The inhibition of N2O production by ocean acidification in cold temperate and polar waters

Andrew P Rees,Ian J Brown,Amal Jayakumar,Bess B Ward

doi:10.1016/j.dsr2.2015.12.006

Andrew P Rees, Ian J Brown + Show 2 more

Open Access

https://doi.org/10.1016/j.dsr2.2015.12.006

Copy DOI

Abstract

The effects of ocean acidification (OA) on nitrous oxide (N2O) production and on the community composition of ammonium oxidizing archaea (AOA) were examined in the northern and southern sub-polar and polar Atlantic Ocean. Two research cruises were performed during June 2012 between the North Sea and Arctic Greenland and Barent Seas, and in January–February 2013 to the Antarctic Scotia Sea. Seven stations were occupied in all during which shipboard experimental manipulations of the carbonate chemistry were performed through additions of NaHCO3−+HCl in order to examine the impact of short-term (48h for N2O and between 96 and 168h for AOA) exposure to control and elevated conditions of OA. During each experiment, triplicate incubations were performed at ambient conditions and at 3 lowered levels of pH which varied between 0.06 and 0.4 units according to the total scale and which were targeted at CO2 partial pressures of ~500, 750 and 1000µatm. The AOA assemblage in both Arctic and Antarctic regions was dominated by two major archetypes that represent the marine AOA clades most often detected in seawater. There were no significant changes in AOA assemblage composition between the beginning and end of the incubation experiments. N2O production was sensitive to decreasing pHT at all stations and decreased by between 2.4% and 44% with reduced pHT values of between 0.06 and 0.4. The reduction in N2O yield from nitrification was directly related to a decrease of between 28% and 67% in available NH3 as a result of the pH driven shift in the NH3:NH4+ equilibrium. The maximum reduction in N2O production at conditions projected for the end of the 21st century was estimated to be 0.82TgNy−1.

Highlights

The effects of ocean acidification (OA) on nitrous oxide (N2O) production and on the community composition of ammonium oxidizing archaea (AOA) were examined in the northern and southern sub-polar and polar Atlantic Ocean
The AOA assemblage in both Arctic and Antarctic regions was dominated by two major archetypes that represent the marine AOA clades most often detected in seawater
Several other archetypes were present, the dominance of AOA1 and AOA3 implies that their physiological characteristics control the rates of nitrification and their sensitivity to OA

Summary

Introduction

The effects of ocean acidification (OA) on nitrous oxide (N2O) production and on the community composition of ammonium oxidizing archaea (AOA) were examined in the northern and southern sub-polar and polar Atlantic Ocean. Environmental effects associated with a changing climate, which include rising temperatures, oxygen depletion and ocean acidification are quite likely to impact the level of this equilibrium (Codispoti, 2010). In the open oceans nitrification is the dominant mechanism for the production of N2O (Freing et al, 2012), and in the limited number of studies that have been reported, this process has been shown to be sensitive to OA in sub-surface marine waters. Hutchins et al (2009) speculated that increasing levels of CO2 may lead to an increase in autotrophic nitrification rates through a reduction of CO2 limitation or CO2 fertilization effect To date this effect has not been observed for the open ocean and the limited number of studies have shown some equivocality in their findings. The NH4 þ :NH3 equilibrium has a pKa of $ 9.2, favouring NH4 þ in an acidifying ocean

Methods

Results

Conclusion