In Coral Skeletons, Microscopic Portraits of Resilience?

 

A scanning helium ion micrograph showing the process of crystal growth in coral. Credit Viacheslav Manichev and Stanislas Von Euw/Rutgers

Coral reefs are sprawling, intricate ecosystems that house an estimated 25 percent of all marine life and can sometimes be seen from space. Yet they are formed by a process invisible to us.

A study published in Science on Wednesday now presents a microscopic picture of the biology that makes corals’ skeletons grow. The findings suggest that coral may be more robust in the face of human-driven ocean acidification than commonly thought.

Corals grow their armor by diligently secreting a chunk of hard skeleton smaller than the width of a human hair each day. This process is called calcification and scientists have debated which parts of it are most important for decades.

One view prioritizes chemical interactions with the seawater. Using ion pumps, corals can possibly decrease the acidity of seawater enough that calcium carbonate — the stuff of limestone and chalk and the basis of coral skeletons — forms spontaneously. Under these circumstances, if oceans become more acidic — a potential consequence of human-emitted carbon dioxide in the atmosphere being absorbed by the seas — coral may struggle to form a skeleton.

The alternative view contends that calcification is primarily a biological process, coordinated by proteins similar to the ones that help us make our teeth and bones. The new study provides evidence for this perspective and some hope for corals in a world with more carbon.

“Coral is not just a rock,” said Paul Falkowski, a professor of marine sciences at Rutgers University and senior author of the study. “And because of that, we’re pretty confident that they’ll be able to continuing making their skeletons even if the ocean becomes slightly more acidic.”

Not all scientists agree.

“The problem is, we have lots of data that show many coral species are very sensitive to environmental change,” said Alexander Venn, a senior scientist at the Scientific Center of Monaco, who was not involved in the study. “While this paper builds a strong model for the biological control of calcification, there are still pieces of the puzzle missing.”

Dr. Falkowski and his colleagues used ultrahigh-resolution microscopic imaging and techniques for observing the structure of molecules to study skeletal branches from smooth cauliflower coral, a well-studied species common in the Indo-Pacific.

The result is a model of coral calcification that starts with a malleable form of calcium carbonate, called amorphous calcium carbonate.

The researchers say they believe that amorphous calcium carbonate is initially formed by proteins. Through a process not yet fully understood, little balls of the material then give way to aragonite, the form of calcium carbonate that makes up a mature coral skeleton.

Similar transitions have been observed in sea urchins and shellfish, and some scientists even suspect amorphous calcium carbonate may be a common precursor for calcification across the tree of life.

 

A scanning helium ion micrograph shows amorphous particles at the center surrounded by aragonite crystals. The transition from this first form of calcium carbonate to the second is not well understood. Credit Viacheslav Manichev and Stanislas Von Euw/Rutgers

“When we precipitate aragonite in the lab, just in a bucket of seawater, it forms this very characteristic pattern with very long, needle-shaped crystals,” said Nicola Allison, a lecturer in earth sciences at the University of St. Andrews, who did not participate in the research.

“This is the first report of amorphous calcium carbonate in coral, and it really does suggest the organism is able to control how solid material is deposited,” she added.

Alex Gagnon, an assistant professor of oceanography at the University of Washington who was not involved in the research, suggested it was an oversimplification to take seawater chemistry out of the equation. Acid dissolves calcium carbonate, so the more acidic the ocean is, the more difficult it is for corals to organize that first bit of skeleton.

“At the end of the day, the fundamental rules of chemistry and physics still apply,” he said.

It’s true that corals lose calcium carbonate in a more acidic environment — but they maintain the ability to grow back that skeleton, “which is good news,” Dr. Falkowski said.

Given current projections of ocean warming and acidification, he is more concerned about warming, which stresses the algae living inside corals and causes coral bleaching.

That said, Dr. Falkowski acknowledges that the cause of warming and acidification is one and the same: carbon emissions from fossil fuel burning. “For all intents and purposes, they’re linked,” he said.

