Too much of a good thing: peeling corals need protection

Coral bleaching. These two words fill many Australians with despair—and for good reason. Our beloved Great Barrier Reef has endured three major bleaching events in the last five years.

When corals come under stress, the microscopic algae that give corals their vibrant colours are expelled or die. Although the corals may survive, they are left bone-white and vulnerable to other stressors.

The bad news is that heat stress isn’t the only threat facing corals, as new Fellow of the Australian Academy of Science Dr Jenny Stauber and her team have discovered.

Manganese: a common metal with a weird effect on corals

When there is too much of the metal manganese in the water, corals don’t bleach. What happens instead could be straight out of a horror movie: the coral tissue peels away, leaving just a bare exposed skeleton.

Corals can often recover from bleaching events if conditions improve. But for corals exposed to manganese, there isn’t much hope. “We don’t think they will be able to recover,” Stauber says. This is something her team is investigating.

These results are surprising scientists. “Manganese is really common in the environment,” says Stauber. “It's a common metal. It's found in soil and water and sediments and it's usually thought not to be very toxic.” 

But the water that flows out from some mining sites into coastal waters can sometimes contain quite a lot of manganese—possibly enough to risk corals close to the shore. “We've been doing some experiments to try to determine what concentration of manganese will cause this unusual effect,” Stauber says. “We know manganese affects adult corals in short-term exposures, but we want to see what happens during longer exposure times.”

Measuring chemicals for accountability

Finding the amount of manganese that harms corals will help Stauber build a case for setting an environmental guideline value. Such values make companies and governments accountable for what is released into the environment.

If the world agrees on a guideline value, the results can be spectacular. When scientists discovered certain chemicals were eating a hole into our ozone layer, the Montreal Protocol put the brakes on their production. And it worked.

Guideline values for fresh and marine water quality set out how much of a chemical is acceptable in an environment. Much more than a few numbers on a page, they can lay the foundation for our relationship with nature. The guideline values help us decide whether we need to try and remove some or all of a contaminant, or whether we can leave it there.

Setting them is a balancing act. If they’re too strict, industrial development grinds to a halt. It’s like resigning yourself to eating salads for the rest of your life because there’s a small chance cooking with gas will set your house on fire.

“But if we are under-protective, that’s just as bad,” Stauber says. “We need to have these guidelines that are just the right balance between sustainable industry development and protecting our environment.”

The science of protecting nature

To strike that balance, ecotoxicologists like Stauber can’t stop at measuring the amount of a chemical in the environment. “We need to know what the chemical form is … and then whether it can get into organisms to cause toxic effects,” Stauber says.

Metals like manganese can be tested on whole organisms or living cells in a lab to get clues on their effect on animals or plants. Called bioassays, in these tests “the organism is an indicator of the contaminant’s effect,” says Stauber.

Taking sediment samples of the ocean floor to test for environmental DNA—a sort of biological footprint of organisms—is another powerful tool. It shows what kinds of organisms lived in the environment and whether the community is diverse. When viewed alongside the amount of contaminants in that environment, scientists can get an idea of whether contaminants have affected the organisms that live there.

The science is constantly evolving. “There are a lot of contaminants now that we would never have thought of or been able to detect previously,” says Stauber.

“We've got all of these new tools now that we didn't have before,” she says. And as the science improves, we can make better decisions about how to protect the most precious resource we have: the natural environment.