Can corals survive in warmer seas?

Australia’s Great Barrier Reef narrowly missed getting an “in danger” listing from UNESCO in May. But clamping down on coastal development and cleaning up the water may not be enough to make the world’s largest coral reef safe.

If the climate warms by 2°C it will threaten many of the more than 400 species of coral that live on the reef, and could shrink the world’s reefs by two thirds, according to a 2013 Nature Climate Change paper. 

Corals have adapted to huge climatic changes over the millennia, but it is not known if they can adapt to similar changes over decades. If corals can’t adapt quickly enough on their own, could science help?

Most reef-building corals are part animal (polyp) and part plant (zooxanthellae) – a symbiotic marriage born of the need to survive in nutrient-poor tropical waters. The polyps are related to sea anemones. Tiny translucent sacs, they have a limestone base that attaches them to rocks (or each other) and a tentacle-covered mouth to snag prey. They share their waste nitrogen and phosphorus with their zooxanthellae partners and build complex structures that allow the algae-like zooxanthellae to access light. In exchange, the zooxanthellae share the sugars they make via photosynthesis. 

The coral marriage can cope with short, sharp blasts of heat during a hot summer spell. But when the temperature rise is sustained – even by less than a degree – the coral’s stress levels erupt. Sadly, the zooxanthellae and polyps then part ways, leaving the coral ghostly white. It’s called coral bleaching. 

Coral polyps die when bleaching events are too sustained or frequent.

Scientists don’t entirely understand why coral responds so drastically. One theory is that heat and bright light makes zooxanthellae hyperactive. With photosynthesis in overdrive, the excess energy is released as protons. These generate free radicals that could poison the polyps. 

Losing their zooxanthellae partners is not an automatic death sentence for corals. When the weather cools corals can grow new plant partners from algal remnants. 

Coral polyps die when bleaching events are too sustained or frequent. For instance, reefs across the Pacific and Indian Oceans, the Caribbean and the Red Sea were bleached during an El Nino event in 1997/98. Some 16% of the world’s coral died. 

The Intergovernmental Panel for Climate Change predicts sea temperatures will rise by between 1°C  and 4°C by 2100 – bad news for reefs. 

Can corals adapt in less than 100 years? The Great Barrier Reef’s saving grace may lie in its vast size – measuring about 2,300 kilometres from north to south, it is about the same length as Italy. Its northerly corals are already adapted to hotter waters. A paper in Science in June suggested scientists could help cool water corals learn some lessons from their hot water cousins. 

Princess Charlotte Bay is almost 350 kilometres north of Cairns, with sea temperatures 2°C  higher than the main stretch of reef. Here the branching coral species Acropora millepora thrives thanks to adaptations evolved by its polyps. Could their genes also help more southern corals?

A team co-led by coral researcher Line Bay from the Australian Institute of Marine Science in Townsville, and Mikhail Matz at the University of Texas at Austin, tested that idea. The team crossed the northerner with the same species living 540 kilometres further south at Orpheus Island. 

The offspring were 10 times more resistant to heat stress than their sensitive southern parents. Many of the heat tolerance genes bequeathed by the northern polyps appeared to come from the mitochondria, the organelles responsible for energy production. Matz and Bay suspect these mitochondrial genes might help polyps resist free radical stress induced by overheating – something they plan to explore next.

The study is the first to show heat tolerance can be inherited. “It opens up possibilities for rapid evolution,” says Bay. Matz suggests this is likely to happen naturally as the northern larvae can disperse over vast distances. But the process “can be jump-started by humans moving adult corals”, he says.

And it’s not only polyps that have evolved to cope with the heat. University of Southampton biological oceanographer Jörg Wiedenmann studied corals living in the  Persian Gulf, where water temperatures regularly reach 35 °C –
and found a new type of zooxanthella that could take the extreme heat. He and colleagues published their new species, Symbiodinium thermophilum, in a February issue of Scientific Reports. But the corals’ exceptional heat tolerance was strongly coupled to their ability to survive the Gulf’s high salinity. Taken out of super salty seawater, they died.

So moving coral colonies and their zooxanthellae is not likely to be a panacea. Bay says: “Assisted colonisation has been suggested before. But we’re not yet at the stage where we can say ‘yep, we’re ready to go, there’s no problem with this’.” 

Ultimately, whether they relocate naturally or with help, there’s a limit to how far corals can move from the equator, adds Queensland Museum coral researcher Paul Muir in Townsville. 

In a paper published in Science in June he and colleagues analysed coral depth distributions and found that they are limited by winter sunlight. The sunlight at higher latitudes is too weak for zooxanthellae to photosynthesise, because not enough light penetrates to the depths where the corals grow. Moving coral to shallower water wouldn’t help, he adds. Not all corals can survive exposure at low tide – waves and swells tend to shatter their delicate skeletons. 

As oceans warm, corals will need to adapt to multiple pressures. More acidic oceans – caused by dissolved carbon dioxide – and higher temperatures tend to go hand in hand, Bay says. She plans to see if heat-tolerant corals such as those in Princess Charlotte Bay can also cope with more acidic seas.

But breeding strategies and relocating corals are unlikely to help corals adapt quickly enough. Wiedenmann, Bay and Muir say that aside from limiting our CO₂ emissions, the most important help we can give the corals is to keep their existing homes pristine by reducing fertiliser run-off from creeks and not dredging the sea floor. These activities trigger algal blooms and make the waters murky, depriving zooxanthellae of sunlight. 

Australia has agreed to do that. UNESCO will be back in 2017 to check we have kept our promise.