Regulation of nitrous oxide production in low-oxygen waters off the coast of Peru

Oxygen-deficient zones (ODZs) are major sites of net natural nitrous oxide (N2O) production and emissions. In order to understand changes in the magnitude of N2O pro- duction in response to global change, knowledge on the indi- vidual contributions of the major microbial pathways (nitrifi- cation and denitrification) to N2O production and their regu- lation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N2O production to oxygen and organic matter was investigated using 15N tracer experiments in combina- tion with quantitative PCR (qPCR) and microarray analysis of total and active functional genes targeting archaeal amoA and nirS as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N2O production with a mean of 8.7 nmolL−1 d−1 but up to 118±27.8 nmolL−1 d−1 just below the oxic-anoxic inter- face. The highest N2O production from ammonium oxida- tion (AO) of 0.16±0.003 nmolL−1 d−1 occurred in the up- per oxycline at O2 concentrations of 10-30 µmolL−1 which coincided with the highest archaeal amoA transcripts/genes. Hybrid N2O formation (i.e., N2O with one N atom from NH+4 and the other from other substrates such as NO−2 ) wasthe dominant species, comprising 70 %-85% of total pro- duced N2O from NH+4 , regardless of the ammonium oxidation rate or O2 concentrations. Oxygen responses of N2O production varied with substrate, but production and yields were generally highest below 10 µmolL−1 O2. Particulate organic matter additions increased N2O production by denitri- fication up to 5-fold, suggesting increased N2O production during times of high particulate organic matter export. High N2O yields of 2.1% from AO were measured, but the over- all contribution by AO to N2O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N2O production process in low-oxygen conditions fueled by or- ganic carbon supply, which implies a positive feedback of the total oceanic N2O sources in response to increasing oceanic deoxygenation.