Ocean Acidification Disturbing The Nitrogen Cycle- How Rude!

Atmospheric carbon dioxide concentrations are projected to double over the next century, which is predicted to decrease seawater pH on average by 0.3-0.4. This study showed that ocean acidification will alter the central process of the nitrogen cycle, nitrification. Nitrification produces both nitrous oxide (a greenhouse gas) and more importantly, oxidized forms of nitrogen, which is used by phytoplankton and other microorganisms in the ocean. Ammonia Oxidizing Archaea (AOA) and Ammonia Oxidizing Bacteria (AOB) carry out the first stage of this process, converting ammonium into nitrite. From the results of this study it can be predicted that in 20-30 years, ammonia oxidation rates by AOA and AOB will decrease by 3-44% in response to the expected 0.1 decrease of seawater pH. 

To make this discovery, a series of experiments took place, each focused on a water samples taken from three locations each simulating different pH characteristics of water masses. Samples were taken from two oligotrophic seas and a subtropical gyre, from the eutrophic zone. The pH exposures mimicked the average conditions expected later this century. Rates of oxidization were measured by adding a stable isotope tracer to the seawater samples and the accumulation of this isotope in the oxidized product pools after incubation were calculated into rates using an equation. Percentage changes in ammonia oxidation rates with the projected changes in ocean pH could then be determined by using a graph.

It is likely that the decrease in pH slowed down ammonia oxidizing rates because it affected the enzyme activity directly by disrupting the function of ammonia mono-oxygenase, which ammonia is transferred and broken down by. A study on confirming if this is the reason would be of great use to enable predictions of what other important enzymes in marine organisms a lowered pH could effect.

This shift in chemical forms of nitrogen supplies to more ammonium than nitrate in the upper water column could mean nitrate supported organisms, such as diatoms, would be at a disadvantage; therefor effecting oceanic food webs, fisheries and carbon export to the deep sea.  However, not all results would be negative, as with the 3-44% decrease in oceanic nitrification, less nitrous oxide will be produced, which will hopefully cushion the increased fossil fuel combustion and industry processes effects. But it also has to be considered that along with ocean acidification, other climate change players may affect nitrifying abilities such as upper ocean temperature and oxygen concentration, which should now also be explored.  I feel that a study like this should involve a repeat mesocosm experiment using cultured AOA and BOA, run alongside the samples taken from the field. This would be of importance as differing substrate availability, light and dissolved oxygen concentrations in the sea water samples could have affected nitrification rates seen in this study.

I think it's important to mention that the authors found that in their study AOA and AOB from regions of recorded lower pH are not immune to future acidification, as their oxidizing rates showed no different to water samples that have not been previously acclimatised to lower pH. This has even more implications that the acclimatisation ability of microbes will not be fast enough for climate change.

Beman, J. M., Chow, C. E., King, A. L., Feng, Y., Fuhrman, J. A., Andersson, A., ... & Hutchins, D. A. (2011). Global declines in oceanic nitrification rates as a consequence of ocean acidification. Proceedings of the National Academy of Sciences, 108(1), 208-213.

Available at: http://www.pnas.org/content/108/1/208.full