Sustained bacterial (N₂O) reduction at acidic pH

Abstract Nitrous oxide (N 2 O) is a climate-active gas and emissions from terrestrial ecosystems are concerning. Microbial reduction of N 2 O to dinitrogen (N 2 ) is the only known consumption process and has been studied extensively at circumneutral pH; however, N 2 O reduction under acidic conditions is thought to be limited. Global soil acidification, accelerated by anthropogenic practices, introduces high uncertainty into N 2 O emission budgets. We obtained an enrichment culture from an acidic tropical forest soil that robustly reduces N 2 O to N 2 at pH 4.5 with the addition of pyruvate and hydrogen. Consecutive transfers at pH 4.5 yielded a co-culture and temporal analyses revealed a bimodal growth pattern with a Serratia sp. growing during the initial pyruvate fermentation phase followed by growth of a novel Desulfosporosinus sp. via hydrogenotrophic N 2 O reduction. The Desulfosporosinus sp. produced (3.1 ± 0.11) × 10 8 cells per mmol of N 2 O consumed, on par with growth yields reported for clade II N 2 O reducers at circumneutral pH. Genome analysis identified a clade II nos gene cluster, but an incomplete pathway for sulfate reduction, a hallmark feature of the genus Desulfosporosinus . Physiological and metabogenomic characterization revealed interspecies nutritional interactions, with the pyruvate fermenting Serratia sp. supplying amino acids as essential growth factors to the Desulfosporosinus sp. The co-culture reduced N 2 O between pH 4.5 and 6 but not at or above pH 7, contradicting the paradigm that sustained microbial N 2 O reduction ceases under acidic pH conditions, thus confirming a previously unrecognized N 2 O reduction potential in acidic soils. Significance Statement Processes generating N 2 O occur over a broad pH range spanning pH 3 to 12; however, the current paradigm assumes that microbial N 2 O consumption is limited to circumneutral pH (6 to 8). The imbalance between N 2 O production versus consumption has increased the atmospheric concentration of this climate active gas by 17 % over the last 100 years, and accelerated emissions due to global soil acidification are a major climate concern. From acidic soil, we obtained a bacterial culture harboring a novel Desulfosporosinus species that effectively reduces N 2 O at pH 4.5, but not at or above pH 7. The discovery of an N 2 O reducer adapted to acidic pH conditions has far-reaching implications for predicting, modeling, and potentially managing N 2 O emissions from low pH ecosystems. Note for publisher (this text will be removed prior to publication) This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ).