9:15 AM - 9:30 AM
[MIS14-02] Response of N2O production by nitrifying bacteria to ocean acidification
Keywords:nitrous oxide, nitrification, ocean acidification
Ocean acidification (OA) by increasing atmospheric CO2 has a serious impact on marine organisms and biogeochemical processes. Recent studies showed that the rate of ammonia oxidation and that of nitrous oxide (N2O) production in aerobic seawater change in response to OA. Under aerobic condition, N2O is mainly produced in nitrification either as a byproduct during the oxidation of hydroxylamine (NH2OH) to nitrite (NO2−) or as a product of partial reduction of NO2−. Since N2O is a strong greenhouse gas and the dominant stratospheric ozone-depleting substance, it is crucial to understand the response of N2O production by nitrification to OA in order to predict future N2O emission from the ocean. The purpose of this study was revealing how N2O production by marine nitrifying bacteria depends on pH under several different dissolved oxygen (DO) conditions with respect to the reaction rate and reaction pathways through concentration and isotope ratio analyses of N2O. The isotope ratios, intramolecular 15N distribution (SP value) in particular, can provide us insights of its production pathways.
Nitrosococcus oceani strain NS58 was incubated in 1-L artificial seawater medium with initial NH4+ concentration of 38 mM at 25°C for 50–70 hours during its exponential growth. Total 16 experiments were conducted changing combination of DO (100%, 70%, 35%, and 3% saturation with respect to the atmosphere) and pH (8.3, 8.0, and 7.7) conditions. The DO and pH were kept constant using a continuous culturing system equipped with stirrer, DO/pH sensors, acid/base adding pumps, and flow controllers for carrier gas. Concentration of N2O was monitored using an infrared laser-based analyzer connected to the gas outlet of the incubation tank. Gas samples were collected at the outlet of the concentration analyzer using a 1-L glass flask at 2–20 h interval, and were measured for N2O isotope ratios with a preconcentration-GC-IRMS system. Simultaneously, the medium was also sampled and measured for concentrations of cells, NH4+, and NO2− by UV/VIS absorption spectroscopy.
Up to 50% of initial NH4+ was converted to NO2− during the experiments and concentration of N2O at the outlet of gas phase was between 0.1–2 ppm. Ammonium oxidation rate (VNO2) and N2O production rate (VN2O) calculated from the temporal change of their concentrations and flow rate of the carrier gas ranged 0.4–3.5×10−14 mol h−1 cell−1 and 1–11×10−17 mol h−1 cell−1, respectively. When compared at t = 48 h, both VNO2 and VN2O were higher at pH 7.7 than at pH 8.0 and 8.3 for all the four DO conditions, although VNO2 was decreased under lower DO and VN2O was highest at DO 35%. Isotopic ratios of N2O, d15Nbulk, d18O, and SP respectively ranged −57–−10‰, 10–35‰, 11–31‰. The d15Nbulk tended to be low at lower pH, implying increased isotope fractionation by change in rate-determining step. The d18O did not show significant difference between the three pH conditions under DO 100%, but it showed temporal decrease under DO 35% and 3% with positive correlation with pH. The SP showed similar temporal decrease under DO 35% and 3%, but its positive pH dependence was only observed under DO 3%, whereas it was constant under other DO conditions. These results suggest that N2O production by NH2OH oxidation and NO2− reduction pathways could be both increased by acidification, and the latter pathway might be more sensitive to pH under low DO conditions.
Nitrosococcus oceani strain NS58 was incubated in 1-L artificial seawater medium with initial NH4+ concentration of 38 mM at 25°C for 50–70 hours during its exponential growth. Total 16 experiments were conducted changing combination of DO (100%, 70%, 35%, and 3% saturation with respect to the atmosphere) and pH (8.3, 8.0, and 7.7) conditions. The DO and pH were kept constant using a continuous culturing system equipped with stirrer, DO/pH sensors, acid/base adding pumps, and flow controllers for carrier gas. Concentration of N2O was monitored using an infrared laser-based analyzer connected to the gas outlet of the incubation tank. Gas samples were collected at the outlet of the concentration analyzer using a 1-L glass flask at 2–20 h interval, and were measured for N2O isotope ratios with a preconcentration-GC-IRMS system. Simultaneously, the medium was also sampled and measured for concentrations of cells, NH4+, and NO2− by UV/VIS absorption spectroscopy.
Up to 50% of initial NH4+ was converted to NO2− during the experiments and concentration of N2O at the outlet of gas phase was between 0.1–2 ppm. Ammonium oxidation rate (VNO2) and N2O production rate (VN2O) calculated from the temporal change of their concentrations and flow rate of the carrier gas ranged 0.4–3.5×10−14 mol h−1 cell−1 and 1–11×10−17 mol h−1 cell−1, respectively. When compared at t = 48 h, both VNO2 and VN2O were higher at pH 7.7 than at pH 8.0 and 8.3 for all the four DO conditions, although VNO2 was decreased under lower DO and VN2O was highest at DO 35%. Isotopic ratios of N2O, d15Nbulk, d18O, and SP respectively ranged −57–−10‰, 10–35‰, 11–31‰. The d15Nbulk tended to be low at lower pH, implying increased isotope fractionation by change in rate-determining step. The d18O did not show significant difference between the three pH conditions under DO 100%, but it showed temporal decrease under DO 35% and 3% with positive correlation with pH. The SP showed similar temporal decrease under DO 35% and 3%, but its positive pH dependence was only observed under DO 3%, whereas it was constant under other DO conditions. These results suggest that N2O production by NH2OH oxidation and NO2− reduction pathways could be both increased by acidification, and the latter pathway might be more sensitive to pH under low DO conditions.