Acid oceans can trigger thicker shells

Raising new questions about the effects of CO2 on marine life, scientists have found that creatures like crabs, shrimp and lobsters unexpectedly build more shell when exposed to oceans with high levels of atmospheric carbon dioxide.

Because excess CO2 dissolves in the ocean — causing it to ‘acidify’ — researchers have been concerned about the ability of certain organisms to maintain the strength of their shells. Carbon dioxide is known to reduce the amount of carbonate ions in seawater — one of the primary materials that marine organisms use to build their calcium carbonate shells and skeletons.

But a study led by former Woods Hole Oceanographic Institution (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.

This may be because the total amount of dissolved inorganic carbon available to them 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. “They were somehow able to manipulate CO2… to build their skeletons.”

“We were surprised that some organisms didn’t behave in the way we expected under elevated CO2,” said co-author Anne L Cohen. “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.” Current atmospheric CO2 levels are about 380 ppm, she said.

But some creatures, such as the soft clam and the oyster, showed a clear reduction in calcification in proportion to increases in CO2.

The researchers caution, however, that acidification’s overall impact may be more complex than it appears. For example, Cohen says that available food and nutrients such as nitrates, phosphates and iron may help dictate how some organisms respond to carbon dioxide.

“I wouldn’t make any predictions based on these results,” said Cohen. “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.”

The research is published in Geology.