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Imagine sitting in a pool of water that seesaws between too hot and too cold and being unable to control your body temperature. Now substitute temperature with acidity. Congratulations, you’re a mussel—and you’re stressed out.
The acidity of ocean water can vary due to tides, location, and the time of day or year. But the shallow water along the coast is more susceptible to ups and downs in acidity than the open ocean. And recently, scientists, including marine biologist Ceri Lewis at the University of Exeter, in the United Kingdom, have been paying more attention to how this variability in acidity may affect marine life now and in the increasingly acidic ocean of the future.
In recent experiments, Stephanie Mangan, who was a master’s candidate working with Lewis, examined how blue mussels responded to seawater at current acidity levels (at pH 8.1), and water with an acidity 150 percent higher, around pH 7.7. This more acidic water is expected by century’s end because of anthropogenic climate change. But unlike most ocean acidification studies, which plop marine life into seawater with steady acidity, Mangan’s experiments also tested how mussels fare when the acidity fluctuates.
First, Mangan looked at how the mussels responded to a kind of shock treatment: the water started at pH 8.1, and over six hours dropped to 7.1, before climbing back up to 8.1 over another six hours. She and her colleagues found the mussels had very little ability to cope with the fluctuations by regulating their own internal acidity levels. “What happens inside the mussel almost parallels what the seawater is doing,” Lewis says.
In contrast, humans have a complex system that maintains blood acidity at a relatively steady level around pH 7.4. A change in blood acidity of just 0.1 pH, however, is incredibly dangerous and potentially fatal. A change in internal acidity isn’t as serious for mussels, but it’s not good for them either.
Mangan also ran a version of this experiment over two weeks. In some cases, the mussels lived in seawater that was locked at current acidity levels. In other cases, they were in water fixed at levels representative of the future. In a subset of each of these tests, Mangan also set the acidity to fluctuate twice daily, rising and falling by about 0.5 pH, like an acidic rollercoaster ride. The experiment was meant to expose the mussels to fluctuating acidity similar to what they might experience in shallow tidal estuaries.
The researchers had expected the more acidic water to be more stressful to the mussels than the present-day conditions. But, after evaluating several stress markers, they determined that was generally not the case.
“What our data showed was that actually the variability mattered more than the total change,” Lewis says. Mussels living in seawater where the acidity yo-yoed worked harder and burned more energy to keep running basic metabolic processes, even under present acidity. But when the acidity level remained steady—whether at present or future conditions—mussels responded similarly.
Jon Havenhand, a marine scientist who has studied the effects of fluctuating acidity on barnacles, welcomed the contribution to an underrepresented corner of ocean acidification research. Still, he emphasizes that the few studies looking at the variability in acidity, rather than just the net change, have yielded mixed results. “This [variability] might be really important, and we don’t have enough data to know,” Havenhand says.
Lewis acknowledges the pressing need for more data. In particular, she says, scientists need better measurements of the ocean’s actual short-term fluctuations in acidity, which this study lacked. She’s looking for funding to put in place sensors to remotely and continuously monitor coastal conditions. “I think it’s really important that we get a good sense of how variable are coastal environments,” she says, especially for aquaculture species like mussels.