The prolonged, extensive emission of greenhouse gases over the next several decades could have significant impacts on ocean life, according to a study by UC Merced marine biologist Michael Beman.
Increases in carbon dioxide emissions — exacerbated by the burning of fossil fuels and other human activities — are making ocean water more acidic, and Beman's study shows that the increased acidity will fundamentally alter the way nitrogen cycles throughout the sea.
Because nitrogen is an important nutrient for all organisms, this could ultimately have significant impacts for all forms of marine life.
"There is growing concern about this issue because human activities are modifying ocean pH so rapidly," Beman said. "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."
Beman's study — funded by the National Science Foundation and co-authored by a team of researchers from the University of Hawaii, University of Southern California and the Bermuda Institute of Ocean Sciences — will be published this week in the prestigious journal, Proceedings of the National Academy of Sciences  (PNAS). Beman conducted the studies while at the University of Hawaii, before coming to UC Merced  in 2009.
During the study, Beman and his coworkers 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.
The results showed that when the pH of the water was 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, 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 and another example of UC Merced researchers  addressing society's most challenging problems.
"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 said.
"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."
Co-authors on the PNAS paper were David A. Hutchins, Cheryl-Emiliane Chow, Andrew L. King, Yuanyuan Feng and Jed A. Fuhrman of USC, Andreas Andersson and Nicholas R. Bates of the Bermuda Institute of Ocean Sciences, and Brian N. Popp of the University of Hawaii.