Unlocking causes of past mass extinction events is a nifty – if not controversial – trick. But forecasting the future while also explaining the geologic past is even niftier. And that is just what a new study attempts to do by documenting experimental effects of ocean acidification upon shelled marine invertebrates.
The study, published Dec. 1 in Geology and led by a University of North Carolina scientist, reports a spectrum of positive to negative responses across seven major groups of calcifying marine organisms. It also offers supporting evidence for understanding patterns of past mass extinction — and survival — seen 251 million years ago at the Permian-Triassic boundary.
UNC marine scientist and lead author Justin Ries said that he and his team wanted to reconstruct conditions from the past to test whether carbon dioxide-induced climate change might have triggered past extinction events that removed certain shelled marine organisms from the fossil record while spurring diversification of others. Exactly why some species survive extinction events and others live or even thrive is a key question that scientists wrestle with. Ries said that an emerging body of research suggests that the Permian-Triassic extinction may have occurred because of a massive ocean acidification event. Also known as the “Great Dying,” this event resulted in an estimated 96 percent of species disappearing from the oceans.
To test the patterns of survival and extinction known from the fossil record, Ries and his colleagues from Woods Hole Oceanographic Institution in Massachusetts devised experimental tanks holding seawater and then adjusted the pH to match today’s atmospheric CO2 levels (440 parts per million), as well as elevated levels that existed 110 million years ago, during the mid-Cretaceous (2850 ppm).
“We chose the Cretaceous time point because it represents a CO2 maximum in the geologic past,” Ries said. Conditions in the Cretaceous included increased volcanic activity and rapid ocean crust production and sea floor spreading. This geologic activity pumped massive amounts of carbon dioxide into the atmosphere, spiking global concentrations which may have led to oceans being more acidic than today’s.
For 60 days, the researchers reared marine organisms representing the major invertebrate calcifying groups that inhabit the seafloor — such as urchins, algaes, clams, lobster and crabs — and then they compared the animal’s calcification rates under the modern and elevated carbon dioxide conditions. The most unexpected finding was that some crustaceans, like American lobsters, shrimp and blue crabs, grew bigger and heavier shells under the mid-Cretaceous carbon dioxide levels. The larger shells mean that the animals were increasing their rate of calcification despite the increased seawater acidity. Other species like pencil urchins and hard and soft clams grew thinner shells or suffered their hard parts dissolving under the acidified seawater conditions.
Andrew Knoll, an evolutionary biologist at Harvard University, said he thought Ries’s paper moved the field forward in important ways, toward thinking about the differential responses organisms may have to changing ocean pH. “I’m pleased, if not entirely surprised, that the responses documented in Justin’s experiments are pretty much in accord with patterns of extinction and survival at the Permian-Triassic boundary, when CO2 is thought to have reached transiently high levels,” Knoll wrote in an email. “For example, during the Permian-Triassic extinction, corals disappear completely, mollusks show a limited response, and arthropods actually increase in diversity.”
Bob Steneck, a marine biologist at the University of Maine in Orono said that the Permian-Triassic boundary was “absolutely the event to look at” to test patterns of extinction and survival in shelled marine organisms. “Of the five big extinction events, that was the biggest,” Steneck said. “And following the P-T extinction, the early Triassic was almost devoid of calcifying organisms. Theirs is the right kind of experiment to do for the right reasons; it’s just that the results are so surprising.”
Steneck said some of the organisms in the experiment responded in ways that scientists would expect. For example, many of the corals suffered eroded exoskeletons under the higher seawater acidity. “But what is harder to understand,” he said, “is why… as the oceans acidify, why is it that some organisms would calcify more robustly?”
Ries said the pattern of how the species fared in the differing CO2 scenarios was dependent upon several factors. He’d hypothesized that because organisms make different kinds of calcium carbonate shells, their vulnerability to acidified seawater would vary with their shell’s specific mineralogy. The three most common forms of calcium carbonate shells are aragonite, high-magnesium calcite and low-magnesium calcite. Because aragonite is more likely to dissolve when exposed to even slight changes in pH that increase the acidity, Ries said it made sense that animals using this form of calcium carbonate would be more vulnerable. And because low-magnesium calcite can hold up to slight changes in pH, it also made sense that animals using this form would be more resistant to acidified waters. To a large extent, they found this to be true.
“But there were some other factors at play,” Ries said. Animals that had a layer of tissue between their exoskeleton or shell — what scientists might call an epidermis, epicuticle or a periostracum — and the seawater appeared to fare better and the tissue seemed to play a powerfully protective role. And if animals were able to control pH at their calcification sites, literally buffering the acid surrounding it, they also fared much better, but with unknown energy costs. The presence of photosynthetic processes also appeared to benefit certain species and confer some protection; this even led to greater calcification in corals under the intermediate CO2 levels.
The team’s results are both forward and backward looking. They foretell a complex future of mixed responses among species to ocean acidification, and they also support the idea of preferential survival of certain species during CO2-induced climate change.
“Both biologists and paleobiologists have long predicted that ocean acidification will result in winners and losers in the oceans,” said Knoll, the evolutionary biologist at Harvard. “Justin’s research helps us to understand who the winners and losers might be.”
Atmospheric carbon dioxide concentrations drive seawater acidity because the oceans act like a giant sponge soaking up CO2. But carbon dioxide dissolves in seawater and forms carbonic acid, lowering the ocean’s pH. Scientists have documented a 0.1 unit change in the oceans’ surface pH since the Industrial revolution, and they expect it to decline up to 0.5 units by the end of this century – leading to questions about how an acidified ocean will affect the creatures living there.
Today’s increasing concentrations of atmospheric CO2 are widely believed to be caused by humans burning of fossil fuels. But if the current trajectory of CO2 building up in the atmosphere does not change, then within 500 to 700 years our planet’s oceans could return to the acidified conditions that existed in mid-Cretaceous. This means that patterns of past mass extinctions — thought to have been triggered by spikes in atmospheric CO2 — could be repeated.