Ocean acidification, the result of roughly a third of global CO2 emissions dissolving into the seawater and lowering its pH, has complicated and poorly understood consequences for ocean ecosystems. Scientists already know that a drop in ocean pH affects the carbon cycle, reducing the carbonate ions that organisms like corals, mollusks and crustaceans use to build shells and external skeletons. Now, a new study shows that a CO2-induced increase in acidity also appears to disrupt the marine nitrogen cycle. The finding, to be published December 21 in the Proceedings of the National Academy of Sciences, could have ramifications for the entire ocean food web.
The authors of the study examined a specific step in the marine nitrogen cycle, called nitrification, in which microorganisms convert one form of nitrogen, ammonium, into nitrate, a form plants and other marine microorganisms require to survive. Previous research studies on experimentally acidified freshwater and in the laboratory have suggested that reduced pH slows nitrification, and one study in coastal ocean waters showed that large pH decreases did the same. But no one had tried to experimentally simulate the more subtle pH changes predicted to occur in oceans due to the increase in atmospheric CO2 expected over the next 20-30 years, says lead author J. Michael Beman, a professor of oceanography and biogeochemistry at the University of California Merced.
Beman and his colleagues collected samples (six in total) from four separate ocean research locations in the Atlantic and Pacific Oceans, and induced pH decreases ranging from 0.05 to 0.14 in the experimental samples—either by bubbling CO2 through the bottles or adding dilute acid. The experimental nitrification rates were then compared to those in the controls. In the bottles to which CO2 was added, explains Beman, "basically, we exposed them to the future atmosphere in terms of CO2 composition." The group treated some samples with acid "to make sure the effect we were observing wasn’t driven by experimental approaches."
Nitrification decreased, compared to controls, in all experimental cases, with the effect ranging from an 8 percent reduction to a 38 percent reduction. "What we saw is almost uniform across the ocean, or at least in all the experiments we conducted, which seems to suggest this is fairly consistent effect," says Beman. Importantly, in some cases the change was quite large. "So it could have a pretty substantial effect on how nitrogen is cycled in the ocean," he says.
One potentially positive effect would be a reduction of nitrous oxide—marine nitrification is a relatively big source of this greenhouse gas. "But the larger, much more difficult things to predict are the connections to other organisms and processes," says Beman. Less nitrification would make fewer nitrates available to the plants and other organisms that use them to make vital proteins, making it more difficult for them to thrive. This in turn means less food would be available to the animals that eat those nitrate-using organisms, and so on up the food web. But the food web is complex, and the precise implications of the study's results are still unclear, he says.
Beman says this experiment is a first step toward gaining a more detailed understanding of the ramifications of reduced nitrification on ocean ecosystems. "It gives us an envelope of different possible scenarios," to test in computer models that can account for other global changes in biogeochemistry, like decreasing oxygen concentrations, which may happen simultaneously with reductions in pH. He hopes such models will help researchers figure out how modified pH and corresponding changes in nitrification "percolate through the system."
More broadly, the results are a reminder that ocean acidification is bound to influence other nutrient cycles besides the carbon cycle, with potentially profound ecological consequences. “Some of these nutrient cycling processes ultimately affect the entire food web," notes Beman. "So I would argue it is worth examining them in more detail, to try to figure out what sorts of effects we might expect to see."
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