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Ocean acidification may be making it harder for plankton like Neogloboquadrina dutertrei to build robust shells.

L. Fox et al., Scientific Reports (2020) 10.1038

Samples from famed 19th century voyage reveal ‘shocking’ effects of ocean acidification

Not everyone would consider 150-year-old plankton specimens a treasure with “cutting-edge” research potential. But that’s precisely what Lyndsey Fox thought when she discovered a cache of single-celled, shell-building foraminifera deep in storage at London’s Museum of Natural History. Now, the Kingston University micropaleontologist and colleagues have shown that the samples, collected during the pioneering 1872–76 expedition of the HMS Challenger, hold valuable insights about modern-day climate change: Their shells are far thicker than those of today’s foraminifera, which are thinning in our increasingly acidic oceans.

Scientists have known for years that ocean acidification—a drop in pH that occurs when excess carbon dioxide in the atmosphere dissolves in seawater—brings bad news for most marine life. Acidic waters eat away at the calcium carbonate shells and exo-skeletons of organisms from crabs to corals and make it harder for them to build such structures in the first place. But most evidence has come from lab experiments that last no longer than a few years. Scientists haven’t been able to examine the long-term impacts of acidification in the open ocean—until now.

To compare the Challenger samples to modern-day specimens, the researchers focused on two species of plankton—Neogloboquadrina dutertrei and Globigerinoides ruber—collected during Tara Oceans, a 2011 expedition to the eastern equatorial Pacific Ocean. Using detailed notes from the Challenger expedition as their guide, they pinpointed the precise locations and time of year that the Challenger samples were taken and selected comparable samples from the Tara voyage.

Next, the researchers used a computerized tomography scanner to create precise 3D images of the shells, which are less than 1 millimeter in diameter. They discovered that on average, all modern specimens had thinner shells than the historic specimens, up to 76% thinner in N. dutertrei, they reported last week in Scientific Reports. Some modern specimens had shells so thin that the team was unable to image some portions. “I was a little bit shocked to see how dramatic the results were for some species,” Fox says.

Based on past experiments, the researchers say ocean acidification is likely to blame. They acknowledge, however, that other factors related to ocean acidification, including warmer water and lower amounts of oxygen, could also play a role.

“This is a really neat demonstration,” says Lukas Jonkers, a paleo-oceanographer at the University of Bremen who studies planktonic foraminifera. He says most of what’s known about ocean acidification comes from observations made after the 1950s, when ocean pH was already dropping. “They’re sitting on a unique treasure there at the Natural History Museum.” Jonkers hopes the team will expand its work to include additional specimens and locations.

The researchers plan to do just that, using the “thousands and thousands of jars” of old plankton stored at the museum. “There’s more than any one person can work on in a lifetime,” Fox says. She hopes the research will inspire more scientists to explore often-overlooked museum collections: “There’s lots of great materials stored in museums that are just waiting to be studied.”

*Correction, 5 February, 1:20 p.m.: A previous version of this story incorrectly said the Challenger shells were 76% thicker than the modern-day shells.