Coral bleaching in the Maldives in the Indian Ocean. Deeper coral may stand the best chance against climate change. Photo / Paul Muir
Coral bleaching in the Maldives in the Indian Ocean. Deeper coral may stand the best chance against climate change. Photo / Paul Muir
It's one of the cruellest impacts climate change can have on our natural ecosystems.
Yet mass coral bleaching may not be a death sentence for all of the delicate reef systems it touches, researchers say.
Coral bleaching, named because of the lightened-white appearance the coral takes when it expels algae from its tissue, is driven by rising sea temperatures.
"Thermal stress caused by global warming is as big of a threat to coral reef communities as overfishing and pollution," Massey University marine ecologist Dr David Aguirre explained.
"While the loss of its alluring colour is unfortunate for those who admire its beauty, the effect of this bleaching on the coral's survival is far more serious."
This was because the algae offered a significant amount of the nutrients needed to perform photosynthesis, which was how the coral feed themselves.
"While the coral is not necessarily going to die, it loses its main source of food and is at a higher risk of starvation."
The longest global bleaching event ever recorded, between 2014 and 2016, hit 90 per cent of coral on the world-famous Great Barrier Reef, killing somewhere between 29 and 50 per cent of it and signalling a stark message for the future.
But a new study, led by Australian Museum researcher Dr Paul Muir and co-authored by Aguirre, looked to the stunning coral reefs in the Maldives islands in the Indian Ocean and found, importantly, that bleaching events might not affect all parts of reefs equally.
Massey University's Dr David Aguirre is now investigating the impacts of climate change on corals and the distribution of marine species, including our Kermadec Islands. Photo / seacologynz.com
By studying the individual responses of 6859 corals from 28 reefs in certain depths and conditions, the researchers were able to pin-point which of 191 species studied were least and most vulnerable.
They found that the deepest occurring individuals of each population had the greatest potential to survive and drive reef recovery and picked out those likely to dominate following repeated events.
Over the recent bleaching disaster, they found that 83 per cent of coral cover was severely affected on Maldivian reefs at a depth of 3-5m, but significantly reduced effects at 24-30m.
Analysis of 153 species' responses further showed depth, shading and species identity had strong, significant effects on susceptibility.
Aguirre said the work had wide-reaching implications for future coral reef research.
"Current surveys for reef health focus on aerial and shallow in-water surveys that provide data spanning thousands of kilometres, our survey shows the importance of studying the reefs in entirety to see where populations may be bucking the trend and surviving."
The new insight was critical to understanding how reefs would respond to climate change - and may even show us where our best efforts to preserve reefs lie, he said.
The new insights were critical to understanding how reefs would respond to climate change - and may even show us where our best efforts to preserve reefs lie. Photo / seacologynz.com
"If we cannot protect the shallow reef, maybe we can protect the deep and the coral which inhabit it."
Aguirre was now investigating the impacts of climate change on corals and the distribution of marine species, including our Kermadec Islands.
"New Zealand's own Kermadec Islands host one of the highest latitude coral reef communities in the world, and while fishing and pollution are not a problem at the Kermadecs, due to their status as marine protected areas, we need to understand more about how coral bleaching effects communities if we are to have any chance of mitigating such effects.
"Particularly, for areas such as the Kermadecs which are hypothesised as potential refuges for corals escaping tropical regions that are becoming increasingly hostile for corals."
Coral genomes equal to 30,000 volumes of War and Peace
Meanwhile, scientists have just made another major breakthrough in discovering what makes some corals the ultimate survivors in the face of climate change.
Just-published research into the tiniest components of the world's largest living organism, coral DNA, has yielded a world first that can be used to inform future coral reef restoration.
The world's largest coral genomics sequencing project, the five-year Sea-quence project, has genetically sequenced a whole coral organism - the Porites lutea species, a type of massive boulder coral - for the first time.
Coral on the Great Barrier Reef. Photo / Spirit of Freedom
That included the coral animal, the tiny plants, or zooxanthellae, that live in its tissue, and associated microbes including bacteria and viruses.
The Australian research team have also successfully sequenced the eight other forms of coral in the so-called "GBR 9", or the nine Great Barrier Reef coral genomes.
"Scientists have long believed the secret to understanding why some corals survive and bounce back from threats like warming oceans is linked to their genes, just as human genetics is revealing the keys to different health issues for us," Great Barrier Reef Foundation managing director Anna Marsden said.
"When the foundation initiated this project with our partners, only two types of corals had ever been genetically sequenced, so this was a huge gap in our scientific knowledge in the quest to protect our Great Barrier Reef and coral reefs globally.
Scuba diving on Australia's Great Barrier Reef. Photo / Tourism and Events Queensland
"We've now unlocked the genetic data for nine different forms of coral which is important because we know that different corals have different tolerances to stress."
The feat was also remarkable in that it took just five years to download the corals' genetic make-up in just five years, compared with the 13 years it took to sequence our own human genome.
And the coral genomes turned out to be much more complex than first thought.
The genetic information from just one coral species alone would fill 30,000 volumes of Leo Tolstoy's epic War and Peace, and two different species of coral were 100 times more genetically different than humans and chimpanzees.
Mass bleaching has occurred in the Great Barrier Reef as a result of global warming. Photo / Supplied
Scientists would now be able to use this information to reveal which genes contributed to making some corals more resilient than others.
"Sequencing the whole coral organism for the first time is another giant leap forward because corals' health and resilience depend on the symbiotic relationship between the coral animal and the plants and microbes that live within it," Marsden said.
"It's possible that the key to long term coral survival could lie within the relationship between the host and its houseguests' genes and we've just opened the door to testing that theory."
Another step change had been achieved in developing a new methodology for analysing coral genomes faster, more efficiently and cost effectively.
"With around 600 types of soft and hard corals found on the Great Barrier Reef alone, the fact that we can now fast track the whole process and make it easier for other coral species to be sequenced, is a remarkable outcome.
"This project has really set the benchmark and platform for future coral genomics research to discover which corals are best able to survive and live with the impacts of climate change and why."