Sumber : nytimes

Great Barrier Reef sharply declines in north but signs coral recovering elsewhere

Australian Institute of Marine Science says reef’s capacity to recover under threat from climate change and pollution

Parts of the Great Barrier Reef not regularly affected by problems such as cyclones have demonstrated the reef still has the ability to regenerate, with a survey showing sharp declines in coral cover in the north but increases elsewhere.
However, the latest results from the Australian Institute of Marine Science (Aims), collected by divers visiting 243 individual reefs, do not include the losses caused by bleaching this year, or the effects of cyclone Debbie, both of which killed coral in the central section.
Up until March 2017, the results show that coral cover over the whole Great Barrier Reef improved dramatically between 2012 and 2016 but then suffered an unprecedented loss from the 2016 bleaching, with about a quarter of the reef’s coral being killed.
The head of the long-term monitoring program at Aims, Hugh Sweatman, said the capacity of the reef to recover was under threat from climate change, as well as other chronic stressors such as pollution.
“The predictions under climate change are that cyclones will get more intense, which means there will be more damage, so they will require longer to recover,” Sweatman said. In addition, pollution and warmer waters are slowing the rate of recovery.

“So you’ve got this double whammy. The time for recovery is likely to get less and the rate of recovery is going to be impeded.
“So what I’m saying is it’s OK so far but there are limits to this.”
Sweatman also noted the recovery in the southern section of the Great Barrier Reef was partly a result of those reefs being mostly dominated by fast-growing corals, which are easily damaged by cyclones but recover relatively quickly.
Richard Leck, a campaigner at WWF, said the results gave hope to conservationists.
“There is still resilience in the system,” Leck said. “This data shows that if the reefs aren’t exposed to underwater heatwaves, if they’re not subject to major pollution events, and they’re not hit by a cyclone, then they recover really well and that should give us a lot of hope that the resilience of the Great Barrier Reef is still there and, if we reduce those threats, we should see recovery reef-wide.”

 Trends in mean hard coral cover for the whole Great Barrier Reef up until March 2017, from the Australian Institute of Marine Science’s long-term monitoring program. N indicates the number of reefs contributing to the analyses; blue shading represents 95% certainty. Photograph: Australian Institute of Marine Science 

That loss as a result of the 2016 bleaching event was driven almost entirely by coral mortality in the northern third of the Great Barrier Reef, where there was ongoing loss since 2013 caused by two cyclones and the continuing crown-of-thorns starfish outbreak.
As a result, the northern Great Barrier Reef has lost the majority of its coral, declining from about 25% coral cover in 2010 to just 10%. “This level of decline is unprecedented in the 30+ year time series,” the report said.

 

Trends in mean hard coral cover for the central Great Barrier Reef up until March 2017, from the Australian Institute of Marine Science’s long-term monitoring program. N indicates the number of reefs contributing to the analyses; blue shading represents 95% certainty. Photograph: Australian Institute of Marine Science 

Sweatman said next year’s results will likely reveal even more significant coral loss in that region, following the most recent bleaching and cyclone Debbie.
In the southern section of the Great Barrier Reef, the latest results show continued improvement in coral cover. In that region, coral cover had reduced from almost 45% coverage in the mid-1980s, to below 10% in 2011. The latest results show coral cover has continued to recover, to more than 30%.

 

Trends in mean hard coral cover for the Southern Great Barrier Reef up until March 2017, from the Australian Institute of Marine Science’s Long Term Monitoring Program. N indicates the number of reefs contributing to the analyses; blue shading represents 95% certainty. Photograph: Australian Institute of Marine Science  

The report notes that impacts from the latest bleaching will be captured in future Aims long-term monitoring program updates.
Imogen Zethoven, from the Australian Marine Conservation Society, said it was “heartening to see coral cover increase significantly in the southern third of the Great Barrier Reef”.
“It’s not too late to save our reef but the federal government must stop the Adani coalmine, reject all new coalmines and switch to 100% renewable energy as a matter of urgency,” Zethoven said.


Sumber : TheGuardian