Chemical formation of hybrid di-nitrogen calls fungal codenitrification into question

Abstract

Removal of excess nitrogen (N) can best be achieved through denitrification processes that transform N in water and terrestrial ecosystems to di-nitrogen (N2) gas. The greenhouse gas nitrous oxide (N2O) is considered an intermediate or end-product in denitrification pathways. Both abiotic and biotic denitrification processes use a single N source to form N2O. However, N2 can be formed from two distinct N sources (known as hybrid N2) through biologically mediated processes of anammox and codenitrification. We questioned if hybrid N2 produced during fungal incubation at neutral pH could be attributed to abiotic nitrosation and if N2O was consumed during N2 formation. Experiments with gas chromatography indicated N2 was formed in the presence of live and dead fungi and in the absence of fungi, while N2O steadily increased. We used isotope pairing techniques and confirmed abiotic production of hybrid N2 under both anoxic and 20% O2 atmosphere conditions. Our findings question the assumptions that (1) N2O is an intermediate required for N2 formation, (2) production of N2 and N2O requires anaerobiosis, and (3) hybrid N2 is evidence of codenitrification and/or anammox. The N cycle framework should include abiotic production of N2.

Dominance of bacterial ammonium oxidizers and fungal denitrifiers in the complex nitrogen cycle pathways related to nitrous oxide emission

“…A role of fungi in N 2 O emissions has recently been reported (Mothapo et al., ; Shoun, Fushinobu, Jiang, Kim, & Wakagi, ; Wu et al., ), albeit in crop and management systems other than sugarcane. In contrast to bacteria, fungi do not have genes encoding nitrous oxide reductase ( nosZ ), which reduces N 2 O to N 2 , and thus fungal denitrification terminates at N 2 O (Phillips et al., ; Shoun et al., ). Therefore, an increase in fungal denitrification might increase N 2 O emissions.…”

“…A role of fungi in N 2 O emissions has recently been reported (Mothapo et al, T A B L E 5 Spearman's correlation coefficients between N 2 O emission flux (mg m À2 day À1 ) and the ratios of the abundances of nitrifier (archaeal and bacterial amoA) and denitrifier (fungal and bacterial nirK, bacterial nirS and nosZ, total bacterial 16S rRNA, and total fungal 18S rRNA) genes in the rainy (RS) and dry seasons ( 2015; Shoun, Fushinobu, Jiang, Kim, & Wakagi, 2012;Wu et al, 2017), albeit in crop and management systems other than sugarcane. In contrast to bacteria, fungi do not have genes encoding nitrous oxide reductase (nosZ), which reduces N 2 O to N 2 , and thus fungal denitrification terminates at N 2 O (Phillips et al, 2016;Shoun et al, 2012). Therefore, an increase in fungal denitrification might increase N 2 O emissions.…”

Seasonal and Spatial Variation in the Location and Reactivity of a Nitrate‐Contaminated Groundwater Discharge Zone in a Lakebed

“…(5) Additionally, the zone of reactivity moves spatially with time as the plume discharge location changes with changing lake and water table levels, which rise (black arrows) and fall (white arrows) as precipitation and evapotranspiration vary seasonally and over long periods, subjecting the microbial populations at any fixed location within the lakebed to continually changing geochemical regimes. mediated by nitrosation (Phillips et al, 2016), anoxic NO 2 − oxidation to NO 3 − by anammox in the deeper sediments (Kartal et al, 2012), and nitrification in the surface sediments, where O 2 is present in low concentration. Additional tracer tests with 15 N-labeled substrates would be needed to discern the extent to which these other processes are occurring.…”

Phylogenomics reveals dynamic evolution of fungal nitric oxide reductases and their relationship to secondary metabolism

“…Fungi also fail to generate anoxic growth yields proportional to the quantity of inorganic N reduced in pure culture (6, 21–23), and no significant relationship was detected between fungal denitrification activity and fungal biomass in anoxic soil incubations (24). Above all, partitioning techniques (antibiotic inhibition and isotope site preference) used to estimate fungal and bacterial contributions to N 2 O emissions are biased and often lack corroborating evidence in conjunction with their application, suggesting fungal contributions to N 2 O emissions are substantially inflated (5, 25–27). For example, antibiotics are often criticized for lacking both generality and specificity, but the expected biases resulting from the exclusive use of antibiotic inhibition techniques to assess fungal contributions to N 2 O emissions remain unaccounted for.…”