Increase in ocean acidity affects the marine nitrogen cycle

More carbon dioxide in the atmosphere (CO2) means more carbon dioxide dissolves in the oceans. As that happens the acidity of the oceans increases (technically, the pH decreases). This process is chemically obvious and the data indicates that ocean acidification is occurring and increasing. The impact on life in the seas is complex. Many studies indicate that carbon cycle is affected. In particular, many ocean species such as corals and mollusks rely on carbonate to produce shells and skeletons. Carbonate is destroyed by increasing acidity.

Increasing ocean acidity appears to also affect the nitrogen cycle. A new study published December 21 in the Proceedings of the National Academy of Sciences (PNAS) [ Global declines in oceanic nitrification rates as a consequence of ocean acidification], from a consortium of research institutions led by J. Michael Beman of the University of California Merced, indicates that a particular phase of the nitrogen cycle called nitrification is negatively affected by ocean acidification. Nitrogen is used by all life as an essential component of DNA, RNA, amino acids, proteins, and other aspects of organic material. Earth’s atmosphere is mostly nitrogen, but in this gaseous form it is not usable by most living things. However, through the action of various bacteria, atmospheric nitrogen is converted into biologically useful nitrate. In water, the process of nitrification is slowed as the acidity of the water increases.

To quantify the effect, Beman and his associates collected six water samples from four ocean locations in the Pacific and Atlantic. In part of the samples the water was carefully acidified (either by introducing carbon dioxide or by adding a weak acid). Then the samples were tested for nitrification levels (against the untreated control samples). Nitrification decreased in a range from 8 to 38 percent – significant results with many potential effects.

This is where the study cannot provide evidence. It shows that acidification of ocean water affects nitrification. What does the drop in biologically available nitrogen do to the plant and animal life of the sea? It will affect their ability to produce proteins. There will be less food for animals that eat nitrogen using organisms. However the food web is complex and this study does not attempt to unravel the potential consequences beyond simple speculation.

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.”

[Source: Scientific American]

In other words, like so much of the moving target of climate change, the effect of increasing CO2 on ocean acidification – especially in reference to changes in the nitrogen cycle – needs more study.

Research Spectrum

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