Ocean’s acidity jolts marine microbes

USC (US) — Increases in carbon dioxide emissions—exacerbated by the burning of fossil fuels and other human activities—are making sea water more acidic and will ultimately have significant impact on marine life.

“There is growing concern about this issue because human activities are modifying ocean pH so rapidly,” says Michael Beman, marine biologist at University of California, Merced.

“While we do not know what the full effects of changing the nitrogen cycle will be, we performed experiments all over the world and believe that these changes will be global in extent.”

Findings are reported in Proceedings of the National Academy of Sciences.

For the study, Beman and colleagues decreased the pH level of ocean water—making it more acidic—in six total experiments at four different locations in the Atlantic and Pacific oceans: two near Hawaii, one off the coast of Los Angeles, one near Bermuda and two in the Sargasso Sea southeast of Bermuda.

In every instance, when the pH was decreased, the production of the oxidized forms of nitrogen used by phytoplankton and other microorganisms also decreased. That nitrogen is produced through the oxidation of ammonia in seawater by microscopic organisms.

When the pH of the water decreased from 8.1 to 8.0 — roughly the decrease expected over the next 20 to 30 years—ammonia oxidation rates decreased by an average of 21 percent over the six experiments, with a minimum decrease of 3 percent and a maximum of 44 percent.

Such a reduction could lead to a substantial shift in the chemical form of nitrogen supplied to phytoplankton, the single-celled aquatic “plants” that form the base of the ocean’s food web.

The decrease in nitrogen would likely favor smaller species of phytoplankton over larger ones, possibly creating a domino effect throughout the food web.

This is an important step in furthering science’s understanding of how continued increases in greenhouse gas emissions will affect marine life on a global scale.

“What makes ocean acidification such a challenging scientific and societal issue is that we’re engaged in a global, unreplicated experiment—one that’s difficult to study and has many unknown consequences,” Beman says.

“Nevertheless, our results can be used to estimate the potential impacts of acidification on the marine nitrogen cycle and on marine life in general. These effects could be substantial and deserve additional study.”

Researchers from the University of Southern California, the University of Hawaii, and the Bermuda Institute of Ocean Sciences contributed to the study, which was funded by the National Science Foundation.

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