One of the most devastating impacts of climate change is the acidification of seawater caused by the uptake of carbon dioxide (CO2) in the atmosphere. Ocean acidification is thought to be particularly devastating to hard-shelled creatures and has cast the future of coral reefs—and the troves of biodiversity they support—in doubt. A new study of the evolution of corals and their close relatives confirms that the threat is very real: In high-CO2 environments of previous eras, reef-building corals all but disappeared.
But the study also adds an important and perhaps encouraging new wrinkle to the story. The paleoclimate research finds that different subgroups of anthozoans—the evolutionary class that includes corals and sea anemones—were favored by different ocean chemistry regimes. On multiple occasions, as conditions became too warm and acidic for reef-building corals to thrive, soft-bodied corals and anemones were able to flourish and diversify.
An Ancient Lineage
Using a bioinformatics-based approach to genomic analysis, coupled with a new technique developed for isolating conserved DNA segments, researchers compared over 1,700 regions of anthozoan genomes from hundreds of museum specimens from around the world. This method allowed researchers to create a new phylogeny, or evolutionary history, of Anthozoa dating back to the Precambrian Era, about 770 million years ago.
“Anthozoa has been around longer than was previously known, about 250 million years older than the first undisputed fossil evidence,” said Andrea Quattrini, curator of corals at the Smithsonian’s National Museum of Natural History and the study’s lead author. Quattrini and her coauthors were able to show how the marine organisms’ evolution through time tracked with data on paleoclimate ocean conditions and the record of global mass extinction events.
“Earlier studies that tried to date this lineage had very wide ranges on the dates,” said Marymegan Daly, a biology professor at Ohio State University who was not part of the study. “Getting these kinds of results has been very difficult, and having a very data-rich, well-supported evolutionary tree is a huge advance.”
The study, published in Nature Ecology and Evolution in August, highlights how changes in ocean chemistry have played a strong role in the evolution of anthozoan skeletal features. Stony corals build their hard skeletal structures from a mineral known as aragonite. This form of calcium carbonate is prevalent in ocean water only when magnesium concentrations are relatively high.
Aragonite structures are hard but dissolve more readily than those made of calcite, the other predominant form of calcium carbonate. Calcite is the mineral form used by soft-bodied coral species such as sea fans.
Over geologic time, the world’s oceans have cycled between eras of aragonite and calcite dominance. Although driven largely by tectonic movements and rates of seafloor spreading, these shifts are also correlated with climate: Aragonite seas tend to occur in cooler, lower-CO2 conditions, and calcite seas occur when CO2 levels rise.
The first appearance of reef builders, around 400 million years ago, occurred following a calcite-to-aragonite transition, and only in aragonite eras have coral reefs been able to flourish.
Although mineral cycling gave rise to long eras of ocean chemistry more favorable to hard- or soft-bodied species, anthozoan evolution has also been punctuated by mass extinction events, often related to climate change. On at least five occasions in geologic history, stony corals and reefs were replaced by more tolerant or adaptable species, leading to new evolutionary trajectories.
“We see increases in diversification rates following reef crises and mass extinctions, suggesting that empty niches provided as reef-building anthozoans went extinct enabled some other lineages to thrive,” Quattrini said.
Something like this pattern appears to be under way today, noted study coauthor Estefanía Rodríguez, a curator at the American Museum of Natural History. “Numerous stony coral reefs in the Caribbean are already being replaced by forests of sea fans.”
The authors note that even through crises and calcite sea eras, at least some lineages of stony corals have been able to persist and bounce back when conditions improved. But how they have done so remains something of a mystery.
“It is possible that some were able to calcify aragonite skeletons during calcite seas, if they had strong physiochemical control over calcification,” Quattrini said. In addition, “paleontological evidence has suggested that high latitudes or deep-sea environments may have served as refugia for corals.”
—Scott Norris ([email protected]), Science Writer