Climate Change Metagenomic Record Index

    Ammonia oxidizing microorganisms are an important source of nitrous oxide (N2O) in aquatic environments

    MGnify Record MGYS00001664

    Description
    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.

    Related Publications

    Pubmed Record 28119667

    Abstract Text
    Ammonia-oxidizing microorganisms are an important source of the greenhouse gas nitrous oxide (N2O) in aquatic environments. Identifying the impact of pH on N2O production by ammonia oxidizers is key to understanding how aquatic greenhouse gas fluxes will respond to naturally occurring pH changes, as well as acidification driven by anthropogenic CO2. We assessed N2O production rates and formation mechanisms by communities of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in a lake and a marine environment, using incubation-based nitrogen (N) stable isotope tracer methods with 15N-labeled ammonium (15[Formula: see text]) and nitrite (15[Formula: see text]), and also measurements of the natural abundance N and O isotopic composition of dissolved N2O. N2O production during incubations of water from the shallow hypolimnion of Lake Lugano (Switzerland) was significantly higher when the pH was reduced from 7.54 (untreated pH) to 7.20 (reduced pH), while ammonia oxidation rates were similar between treatments. In all incubations, added [Formula: see text] was the source of most of the N incorporated into N2O, suggesting that the main N2O production pathway involved hydroxylamine (NH2OH) and/or [Formula: see text] produced by ammonia oxidation during the incubation period. A small but significant amount of N derived from exogenous/added 15[Formula: see text] was also incorporated into N2O, but only during the reduced-pH incubations. Mass spectra of this N2O revealed that [Formula: see text] and 15[Formula: see text] each contributed N equally to N2O by a "hybrid-N2O" mechanism consistent with a reaction between NH2OH and [Formula: see text], or compounds derived from these two molecules. Nitrifier denitrification was not an important source of N2O. Isotopomeric N2O analyses in Lake Lugano were consistent with incubation results, as 15N enrichment of the internal N vs. external N atoms produced site preferences (25.0-34.4permille) consistent with NH2OH-dependent hybrid-N2O production. Hybrid-N2O formation was also observed during incubations of seawater from coastal Namibia with 15[Formula: see text] and [Formula: see text]. However, the site preference of dissolved N2O here was low (4.9permille), indicating that another mechanism, not captured during the incubations, was important. Multiplex sequencing of 16S rRNA revealed distinct ammonia oxidizer communities: AOB dominated numerically in Lake Lugano, and AOA dominated in the seawater. Potential for hybrid N2O formation exists among both communities, and at least in AOB-dominated environments, acidification may accelerate this mechanism.