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Ocean acidification alters nitrogen cycling in world seas

December 21, 2010 - Washington

Scientists say that increasing acidity in the sea's waters may fundamentally change how nitrogen is cycled in them.

Nitrogen, one of the most important nutrients in the oceans, is used by all organisms to make proteins and other important compounds.

One of these groups, the ammonia oxidizers, plays an important role in determining which forms of nitrogen are present in the ocean. In turn, they affect the lives of many other marine organisms.

"Ocean acidification will have widespread effects on marine ecosystems, but most of those effects are still unknown," said David Garrison of the National Science Foundation (NSF)'s Biological Oceanography Program.

"This report that ocean acidification decreases nitrification (the amount of nitrogen) is extremely important because of the crucial role of the nitrogen cycle in biogeochemical processes-processes that take place throughout the oceans," said Garrison.

In six experiments spread across two oceans, Michael Beman of the University of Hawaii and colleagues looked at the response of ammonia oxidation rates to ocean acidification.

In every case where the researchers experimentally increased the amount of acidity in ocean waters, ammonia oxidation rates decreased.

These declines were remarkably similar in different regions of the ocean indicating that nitrification rates may decrease globally as the oceans acidify in coming decades, said David Hutchins of the University of Southern California, a co-author of the paper.

Oceanic nitrification is a major natural component of production of the greenhouse gas nitrous oxide. From the seas, nitrous oxide then enters the atmosphere, said Beman.

"All else being equal, decreases in nitrification rates therefore have the potential to reduce nitrous oxide emissions to the atmosphere," he said.

However, nitrous oxide emissions from oceanic nitrification may be altered by other forms of global environmental change such as increased deposition of nitrogen to the ocean, or loss of oxygen in some key areas.

"That could offset any decrease due to ocean acidification, and needs to be studied in more detail," said Hutchins.

The study has also suggested that as human-derived carbon dioxide permeates the sea, ammonia-oxidizing organisms will be at a significant disadvantage in competing for ammonia.

The findings were published in the journal Proceedings of the National Academy of Sciences (PNAS).


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