Heat-resistant coral could help rescue reefs

Stress-tolerant coral from Australia’s tropical northwest may provide a lifeline to reefs further south as climate change warms the ocean, researchers have found.

In the Kimberley region, where tides rise and fall by more than 10 metres, coral needs to be able to cope with daily water temperature fluctuations of up to seven degrees Celsius, and low tide temperatures of 37 degrees. It needs to be tough.

So, could the coral surviving in this hostile environment help to restore bleached reefs where the water is cooler and temperatures steadier, such as Australia’s Great Barrier Reef in Queensland or Ningaloo Reef in Western Australia? 

Verena Schoepf and colleagues from the University of Western Australia carried out a nine-month experiment to find out. There’s good news and bad news. 

First, the good news. 

The team collected healthy colonies of the dominant coral Acropora aspera from Shell Island, Cygnet Bay, Western Australia and put them in large tanks where the water temperature could be fine-tuned.

They simulated the environment at Ningaloo Reef, some 1200 kilometres further south, by dropping the temperature four degrees.

At first, the coral showed signs of cold stress, losing up to 25% of its colour and reducing its photo-chemical efficiency. However, it bounced back to full health after about four months, and importantly retained its ability to withstand temperature fluctuations and heat stress. 

“Our study provides long-term experimental evidence that corals from thermally extreme reefs can retain their superior heat tolerance outside their native temperature range – even under much cooler and thermally more stable temperature regimes,” the researchers write in the journal Nature Communications. 

“These findings have important implications for the ability of more heat-resistant corals to provide ‘genetic rescue’ to coral populations maladapted to rising ocean temperatures.”

Such a rescue mission is likely to be extremely challenging. While spreading stress-resistant coral larvae over threatened reefs might seem the logical approach, models suggest this may not work quickly enough to keep up with climate change. {%recommended 9447%}

It is more likely that resistant corals will need to be moved into place by humans to beat rising temperatures. While this would be logistically difficult, A. aspera is fortunately also able to survive prolonged exposure to air and handling stress. 

And now the bad news. 

The study found that while heat-adapted coral thrives in the severe conditions of its natural environment and manages cooler temperatures well, it cannot cope with things getting any hotter. 

As soon as temperatures in the tanks started to exceed the Kimberley’s maximum monthly mean temperatures in spring, heat stress resulted in significant coral bleaching and compromised health. 

In other words, in its natural environment this resilient coral is itself particularly vulnerable to global warming, and is likely to be one of the earliest casualties of rising sea temperatures.

It appears that if A. aspera does provide an opportunity to help out other reefs, it is an opportunity that needs to be acted on quickly.