Chronic lung infection by Pseudomonas aeruginosa is the major severe complication in cystic fibrosis (CF) patients, where P. aeruginosa persists and grows in biofilms in the endobronchial mucus under hypoxic conditions. Numerous polymorphonuclear leukocytes (PMNs) surround the biofilms and create local anoxia by consuming the majority of O2 for production of reactive oxygen species (ROS). We hypothesized that P. aeruginosa acquires energy for growth in anaerobic endobronchial mucus by denitrification, which can be demonstrated by production of nitrous oxide (N2O), an intermediate in the denitrification pathway. We measured N2O and O2 with electrochemical microsensors in 8 freshly expectorated sputum samples from 7 CF patients with chronic P. aeruginosa infection. The concentrations of NO3 − and NO2 − in sputum were estimated by the Griess reagent. We found a maximum median concentration of 41.8 µM N2O (range 1.4–157.9 µM N2O). The concentration of N2O in the sputum was higher below the oxygenated layers. In 4 samples the N2O concentration increased during the initial 6 h of measurements before decreasing for approximately 6 h. Concomitantly, the concentration of NO3 − decreased in sputum during 24 hours of incubation. We demonstrate for the first time production of N2O in clinical material from infected human airways indicating pathogenic metabolism based on denitrification. Therefore, P. aeruginosa may acquire energy for growth by denitrification in anoxic endobronchial mucus in CF patients. Such ability for anaerobic growth may be a hitherto ignored key aspect of chronic P. aeruginosa infections that can inform new strategies for treatment and prevention.

Nitrous oxide (N2O) is an important greenhouse gas and ozone depleter, but the global budget of N2O remains unbalanced. Currently, ∼25% of the global N2O emission is ascribed to uncultivated tropical soils, but the exact locations and controlling mechanisms are not clear. Here we present the first study of soil N2O emission from the Pantanal indicating that this South American wetland may be a significant natural source of N2O. At three sites, we repeatedly measured in situ fluxes of N2O and sampled porewater nitrate MathClass-open(NO3-MathClass-close) during the low water season in 2008 and 2009. In 2010, 10 sites were screened for in situ fluxes of N2O and soil NO3- content. The in situ fluxes of N2O were comparable to fluxes from heavily fertilized forests or agricultural soils. An important parameter affecting N2O emission rate was precipitation, inducing peak emissions of >3 mmol N2O m−2 day−1, while the mean daily flux was 0.43 ± 0.03 mmol N2O m−2 day−1. Over 170 days of the drained period, we estimated non-wetted drained soil to contribute 70.0 mmol N2O m−2, while rain-induced peak events contributed 9.2 mmol N2O m−2, resulting in a total N2O emission of 79.2 mmol N2O m−2. At the sites of repeated sampling, the pool of porewater nitrate varied MathClass-open(0.002-7.1μmolNO3-gdW-1MathClass-close) with higher concentrations of NO3- (p < 0.05) found in drained soil than in water-logged soil, indicating dynamic shifts between nitrification and denitrification. In the field, O2 penetrated the upper 60 cm of drained soil, but was depleted in response to precipitation. Upon experimental wetting the soil showed rapid O2 depletion followed by N2O accumulation and a peak emission of N2O MathClass-open(2.5 - 3.0mmolN2Om-2day-1MathClass-close). Assuming that the observed emission of N2O from these wetland soils is generally representative to the Pantanal, we suggest that this undisturbed tropical wetland potentially contributes ∼1.7% to the global N2O emission budget, a significant single source of N2O.

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<p><strong>Abstract.</strong> The global nitrous oxide (N₂O) budget remains unbalanced. Currently, ~25 % of the global N₂O emission is ascribed to uncultivated tropical soils, but the exact locations and controlling mechanisms are not clear. In this study, we present the first detailed study of the dynamics of soil nitrogen pools and flux of N₂O from the world's largest wetland Pantanal, South America. At three long-term measurement sites we measured porewater pH, NO₃^&amp;ndash;, NH₄⁺ , N₂O and O₂ as well as N₂O dynamics in soil slurry, and in situ fluxes of N₂O and CO₂. The pool of inorganic nitrogen changed (7.1–92 μg NH₄⁺-N g dw⁻¹, and 0.1–201 μg NO₃^&amp;ndash;-N g dw^&amp;ndash;1) with the seasonal flooding and drying cycles, indicating dynamic shifts between ammonification, nitrification and denitrification. In the field, O₂ penetrated to a depth of 60 cm in dry soil, but O₂ was rapidly depleted in response to precipitation. Soil pH fluctuated from pH 7–7.5 in flooded soil to pH 3.5–4.5 in the same drained soil. Microsensor measurements showed rapid N₂O accumulation reaching >500–1000 Pa in soil slurries due to incomplete denitrification. In situ fluxes of N₂O were comparable to heavily fertilized forest or agricultural soils. The dominating parameter affecting N₂O emission rate was precipitation inducing peak emissions of >3 mmol N₂O m⁻² d⁻¹, while the mean daily flux was 0.43 mmol N₂O m⁻² d⁻¹. Single measurement based screening of in situ activity at 10 Pantanal sites during dry conditions averaged 0.39 mmol N₂O m⁻² d⁻¹. The in situ N₂O fluxes were only weakly correlated (<i>r</i>² = 0.177) with NO₃^&amp;ndash; and pH value, showing a tendency (<i>p</i> = 0.063) for NO₃^&amp;ndash; concentration to be positively correlated with the in situ N₂O flux and a weaker tendency (<i>p</i> = 0.138) for the pH value to be negatively correlated with the in situ N₂O flux. Over 170 days of the drained period we estimated non-wetted drained soil to contribute 70.0 mmol N₂O m⁻², while rain induced peak events contributed 9.2 mmol N₂O m⁻², resulting in a total N₂O emission of 79.2 mmol N₂O m⁻². The total nitrogen loss via emission of NO, N₂O and N₂ was estimated to be 206 mmol N m⁻² over 170 days, representing 0.7–1.6 % of the total nitrogen in the top 6.5 cm soil layer.</p>