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Ocean acidification may disrupt the marine nitrogen cycle

The views expressed are those of the author and are not necessarily those of Scientific American.


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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."

Image Credit: Flickr/qf8





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  1. 1. RonStrong 1:45 pm 12/21/2010

    There have been many very long periods in the past when atmospheric CO2 concentrations were far above anything contemplated by worst case scenarios for anthropogenic CO2 emissions. Often ten times or more current levels.

    Could someone explain the impact of this on sea life at the time?

    It seems rather hard to believe that making sea water slightly less alkaline (current PH is about 8.1) is going to have a major impact. Even worst case scenarios have sea water remaining alkaline, although slightly less so than now, 100 years from now.

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  2. 2. mikesmtn 12:56 am 12/30/2010

    Ocean acidification is poorly studies thus far in comparison to GWs other effects. In geologic history there have been times when CO2 levels were higher than they are now. And in each of those cases average temperatures were greater than they are now. Temps rise and fall with CO2. That is very well known. But ocean acidification is not a clear issue (though it is being studied more and more since it has been implicated in the greatest extinction in Earth’s history). What IS clear is that if our guesses about the near future impacts due to climate change are too rosy (a likely scenario given our tendency to be ever the optimists), especially as regards effects such as ocean acidification, then the drowning of Florida, NY and Bangladesh will seem as minor inconveniences. Extreme ocean acidification (and yes, moving even a few levels toward the acid end of the pH scale constitutes potentially dangerous acidification) could result in the mass killing of life on the surface of this planet. Think about that, choking on clouds of H2S as you die, realizing all around you is dying as well. Happy New Year.

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  3. 3. R.Blakely 3:50 am 12/30/2010

    Like CO2 in the atmosphere, CO2 dissolving in water is so dilute that it simply disappears. Some people seem to have a problem realizing how deep an ocean really is. All the CO2 ever created by man would form a layer less than a millimeter thick on the Earth’s surface.
    And, in fact, CO2 is actually leaving the oceans, due to warming.

    Link to this
  4. 4. jrmodge 3:11 pm 02/7/2011

    I like this kind of post… it gives me more knowledge about whats happening in this earth…thank you every one.. for posting all your thoughts!

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