We tend to associate corals with warm, tropical regions. But there are also species that live in cold, dark, deep waters. What happens to these creatures as climate change warms the ocean and makes the water more acidic?
It's a warm summer day in the German Baltic city of Kiel. But in the basement lab of the GEOMAR ocean research building, it's a chilly 6 degrees Celsius (43 degrees Fahrenheit). The seawater in the tanks is kept at 8 degrees Celsius. That's the temperature the cold-water corals live at in their natural environment, Janina Büscher tells DW.
The young German biologist is researching Lophelia pertusa, a species of coral found all around the world in water depths between 200 and 2,000 meters (656 and 6,562 feet). Some cold-water corals live as deep as 4,000 meters down in the ocean.
White beauties of the deep
Though a coral polyp looks like a plant, it's really an animal - or rather, a colony of animals. Most of the corals in the tanks here are white. But this has nothing to do with the coral bleaching process that is leaving tropical corals pale and dead.
"These healthy corals usually appear in this whitish color," Büscher explains. "They can appear in a reddish color or orange, when certain pigments are present."
Unlike their relatives living in shallow tropical and subtropical waters, these corals - which live in cold, deep water - do not have symbionts. Those are the tiny microalgae that live in the tissue of tropical corals, providing them with food and brilliant colors.
Rather, deep-water corals rely on plankton particles sinking down from the surface for food.
'You want to protect it'
Research into cold-water corals is a relatively young discipline, because it relies on technology to get down to the cold, dark ocean depths.
"For tropical corals, you can just dive. But with these corals, we have to dive down with a vehicle," Büscher explains. "We are lucky to have one here at GEOMAR, the Jago. It's a submersible where you can dive down with a pilot."
Her eyes sparkle as she describes her trips into the deep.
"The atmosphere down there - you can't even describe it, it's unbelievable. So quiet, such a peaceful environment, that you immediately get the feeling you want to protect it," she says.
She collected the corals for the experiments here during an expedition in the Trondheim Fjord in Norway, as part of the German research program BIOACID (Biological Impacts of Ocean Acidification).
"There are huge reefs, and we collect the coral at the edges so as not to disturb the reef itself, to do our experiments here in the lab."
Over the past six months, the scientists artificially increased the temperature of the water and the amount of CO2 in it, and varied the supply of food in different tanks to find out how ocean acidification and global warming affect the deep-sea creatures.
Nature: a complex mix
Their work showed that acidification has a negative effect on the cold-water corals, because they build their skeletons from calcium carbonate. The absorption of CO2 from the atmosphere reduces the carbonate concentration and so the corals grew more slowly. This supports past findings.
But the lab studies also indicated that when the water temperature is increased at the same time, Lophelia pertusa grows faster - which may be able to counteract the negative effects of acidification.
However, corals subjected to water that is both warmer and more acidic at the same time were unable to take up additional food when it was provided.
"The elaborate experimental setup shows that when applied in combination, different climate change drivers can interact in their effects on the corals," Büscher explains.
"In our experiments, we have seen how flexibly Lophelia pertusa reacts, and how different factors influence each other," she goes on. In this particular case, it looks as though warming helped the corals cope with acidification.
But the scientists think that will only work up to a certain point. The corals may only benefit from rising temperatures as long as they stay within the range the species is currently living under.
In many regions of the world, the corals are already at their temperature limit. If temperatures rise further, the effect of warming could turn negative, and could even amplify the effect of ocean acidification, says Büscher.
No time to waste
Büscher is concerned about the future of cold-water coral reefs, which support a wealth of species in the dark, cold water: "These corals build ecosystems, which is important. And if they fail in the future and the reefs are not as stable anymore, then the biodiversity that is kept in these ecosystems will break down or be reduced."
The reefs provide food and shelter for many species of fish.
The corals in her lab tanks coped better than Büscher expected. Still, she is worried they might not be able to survive in the altered oceans of the future, depending on the degree of change in the environmental conditions.
"These are really slow-growing animals," she says. "We did this experiment for half a year. They might be able to cope for six months, but maybe not much longer. Maybe in three years or so, they won't be able to compensate or counteract the ocean acidification anymore."
Controlled lab environments are different from the real world, where conditions vary with the season, the time of day and factors such as nutrient influx or pollution.
Büscher can hardly wait to get back on the research vessel Poseideon and the submersible the "Jago" and out into the ocean, to find out how her beloved sea-dwellers are coping.
But that doesn't mean the rest of us can afford to sit back and wait and see, she says. "If we wait for more detailed insights before we mitigate climate change, it may be too late to preserve the cold-water reefs."