Nitric oxide and nitrous oxide turnover in natural and engineered microbial communities: biological pathways, chemical reactions, and novel technologies

Abstract

Nitrous oxide (N2O) is an environmentally important atmospheric trace gas because it is an effective greenhouse gas and it leads to ozone depletion through photo-chemical nitric oxide (NO) production in the stratosphere. Mitigating its steady increase in atmospheric concentration requires an understanding of the mechanisms that lead to its formation in natural and engineered microbial communities. N2O is formed biologically from the oxidation of hydroxylamine (NH2OH) or the reduction of nitrite (NO−2) to NO and further to N2O. Our review of the biological pathways for N2O production shows that apparently all organisms and pathways known to be involved in the catabolic branch of microbial N-cycle have the potential to catalyze the reduction of NO−2 to NO and the further reduction of NO to N2O, while N2O formation from NH2OH is only performed by ammonia oxidizing bacteria (AOB). In addition to biological pathways, we review important chemical reactions that can lead to NO and N2O formation due to the reactivity of NO−2, NH2OH, and nitroxyl (HNO). Moreover, biological N2O formation is highly dynamic in response to N-imbalance imposed on a system. Thus, understanding NO formation and capturing the dynamics of NO and N2O build-up are key to understand mechanisms of N2O release. Here, we discuss novel technologies that allow experiments on NO and N2O formation at high temporal resolution, namely NO and N2O microelectrodes and the dynamic analysis of the isotopic signature of N2O with quantum cascade laser absorption spectroscopy (QCLAS). In addition, we introduce other techniques that use the isotopic composition of N2O to distinguish production pathways and findings that were made with emerging molecular techniques in complex environments. Finally, we discuss how a combination of the presented tools might help to address important open questions on pathways and controls of nitrogen flow through complex microbial communities that eventually lead to N2O build-up.

Nitric oxide turnover in permeable river sediment

“…NO was produced in the upper 1 mm under oxic conditions, reaching a concentration of 0.13 mmol L 21 . The production of NO in the oxic zone, directly below the sediment surface, has also been observed previously with NO microsensors in marine, permeable sediments, where NO reached peak concentrations of 0.5 mmol L 21 when measured directly in the field and 0.9 mmol L 21 when measured in cores stored for 3 d at 4uC in the laboratory (Schreiber et al 2008). These previous results from marine sediments indicate that NO measurements in cores might overestimate net NO production occurring in the field, possibly due to an increased flux of Fig.…”

“…Sediment from the depth horizon between 0.5 and 1.5 cm was pooled and mixed at a 1 : 1 ratio with artificial freshwater medium (AFmedium; the composition of the medium is given in Schreiber et al [2009]) without any inorganic nitrogen and stored overnight at 4uC. On the following day, the overlying water was discarded, and the sediment was washed again with AF-medium.…”

“…Also, the dissolved inorganic nitrogen concentrations in the water column were comparable between these two sampling periods (NH Microsensor measurements-Undisturbed sediment cores (diameter 5 5.4 cm; overlying water head < 4 cm; sediment depth < 30 cm) for microsensor measurements were retrieved from the sampling site and incubated not longer than 4 d with in situ water in an aquarium in the laboratory at room temperature (, 22uC). Amperometric microsensors for NO, N 2 O, and O 2 and potentiometric liquid ion exchange microsensors for NO concentration profiles were measured as described previously (Schreiber et al 2009). The measurements in the cores were done outside the aquarium under dark conditions at room temperature with a moist air jet directed onto the water surface to induce constant flow.…”

“…Importantly, the sediment strata where NO is produced and how its production is related to the conversion activities of other N-cycle processes (i.e., biogeochemical context) have remained unknown. While N 2 O has been studied with microsensors in aquatic sediments (Meyer et al 2008), only recently did an NO microsensor for measurements in sediments with high spatial resolution (60 mm) and low detection limit (30 nmol L 21 ) become available (Schreiber et al 2008).…”

Nitrogen fertiliser-induced changes in N2O emissions are attributed more to ammonia-oxidising bacteria rather than archaea as revealed using 1-octyne and acetylene inhibitors in two arable soils

“…Two mainly N 2 O-yielding can explain the reason for the higher N 2 O production by AOB than AOA, NH 2 OH oxidation and nitrifier denitrification (Shaw et al 2006;Stein 2011;Schreiber et al 2012). It was reported that N 2 O emission from the oxidation of NH 2 OH contributed very little to total N 2 O production and nitrifier denitrification was believed to be the predominant process for N 2 O production in soils with 50 % < WFPS < 70 % (Kool et al 2010(Kool et al , 2011.…”