Researchers fret that many species of invertebrates will disappear as the oceans acidify due to increased levels of atmospheric carbon dioxide (CO2). But a new study concludes that some of these species may benefit from ocean acidification, growing bigger shells or skeletons that provide more protection. The work suggests that the effects of increased CO2 on marine environments will be more complex than previously thought.
Bottom-dwelling marine critters such as lobsters and corals encase themselves in shells or exoskeletons made from calcium carbonate. Previous studies predict that rising ocean acidity will result in the loss or weakening of these exoskeletons or shells and increase their owner's vulnerability to disease, predators, and environmental stress. But marine scientist Justin Ries of the University of North Carolina, Chapel Hill, hypothesized that not all ocean organisms would respond the same way to acidity because they use different forms of calcium carbonate for their shells.
Ries and two colleagues from the Woods Hole Oceanographic Institution in Massachusetts exposed 18 species of marine organisms to seawater with four levels of acidity. The first environment matched today's atmospheric CO2 levels, and two others were set at double and triple the pre-Industrial CO2 levels, mimicking conditions predicted to occur over the next century. The fourth CO2 level was 10 times pre-Industrial levels. Although CO2 levels won't rise that high in our lifetime, Ries says they could within 500 to 700 years. The atmosphere did contain that much CO2 during the Cretaceous period about 100 million years ago, Ries says. "This is an interval in which many of these organisms lived and apparently did okay, despite the extremely elevated levels of atmospheric CO2 that existed at that time."
Blue crabs, lobsters, and shrimp prospered in the highest CO2 level, growing heavier shells, the researchers report today in Geology. Ries says a bulkier shell might be more resistant to crushing by predators. American oysters, scallops, temperate corals, and tube worms all fared poorly and grew thinner, weaker shells. The biggest losers included clams and pencil urchins; their exoskeletons dissolved at the highest CO2 levels.
Susceptibility to acid depends in part on the type of calcium carbonate the animal makes, the researchers found. But a shell's mineralogy alone was not the only factor. If critters were able to control pH at their calcification sites by buffering the acid in the surrounding water, as the calcareous green algae did, they also fared better. But Ries points out that this coping mechanism takes energy--how much isn't known--which could have side effects such as diverting energy from maintaining an immune response. "The take-home message is that the responses to ocean acidification are going to be a lot more nuanced and complex than we thought," Ries says.
"The thought has been that as ocean's acidify, the cost of calcification will continually go up," and organisms will be less likely to do it, says Robert Steneck, a marine biologist at the University of Maine, Orono. "Their findings are surprising, to say the least."