Coral belches algae under heat stress in ‘violent’

[Australia] Researchers studying bleaching have for the first time captured video of coral from the Great Barrier Reef convulsing before spewing out the algae that lives within it in response to heat stress.

Earlier this year, the ABC’s 7.30 program revealed that 95 per cent of the northern Great Barrier Reef showed evidence of coral bleaching.

Researchers from the Queensland University of Technology (QUT) have now captured exactly how coral reacts to rising water temperatures, using a microscope and digital camera to record detailed time lapse videos.

“It’s really violent,” key researcher Brett Lewis told Catalyst.

“It’s much more violent than we expected.

“They’re not static — and that’s absolutely what our videos are able to show — just how dynamic they actually are.”

How does coral come to bleach?

Coral needs a key partner to survive — microscopic algae plants called zooxanthellae.

Healthy coral takes the algae from the water to live symbiotically inside its own cells, with the algae or symbionts, receiving shelter and carbon dioxide from their host.

In return, the coral gets most of its nutrition from sugars that the algae make through photosynthesis and a brilliant colourful appearance.

However this solar-powered partnership depends on temperature to work and the change of just a few degrees spells the difference between life and death.

Simulating rising sea surface temperatures, QUT researchers tested the response of the coral Heliofungia actiniformi, which is considered to be relatively resilient to bleaching.

Over a week they increased the water temperature of controlled aquaria by four degrees, to peak at 32 degrees Celsius.

But after 24 hours, the mushroom coral expelled their algal symbionts with repeated convulsions, known as pulsed inflation.

“Some of the largest expansions we’ve seen were 340 per cent the size of the actual original tissue,” said Mr Lewis.
Corals are known for using pulsed inflation to get rid of sand but getting rid of algae in this way has not been seen before.

As clouds of algae are pumped into the water, the coral loses its colour and becomes pale.

Above 30 degrees, the algae start to lose their ability to convert solar energy and the energy that cannot be converted becomes a reactive form of oxygen, like peroxide in bleach, inside the coral’s cells.

The very light that coral needs for growth becomes poisonous and after the algae has been pumped out of the system, the tissue is left thin and weak.

Can coral recover from a bleaching hangover?

Once the health of coral is compromised, bacteria and other microbes can cause infection.

“For corals to recover, they don’t just have to take up their symbionts again,” said coral biologist Dr Tracy Ainsworth from James Cook University.

“They have to repair their tissues, they have to fight off these invaders and then they have to select that one microbe that they need to survive.”

However in 2016, many corals did not get the chance.

Bleaching has killed 35 per cent of coral in central and northern parts of the Great Barrier Reef, extensive aerial and underwater surveys have revealed.

“The peak of the bleaching was a big pulse of more or less instantaneous mortality due simply to the heat stress,” said microbiologist Dr David Bourne from the Australian Institute of Marine Science.

“Those corals didn’t die slowly of starvation because they’d lost their symbionts, they actually cooked over a period of just a week or two because the temperatures in the northern Barrier Reef were so extreme.

“There’s also all these knock on effects; six months, twelve months down the track, we’re probably going to see a lot more disease.”

How have conditions changed?

Paradoxically, temperature records from the Great Barrier Reef over the last 27 years showed that while most of the time water got hot enough to bleach coral, the way it warmed made them more heat resistant.

“That small pulse of temperature, a little bit of early warmth and then a cooling-off, was preconditioning the coral to respond better when the big stress came,” said Scott Heron from the US National Oceanic and Atmospheric Administration.

When scientists model future warming though, those protective pulses by the coral disappear with an increase of just half a degree.

“As temperature goes up … then the relaxation period goes up too so instead of it being a relaxation period, that recovery section actually moves into the stress zone, the stress responses,” explained Dr Ainsworth.

“It never gets to relax.”

Experts such as reef ecologist Professor Terry Hughes from the ARC Centre of Excellence for Coral Reef Studies, question whether coral reefs can ultimately recover from such events, with climate models forecasting future warming.

“The worry we have is that these events are now becoming so frequent that the gap between them, the return time is getting shorter and shorter,” he told Catalyst.

“The median return time now throughout the Indo Pacific is only six years and we know it takes 10 to 15 years for the fastest growing corals to bounce back after a severe disturbance like a bleaching event or a cyclone.

“So six years is certainly not long enough for those corals to recover.”

Does all coral react the same?

There is emerging evidence that if coral can survive a bleaching event, some adult corals can then switch their algal symbionts to a tougher species.

Researchers on Lizard Island said weather conditions had created the "perfect storm" for coral bleaching. (Supplied: WWF)
Researchers on Lizard Island said weather conditions had created the “perfect storm” for coral bleaching. (Supplied: WWF)

Marine biologist David Suggett from the University of Technology Sydney studies which microbes work best for coral and why.

“When the corals reproduce, they create baby corals, or planula larvae, they acquire a diversity of symbionts, and then over time, select a species that its learned to live and operate with most successfully,” he said.

“We now know that Symbiodinium has a broad range of genetic diversity and in fact, as many as 400 or 500 species.

“If we can find that little trick, that little trait that these symbionts have acquired to make them either more sensitive or tolerant, we can start searching for that more broadly in the ecosystems.”
Genetic sequencing of algae from the Red Sea and Persian Gulf, where corals live in high salinities and temperatures up to 36 degrees, may also reveal clues as to how they survive.

Should we lend coral evolution a helping hand?

At the Australian Institute of Marine Science, a research team is attempting to create super-corals with super-symbionts that can tolerate higher temperatures without bleaching.

They have taken algae and exposed them to higher temperatures over several generations.

The National Sea Simulator near Townsville is the largest and smartest aquarium in Australia. Australian Institute of Marine Science
The National Sea Simulator near Townsville is the largest and smartest aquarium in Australia.
Australian Institute of Marine Science

“It’s the same as natural selection, but we’re doing it at a much faster pace so that we can get them to evolve in short time periods,” said PhD candidate Leela Chakravarti.

It only took 150 quick generations for the algae to evolve the ability to grow faster and bleach less when exposed to the hot acidic ocean conditions that are projected for later this century.

The bleaching of three different corals with the new evolved algae is compared with ordinary coral at two different temperatures, 27 degrees and 31 degrees.

“We found that the effect of the temperature evolution is a bit smaller … but there was still a significant delay in the bleaching of corals that associated with the evolved algae compared to the unevolved algae,” said Ms Chakravarti.

Could heat resistant corals save reefs?

“People describe the science as controversial because some say you’re playing ‘god’ because we’re manipulating the constitution of the organisms if you like,” said the supervisor for the research, Professor Madeleine van Oppen.

“But we’re not actually using any mechanisms that do not occur naturally — we’re just trying to speed up those natural processes of evolution.”

Others argue that prevention is better than cure.

“No one has ever successfully restored a reef, even at the scale of a kilometre, never mind 800 kilometres,” said Professor Hughes.
“The choice is whether want to just sit back and enjoy these ecosystems while they’re hanging on by a thread or whether we want to literally dive right in and do something proactive,” said Professor Suggett.

 

Photo: Coral off Lizard Island, 100 kilometres north of Cooktown, captured in March 2016. (Supplied: XL Catlin Seaview Survey)

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