Some corals could survive a more acidic ocean

There will be “winners and losers in the oceans” as its chemistry changes. AAP

Internal “pumps” found in some corals may help them to endure gradual ocean acidification, giving rise to hope that coral reefs might escape climatic devastation, according to new research.

A team of scientists from Australia’s ARC Centre of Excellence for Coral Reef Studies, at the University of Western Australia, and France’s Laboratoire des Sciences du Climat et de l’Environnement, has identified a powerful internal mechanism that could enable some corals and their symbiotic algae to counter the adverse impact of a more acidic ocean.

The findings are published in the journal Nature Climate Change.

Carbon dioxide released into the atmosphere is turning the world’s oceans more acidic, at rates thought to exceed those seen during past major extinctions of life, the team said.

But some marine organisms that form calcium carbonate skeletons have an in-built mechanism to cope with ocean acidification – which others appear to lack.

“The good news is that most corals appear to have this internal ability to buffer rising acidity of seawater and still form good, solid skeletons,” said Professor Malcolm McCulloch, from the Australian half of the team. “Marine organisms that form calcium carbonate skeletons generally produce it in one of two forms, known as aragonite and calcite.

"Our research broadly suggests that those with skeletons made of aragonite have the coping mechanism, while those that follow the calcite pathway generally do less well under more acidic conditions.”

The aragonite calcifiers – such as the well-known corals Porites and Acropora – have molecular pumps that enable them to regulate their internal acid balance, which buffers them from the external changes in seawater acid levels.

“But the picture for coral reefs as a whole isn’t quite so straightforward, as the "glue” that holds coral reefs together – coralline algae – appear to be vulnerable to rising acidity,“ Professor McCulloch said.

Also of concern is that a large class of plankton, floating in the open oceans and forming a vital component of marine food webs, appears equally vulnerable to acidification. If so, this could be serious not only for marine life that feeds on them, but also for humans, as it could impair the oceans’ ability to soak up increased volumes of carbon dioxide from the atmosphere. This would cause global warming to accelerate.

One plus, however, is that warming oceans may increase the rates of coral growth among colonies now living in cooler waters, he said.

But the big question was whether corals can adapt to global warming, which is now occurring at an unprecedented rate, at about two orders of magnitude faster than occurred with the ending of the last Ice Age, the team said.

"This is crucial since, if corals are bleached by the sudden arrival of hot ocean water and lose the symbiotic algae which are their main source of energy, they will still die,” Professor McCulloch said.

“It’s a more complicated picture, but broadly it means that there are going to be winners and losers in the oceans as its chemistry is modified by human activities. This could have the effect of altering major ocean ecosystems on which both we and a large part of marine life depend.”

Ross Hill, a marine scientist at The University of New South Wales, said that while the study demonstrated the capacity for some species of healthy corals to withstand acidification, that capacity might not be enough to protect them from extreme carbon dioxide emissions.

“Although corals are animals, they contain single celled algal symbionts which, during daylight hours, consume carbon dioxide to produce organic compounds such as sugars,” Dr Hill said. “During times of high photosynthesis, the pH at the site of calcification is greatly increased, promoting calcification. So in the species capable of achieving this internal pH elevation, ocean acidification may not be as great a threat.

"However, this is limited to daylight hours and requires the algal symbionts to be actively photosynthesising. During the night, pH at the site of calcification will decline and may reach a level which leads to dissolution of the calcium carbonate.”