“Microbial nitrification rates decreased in every instance when pH was experimentally reduced at multiple locations in the Atlantic and Pacific Oceans,” wrote researchers led by University of California, Merced biogeochemist J. Michael Beman December 21 in the Proceedings of the National Academy of Sciences. “Our results suggest that ocean acidification could reduce nitrification rates by 3 to 44 percent within the next few decades … fundamentally altering nitrogen cycling in the sea.”

The findings are the latest in a line of research that’s added a new and troubling implication to ocean acidification, a phenomenon already plenty troubling.

Oceans are Earth’s great CO2 sink, having absorbed one-third of human CO2 emissions over the past two centuries. As a result, the concentration of hydrogen ions has increased, making waters more acidic. Earth’s oceanic pH has dropped by 0.1 in the last century, and is expected to drop by another 0.1 over the next several decades. For those who remember litmus tests in high school chemistry, Earth’s is turning redder.

Corals, mollusks, crustaceans and other creatures with shells made of calcium carbonate are clearly threatened by acidifying waters, which literally corrode their shells. Of the disproportionately small public attention paid to ocean acidification, dissolving shellfish get the most. But a few researchers have started studying what acidification means for ocean microbes — a kingdom of life that makes up most aquatic biomass, is a major driver of Earth’s biogeochemical cycles, and is just starting to be understood.

One key microbial task is nitrification, turning ammonium into nitrite and then into organismally useful nitrates. In a series of laboratory flask experiments, limited ocean trials, and long-term lake trials, researchers have demonstrated that increased acidity seems to reduce microbial nitrification rates. Beman and colleagues conducted the most comprehensive ocean trial to date, taking water samples from waters near Hawaii, Los Angeles, Bermuda and in the Sargasso sea, then adding CO2 and measuring how nitrification changed.

As the waters’ pH decreased from 8.1 to 8.0, or what’s expected in the next 20 to 30 years, ammonium-to-nitrite conversion dropped by an average of 21 percent. “Such a change would have major implications for the global marine nitrogen cycle,” wrote the researchers, noting that nitrification in sunlit waters is responsible for one-third of all organic compounds produced in the ocean.

In addition to altering the total amounts of available nitrates, their composition would also change, favoring some organisms while threatening others. Especially threatened would be diatoms, one of the most common types of algae, which are specialized to metabolize certain types of nitrates.

Specific predictions about what this means were beyond the study’s scope, but it would likely have “potentially important implications for oceanic food webs, fisheries, and carbon export to the deep sea,” wrote the researchers. “As anthropogenic CO2 invades the ocean, pH-driven reductions in ammonia oxidation rates could fundamentally change how nitrogen is cycled and used by organisms in the sea.”

Image: A plankton bloom in the Barents Sea. Shades of green come from diatoms./NASA.

See Also:

• Reactive Nitrogen: The Next Big Pollution Problem
• The Future Of Fertilizer
• Ocean Acidification Gives Young Fish a Death Wish
• New Geoengineering Scheme Tackles Ocean Acidification, Too
• Western Oceans Quickly Going Acidic, Say Scientists
• Rising Ocean Acidity Could Disorient Fish

Citation: “Global declines in oceanic nitrification rates as a consequence of ocean acidification.” By J. Michael Beman, Cheryl-Emiliane Chow, Andrew L. King, Yuanyuan Feng, Jed A. Fuhrman, Andreas Andersson, Nicholas R. Bates, Brian N. Popp, David A. Hutchins. Proceedings of the National Academy of Sciences, Vol. 107 No. 51, December 21, 2010.

Authors: Brandon Keim

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