When coral reefs are healthy, countless nickel-sized coral polyps slowly calcify with minerals from ocean water, forming protective stony structures that act as reef “skeletons.” But as corals die at alarming rates around the world amid ongoing ocean warming and acidification, many reefs are being recolonized by mats of turf algae that spread across the skeletal remains.
In the warm waters of the northern Great Barrier Reef, where conditions reflect future projections of a warming ocean, scientists are studying how calcification at dead, algae-covered reefs differs from that at healthy reefs. Their results so far have revealed a surprising similarity: The algae-dominated communities on the dead reefs can calcify at rates similar to those of healthy corals.
But the similarities ended with the light of day. In darkness, the turf mats—which comprise communities of multiple species of algae, including calcifying algae, and which also provide habitats for calcifying (and noncalcifying) invertebrates—reversed course, dissolving amounts of mineralized material nearly equal to what they produced in daylight. Healthy corals, meanwhile, continued calcifying regardless of lighting, albeit at a slower rate in darkness.
“Corals are the main reef builders,” said Manoela Romanó de Orte, a chemical oceanographer at the Carnegie Institution for Science in Stanford, Calif., who presented the research at AGU’s Fall Meeting 2019 in San Francisco, Calif. “How is it that this algal community is calcifying at the same rate as the corals are?”
Romanó de Orte and her team investigated reef environments at Lizard Island and used a tool not previously used to measure calcification. The Coral In Situ Metabolism and Energetics (CISME, pronounced “kiss me”) device is a pipelike instrument that gently suctions to a small area of a reef, controlling the flow of water in contact with the reef patch. The device measured changes in water chemistry as the corals and turf algae communities went about their business. The researchers could also control light levels in CISME’s enclosed chamber, allowing them to measure differences between day and night.
“The cool thing about that instrument is that it allows you to take discrete measurements on small surfaces,” said Michael Fox, a coral reef ecologist at Woods Hole Oceanographic Institution in Massachusetts who wasn’t involved in the research.
This method differs from the typical method of measuring reef calcification, which involves sampling water across a reef, giving a larger-scale, averaged assessment of overall calcification.
But that averaged water “contains a sign [of calcification] from corals, from calcifying algae, from turf algae—from everything together,” said Romanó de Orte. “You can’t pick apart what component is contributing to the amount of total calcification.”
Using CISME, Romanó de Orte and her colleagues confirmed that calcification was occurring within the turf algae, but it is still unclear exactly what organisms were responsible, she said.
Calcifying algae aren’t unheard of. “There are very important algae on coral reefs that calcify,” Fox said. Typically, however, these species live by themselves and are not mixed in with turf, he said, but sometimes they can be interspersed within turfs and can be “easy to overlook.”
More clear is the reason for the nightly reversal of calcification, which Romanó de Orte said is likely due to algae respiration: While algae photosynthesize during the day, they take up carbon dioxide from the water and release oxygen. At night, however, algae respire like humans, taking up oxygen and exhaling carbon dioxide. The carbon dioxide causes the surrounding water to become slightly more acidic, contributing to mineral dissolution.
“What is accumulating during the day is dissolving during the night,” she said. “So the reef is not growing.”
Reefs of the Future
The Great Barrier Reef lost half of its coral population after a sudden spike in ocean temperatures between 2016 and 2017 led to severe bleaching events. The reefs closest to the equator, like those around Lizard Island, are most susceptible to slight increases in temperature. These warm-water corals might offer a glimpse into what future reefs will look like as ocean temperatures increase around the world.
As waters continue warming, such bleaching events will likely become more common, Romanó de Orte said, which means more dead reefs dominated by turf algae. “This has been happening a lot,” she said. “We’re trying to figure out what reefs will look like in the future.”
Fox said the team’s technique could help produce more accurate estimations of how different reefs are changing. “The CISME tool is going to offer a new resolution for understanding these processes in coral reefs,” he said, adding that future research should focus on identifying the relative contributions of “all the different coral characters,” including calcifying algae and invertebrates that can be important for reef growth.
—Jerimiah Oetting (@joetting13), Science Communication Program Graduate Student, University of California, Santa Cruz