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Shell-building creatures like sea urchins could potentially benefit from acidic ocean waters.

Jeff Rotman/Science Source

Ocean acidification could boost shell growth in snails and sea urchins

The world’s oceans are acidifying rapidly as they soak up massive amounts of the carbon dioxide (CO2) released from burning fossil fuels. That’s bad news for tiny marine critters like coral and sea urchins that make up the base of the ocean food chain: Acidic water not only destroys their shells, but it also makes it harder for them to build new ones. Now, scientists studying sea snails have discovered an unexpected side effect of this acid brew—it can help some of them build thicker, stronger shells by making their food more nutritious.

Often called climate change’s “evil twin,” acidification happens when the ocean absorbs atmospheric CO2. As CO2 dissolves, the process releases hydrogen ions, lowering the water’s pH and increasing its acidity. That acidic water also removes many floating carbonate ions that organisms like mussels and clams use to build their sturdy shells. Under these conditions, it takes more energy for these creatures to make shells thick enough to withstand the added stress.

But some lab studies suggest more food, such as algae, could help strengthen marine organisms’ shells, and thus offset some of the damage caused by ocean acidification. Scientists predict climate change will do just that, because extra CO2 increases the availability of nutrients, like nitrogen, essential to algal growth.

To find out what is happening in the wild, Sean Connell, an ecologist at the University of Adelaide in Australia, and colleagues traveled to underwater CO2 vents off the coast of New Zealand’s White Island (Whakaari). Water near the vents is about as acidic as most of the ocean is predicted to be by the end of the century. The researchers collected five sea snails (Eatoniella mortoni), along with five samples of turf algae, a staple of the sea snails’ diet.

Over 6 years, they compared their samples with sea snails and algae from nearby sites lacking CO2 vents. They measured the thickness and strength of the sea snail shells, and they also measured the protein, carbohydrate, and energy content of the algae, to determine their nutritional quality.

The sea snails at the CO2 vents built shells that were twice as thick and more durable than the shells of snails at the control site, Connell and colleagues report this month in the Proceedings of the Royal Society B. In addition, the algae were four times as abundant and had 11% more protein and carbohydrates than at the control location, meaning the snails had a bigger and more nutritious supply of food.

Connell chalks this up to extra nitrogen availability. The water’s lower pH allows marine plants like algae to absorb more nitrate, a form of nitrogen, enabling the plants to produce more protein. “We recognized that energy governs life,” Connell says. “If these energy connections exist in nature, their discovery could change the way we think about threatened species.”

The study was “elegantly” done, says Iris Hendriks, a marine biologist with the Spanish National Research Council in Madrid. However, she adds, “There’s a lot of ‘buts’ here.” For example, Hendriks wonders whether the findings could apply to organisms that aren’t known to survive in acidic water. Further, she notes, it’s hard to predict what will happen in ecosystems, which have complex—and sometimes conflicting—interactions.

Marine biologist Ulf Riebesell, who leads the biological oceanography department at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, agrees. “The study is unique in showing one organism that benefits based on one food change,” he says, “but it implies this might be a general phenomenon that can be extrapolated to other marine systems. I would be very careful in doing so.”

Despite the idea that some marine organisms can resist the dangers of climate change, Riebesell says biodiversity is still decreasing, especially at CO2 vents, and that could make ecosystems less resilient. “Even if some organisms benefit from warming and acidification, there are still losers,” Riebesell says, “and evolutionary adaptation is not fast enough to compensate for the loss of these losers.”