Surprise: Some creatures build more shell in high CO2, acid waters

US researchers have made a surprise discovery about shell-building creatures in acidified waters: contrary to expectations, some animals build more shell, not less.

As the world’s oceans absorb more carbon dioxide from the atmosphere, they’re becoming increasingly acidic. Scientists have long expressed concern this could harm creatures that build shells, as more acid water means less dissolved calcium carbonate available for shell building.

However, researchers at the Woods Hole Oceanographic Institution (WHOI) scientists report that some shell-building creatures — such as crabs, shrimp and lobsters — unexpectedly build more shell when exposed to ocean acidification caused by elevated levels of atmospheric carbon dioxide.

In a study published in the 1 Dec. issue of Geology, a team led by former WHOI postdoctoral researcher Justin B. Ries found that seven of the 18 shelled species they observed actually built more shell when exposed to varying levels of increased acidification. They speculate this may be because the total amount of dissolved inorganic carbon available to the creatures is actually increased when the ocean becomes more acidic, even though the concentration of carbonate ions is decreased.

“Most likely the organisms that responded positively were somehow able to manipulate … dissolved inorganic carbon in the fluid from which they precipitated their skeleton in a way that was beneficial to them,” said Ries, now an assistant professor in marine sciences at the University of North Carolina. “They were somehow able to manipulate CO2 … to build their skeletons.”

Organisms displaying such improvement also included calcifying red and green algae, limpets and temperate urchins. Mussels showed no effect.

“We were surprised that some organisms didn’t behave in the way we expected under elevated CO2,” said Anne L. Cohen, a research specialist at WHOI and one of the study’s co-authors. “What was really interesting was that some of the creatures, the coral, the hard clam and the lobster, for example, didn’t seem to care about CO2 until it was higher than about 1,000 parts per million (ppm).”

Current atmospheric CO2 levels are about 380 ppm. Climate models predict levels of 600 ppm in 100 years, and 900 ppm in 200 years.

The “take-home message, ” Cohen said, is that “we can’t assume that elevated CO2 causes a proportionate decline in calcification of all calcifying organisms.”

Some organisms — such as the soft clam and the oyster — showed a clear reduction in calcification in proportion to increases in CO2. In the most extreme finding, Ries, Cohen and WHOI associate scientist Daniel C. McCorkle exposed creatures to CO2 levels more than seven times the current level.

This led to the dissolving of aragonite — the form of calcium carbonate produced by corals and some other marine calcifiers.  Under such exposure, hard and soft clams, conchs, periwinkles, whelks and tropical urchins began to lose their shells.

“If this dissolution process continued for sufficient time, then these organisms could lose their shell completely, rendering them defenseless to predators,” Ries said.

“Some organisms were very sensitive, some that have commercial value,” added Cohen. “But there were a couple that didn’t respond to CO2 or didn’t respond till it was sky-high — about 2,800 parts per million. We’re not expecting to see that (CO2 level) anytime soon.”

Despite their findings, the researchers caution that acidification’s overall impact might be more complex than it appears. For example, Cohen said, available food and nutrients such as nitrates, phosphates and iron could help dictate how some organisms respond to carbon dioxide.

“We know that nutrients can be very important,” she said. “We have found that corals for example, that have plenty of food and nutrients can be less sensitive (to CO2). In this study, the organisms were well fed and we didn’t constrain the nutrient levels.

“I wouldn’t make any predictions based on these results. What these results indicate to us is that the organism response to elevated CO2 levels is complex and we now need to go back and study each organism in detail.”

Ries agreed that any possible ramifications are complex. For example, the crab exhibited improved shell-building capacity, and its prey, the clams, showed reduced calcification.

“This may initially suggest that crabs could benefit from this shift in predator-pray dynamics,” Ries said. “But without shells, clams may not be able to sustain their populations, and this could ultimately impact crabs in a negative way, as well.”

In addition, Cohen added, even though some organisms such as crabs and lobsters appear to benefit under elevated CO2 conditions, the energy they expend in shell building under these conditions “might divert from other important processes such as reproduction or tissue building.”

“It’s hard to predict the overall net effect on benthic marine ecosystems,” Ries said. “In the short term, I would guess that the net effect will be negative. In the long term, ecosystems could re-stabilise at a new steady state.

“The bottom line is that we really need to bring down CO2 levels in the atmosphere.”