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.
- Project data type: Other
- Scope:
- Monoisolate
- West Liberty University