title: Acidification Enhances Hybrid N2O Production Associated with Aquatic Ammonia-Oxidizing Microorganisms.
abstract: 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 with15N-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/added15[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] and15[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, as15N enrichment of the internal N vs. external N atoms produced site preferences (25.0-34.4‰) consistent with NH2OH-dependent hybrid-N2O production. Hybrid-N2O formation was also observed during incubations of seawater from coastal Namibia with15[Formula: see text] and [Formula: see text]. However, the site preference of dissolved N2O here was low (4.9‰), 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.
authors.affiliation: Department of Environmental Sciences, University of Basel Basel, Switzerland.
authors.forename: Caitlin H
authors.initials: CH
authors.lastname: Frame
authors.affiliation: Department of Natural Sciences and Mathematics, West Liberty University West Liberty, WV, USA.
authors.forename: Evan
authors.initials: E
authors.lastname: Lau
authors.affiliation: Department of Natural Sciences and Mathematics, West Liberty University West Liberty, WV, USA.
authors.forename: E Joseph
authors.initials: EJ
authors.lastname: Nolan
authors.affiliation: Helmholtz Center for Ocean Research, GEOMAR Kiel, Germany.
authors.forename: Tyler J
authors.initials: TJ
authors.lastname: Goepfert
authors.affiliation: Department of Environmental Sciences, University of Basel Basel, Switzerland.
authors.forename: Moritz F
authors.initials: MF
authors.lastname: Lehmann
keywords: 16S rRNA multiplex sequencing; Lake Lugano; acidification; ammonia oxidation; hybrid nitrous oxide; isotopomer; nitrification; nitrous oxide