Ammonia oxidizing microorganisms are an important source of nitrous oxide (N2O) in aquatic environments, but the impact that acidification has on the rate and mechanism of N2O production by these organisms is not well understood. Here we present evidence from 15N-tracer incubations that acidification (from pH 7.54 to 7.20) significantly enhances N2O production by the ammonia oxidizer community in the shallow hypolimnion (17 m) of Lake Lugano in southern Switzerland. This community is dominated by the ammonia oxidizing bacteria of the genus Nitrosospira. Although ammonia oxidation rates were not significantly different among the pH treatments, the pH reduction did enhance the yield, or the ratio of N2O relative to NOx- (nitrite + nitrate) produced by ammonia oxidizers, from 2.6 × 10-5 to 8.8 × 10-5 mol N-N2O/mol N-NOx- at an O2 concentration of 290 µM, and from 5.7 × 10-5 to 12.1 × 10-5 mol N-N2O/mol N-NOx- at an O2 concentration of 70 µM. The increases were due at lea

Ammonia oxidizing microorganisms are an important source of nitrous oxide (N2O) in aquatic environments, but the impact that acidification has on the rate and mechanism of N2O production by these organisms is not well understood. Here we present evidence from 15N-tracer incubations that acidification (from pH 7.54 to 7.20) significantly enhances N2O production by the ammonia oxidizer community in the shallow hypolimnion (17 m) of Lake Lugano in southern Switzerland. This community is dominated by the ammonia oxidizing bacteria of the genus Nitrosospira. Although ammonia oxidation rates were not significantly different among the pH treatments, the pH reduction did enhance the yield, or the ratio of N2O relative to NOx- (nitrite + nitrate) produced by ammonia oxidizers, from 2.6 × 10-5 to 8.8 × 10-5 mol N-N2O/mol N-NOx- at an O2 concentration of 290 µM, and from 5.7 × 10-5 to 12.1 × 10-5 mol N-N2O/mol N-NOx- at an O2 concentration of 70 µM. The increases were due at least in part to enhanced incorporation of N derived from exogenous NO2- into N2O. Incorporation of N from this exogenous NO2- is consistent with hybrid N2O formation, where the N2O produced contains one ammonia- (NH3-) derived N atom and one nitrite- (NO2-) derived N atom but it is not consistent with nitrifier denitrification (enzymatic reduction of 2 NO2- to N2O). In all incubations, most of the N incorporated into N2O appears to have been derived from NH3 rather than exogenous NO2-. We also present evidence of hybrid N2O formation during similar incubations of seawater (at its unaltered pH) from 200 m depth off the coast of Namibia, a coastal upwelling zone and known hotspot of N2O production whose ammonia oxidizer community is dominated by archaea.

氨氧化微生物是水生环境中一氧化二氮（N2O）的重要来源，但酸化对这类微生物产生N2O的速率与机制的影响尚未得到充分阐释。本研究通过15N示踪（15N-tracer）培养实验提供了相关证据：在瑞士南部卢加诺湖17米水深的浅水湖下层（hypolimnion）中，酸化（pH从7.54降至7.20）显著提升了该生境中氨氧化微生物群落的N2O产生量。该群落以亚硝化螺菌属（Nitrosospira）的氨氧化细菌为主导。
尽管不同pH处理组的氨氧化速率无显著差异，但pH降低确实提升了N2O产率，即氨氧化微生物所产生的N2O与NOx-（亚硝酸盐+硝酸盐）的摩尔比值：在氧气浓度为290 μM时，该比值从2.6 × 10^-5 mol N-N2O/mol N-NOx-提升至8.8 × 10^-5 mol N-N2O/mol N-NOx-；在氧气浓度为70 μM时，该比值则从5.7 × 10^-5 mol N-N2O/mol N-NOx-提升至12.1 × 10^-5 mol N-N2O/mol N-NOx-。
这类提升至少部分源于外源亚硝酸盐（NO2-）来源的氮被更高效地整合进入N2O分子。此类外源NO2-来源氮的整合模式与混合型N2O形成（hybrid N2O formation）机制相符：即所产生的N2O分子包含一个氨（NH3）来源的氮原子与一个亚硝酸盐（NO2-）来源的氮原子，但该模式与硝化反硝化（nitrifier denitrification，即通过酶促作用将2分子NO2-还原为N2O的过程）并不一致。在所有培养实验中，整合进入N2O的氮大部分似乎都源自NH3，而非外源NO2-。
本研究还在纳米比亚沿岸200米水深的海域——该海域为沿岸上升流区，是已知的N2O产生热点，其氨氧化微生物群落以古菌（archaea）为主导——的未酸化天然海水培养实验中，发现了混合型N2O形成的证据。