[Press Release]

• Response of N₂O production process to ocean acidification is verified
• In the western North Pacific, ocean acidification would enhance the production of N₂O
• The new finding refine the prediction of future climate change

Fig. 1. Map of study sites (stars) and picture of seawater sampling at subarctic station KNOT. Water sampler was casted in the sea from the research ship and seawater of active at 100-200 m depth, where microbial nitrification is active, was collected.

The acidification of the western North Pacific Ocean is increasing the natural production rate of N₂O, an ozone-depleting greenhouse gas. That's the finding of a study carried out jointly by scientists at EPFL, Switzerland, Tokyo Institute of Technology (Prof. Naohiro Yoshida and Assoc. Prof. Sakae Toyoda) and Japan Agency for Marine-Earth Science and Technology and appearing recently in Nature Climate Change. The research team conducted on-board seawater incubation experiments in the western North Pacific Ocean, and discovered that the water's lower pH is causing a significant increase in N₂O production. Although it was thought that acidification decreases the natural production rate of N₂O, the new research has discovered that the process appears to work the other way around. The study has just been published in Nature Climate Change.

Background

Today's rising CO₂ emissions are not only causing the global warming but also changing oceans' pH levels, making them more acidic. We can already see the harmful effects in the coral reefs. Yet other chemical processes – whose environmental impact is not fully known – are also being affected. A study by Beman et al. published in the Proceedings of the National Academy of Sciences in 2011 suggested that ocean acidification is lowering the rate at which nitrous oxide (N₂O), an ozone-depleting greenhouse gas (also known as laughing gas), is being produced naturally. Based on this study, it was thought that acidification decreases the natural production rate of N₂O.

The research team collected samples at five different sites off the coast of Japan, from the subarctic region to the subtropical region. Then they lowered the samples' pH levels, triggering the natural process whereby microbes in the water convert ammonium into nitrate, which generates N₂O as a by-product. Acidification did not suppress the N₂O production at all the sites. At the subarctic sites, in particular, the samples showed a decrease in the ammonium-to-nitrate conversion rate, as in the PNAS study, but also an increase in N₂O production. Moreover, they concluded that if pH levels keep falling at the current rate, or 0.0051 units/year – assuming there is no decrease in CO₂ emissions – the N₂O production rate in that part of the Pacific could rise by 190% to 500% by 2100.

The study revealed that N₂O emission from the western North Pacific Ocean would not decrease, but increase by ocean acidification. However, there are production and consumption processes of N₂O other than nitrification tested in this work, and dominant process would be different among oceanic regions. Increase in atmospheric N₂O concentration accelerates global warming and delays the recovery of ozone layer that was damaged by chlorofluorocarbons. Additional research is needed to see the response of other N₂O producing/consuming processes to ocean acidification as well as to study on other oceanic regions.

Fig. 2. Picture of onboard seawater incubation experiment. ¹⁵N-labeled tracer was added to the water sample, and pH was decreased by addition of hydrochloric acid. The manipulated samples are kept in dark at the temperatures same as those at the sampling depth.

Fig. 3. Results of acidification experiments. Horizontal axis shows decrease of pH whereas vertical axis shows rate of nitrification or N₂O production. Symbols with warm and cold colors respectively denote results for subtropiccal and subarctic sites. Decrease in pH is accompanied by decrease in nitrification rate and increase in N₂O production rate.

Fig. 4. Production and consumption processes of N₂O. Red and blue arrows show nitrification, and gray arrows show nitrifier-dnitrification and denitrification. In the region studied in this work, N₂O is mainly produced by nitrification and denitrification. Red-colored substance and arrows are respectively increased and enhanced by ocean acidification whereas blue-colored ones are decreased or suppressed. The present study revealed that the process from NH2OH to N₂O is enhanced by ocean acidification.

 

Acknowledgement

This project was financially supported by Kakenhi (JP23224013, JP15H05822, JP15H05471, and 17H06105) of the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) and the Swiss National Science Foundation (PBNEP2-142954). It utilized samples collected in the research cruises MR13-04, KS-16-8, and YK16-16 of JAMSTEC.

Reference

(This article also appears in Tokyo Tech official website.)