and other reefs nearby came back to life. To understand the stresses corals facing — from pollution and climate change, for example — researchers would like to isolate each problem. Almost everywhere on Earth, corals must endure climate change and human activity. But not in the 1,57,626-square-mile Phoenix Islands Protected Area, created by the US government in 2008.

Shipping lanes skirt the preservation area. Commercial fishing there ceased last year. Randi, who is also the chief scientist for the area’s conservation trust, said the recent protections might have fostered the coral rebound. The algae that live in corals may also be evolving to cope with warmer temperatures, or hardier coral species may be supplanting others, she said.

In a letter published in Nature earlier this year, another global team of researchers reported a similar coral recovery after they reduced the acidity in three lagoons in the southern Great Barrier Reef, off Queensland, Australia. Carbon emissions increase the acidity of seawater. “It’s encouraging, because if we do the right things, health might restore in a pretty responsive manner,” said Rebecca Albright, one of the paper’s authors and a postdoctoral scientist at Stanford University, USA.

But few creatures are more vulnerable to ocean acidification from climate change than corals, which have been declining for decades. About a quarter of the carbon dioxide released into the atmosphere is absorbed by the world’s oceans, measurably lowering their pH levels, Rebecca said.

Forecasters predicted that this year would be the warmest on record, driven by the El Nino weather event that started relatively close to the Phoenix Islands. Late last year, the National Oceanic and Atmospheric Administration declared the third-ever global coral bleaching event, affecting corals in every tropical ocean, and declared that the die-off was being prolonged by El Nino.

The crisis has made the work in the Phoenix Islands even more crucial, said Verena Schoepf, a coral expert at the University of Western Australia. “It’s critical that we understand what happened there, because that would help us understand how corals might be able to cope with climate change in the long run,” she said.

With life in them
Corals are “animal, vegetable and mineral all rolled into one,” Randi said as she held up displays. The fist-size mineral part, beige with white patches, resembles a featherweight stone full of holes. In a small vial of liquid, she points to the animal part: small beads linked by connective tissue. These living beads act as mouths, drinking in nutrients, and make the calcium carbonate that forms their protective home. The animal shares this structure symbiotically with algae that photosynthesise sunlight, producing food that the animal eats.

Warming water can cause the coral to expel its algae, leading to bleaching. Acidic water weakens a coral’s calcium carbonate skeleton, so it cannot contain the coral and grow. And when the nutrients in the surrounding water change, the coral may have trouble getting food.

Adjusting to climate change, Randi said, is not just about getting used to warmer water. A coral depends on its entire context — its ecosystem, the mineral content of the surrounding water, the sociopolitical climate of nearby human populations.

Even old shipwrecks make a difference. As the Pacific grows warmer, rusting hulks alongside the Phoenix Islands seem to be releasing more iron oxides. The corals near the wrecks are not rebounding as well as those farther away, suggesting that the minerals interfere with coral resilience, according to another member of the team, Sangeeta Mangubhai. Climate change isn’t just about heating water, Randi said. “It’s about the little sparks along the way that no one is expecting. We flipped the switch, and now we’re watching the fireworks.”