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Microbial nitrate reduction can be driven by organic carbon oxidation, as well as by inorganic electron donors, such as reduced forms of sulfur and iron. An apparent inverse oxygen isotope fractionation effect was observed during nitrate reduction in sediment incubations from five sampling sites of a freshwater lake, Hongze Lake, China. Incubations with organic and inorganic electron donor additions were performed. Especially, the inverse oxygen isotope effect was intensified after glucose addition, whereas the incubations with sulfide and Fe2+ showed normal fractionation factors. Nitrate reductase encoding genes, napA and narG, were analysed with metagenomics. Higher napA/narG ratios were associated with higher oxygen fractionation factors. The most abundant clade (59%) of NapA in the incubation with glucose was affiliated with Rhodocyclales. In contrast, it only accounted for 8%–9% of NapA in the incubations with sulfide and Fe2+. Differences in nitrate reductases might explain different oxygen isotope effects. Our findings also suggested that large variance of O‐nitrate isotope fractionations might have to be considered in the interpretation of natural isotope records. [ABSTRACT FROM AUTHOR]

Microbial denitrification is a main source of nitrous oxide (N 2 O) emissions which have strong greenhouse effect and destroy stratospheric ozone. Though the importance of sulfide driven chemoautotrophic denitrification has been recognized, its contribution to N 2 O emissions in nature remains elusive. We built up long-term sulfide-added microcosms with sediments from two freshwater lakes. Chemistry analysis confirmed sulfide could drive nitrate respiration in long term. N 2 O accumulated to over 1.5% of nitrate load in both microcosms after long-term sulfide addition, which was up to 12.9 times higher than N 2 O accumulation without sulfide addition. Metagenomes were extracted and sequenced during microcosm incubations. 16 S rRNA genes of Thiobacillus and Defluviimonas were gradually enriched. The nitric oxide reductase with c-type cytochromes as electron donors (cNorB) increased in abundance, while the nitric oxide reductase receiving electrons from quinols (qNorB) decreased in abundance. cnorB genes similar to Thiobacillus were enriched in both microcosms. In parallel, enrichment was observed for enzymes involved in sulfur oxidation, which supplied electrons to nitrate respiration, and enzymes involved in Calvin Cycle, which sustained autotrophic cell growth, implying the coupling relationship between carbon, nitrogen and sulfur cycling processes. Our results suggested sulfur pollution considerably increased N 2 O emissions in natural environments. • N2O accumulation was up to 12.9 times higher after long-term sulfide input. • N2O production was associated more with c-type cytochrome nitric oxide reductases. • cnorB genes similar to Thiobacillus were enriched in both microcosms. • Sulfur oxidation supplied electrons to nitrate respiration. • Microbes sustained autotrophic growth through the Calvin cycle. [ABSTRACT FROM AUTHOR]

Partitioning between nitrate reduction pathways, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) determines the fate of nitrate removal and thus it is of great ecological importance. Sulfide (S2−) is a potentially important factor that influences the role of denitrification and DNRA. However, information on the impact of microbial mechanisms for S2− on the partitioning of nitrate reduction pathways in freshwater environments is still lacking. This study investigated the effects of long-term (108 d) S2− addition on nitrate reduction pathways and microbial communities in the sediments of two different freshwater lakes. The results show that the increasing S2− addition enhanced the coupling of S2− oxidation with denitrification instead of DNRA. The sulfide-oxidizing denitrifier, Thiobacillus , was significantly enriched in the incubations of both lake samples with S2− addition, which indicates that it may be the key genus driving sulfide-oxidizing denitrification in the lake sediments. During S2− incubation of the Hongze Lake sample, which had lower inherent organic carbon (C) and sulfate (SO 4 2−), Thiobacillus was more enriched and played a dominant role in the microbial community; while during that of the Nansi Lake sample, which had higher inherent organic C and SO 4 2−, Thiobacillus was less enriched, but increasing abundances of sulfate reducing bacteria (Desulfomicrobium , Desulfatitalea and Geothermobacter) were observed. Moreover, sulfide-oxidizing denitrifiers and sulfate reducers were enriched in the Nansi Lake control treatment without external S2− input, which suggests that internal sulfate release may promote the cooperation between sulfide-oxidizing denitrifiers and sulfate reducers. This study highlights the importance of sulfide-driven denitrification and the close coupling between the N and S cycles in freshwater environments, which are factors that have often been overlooked. Image 1 • Long-term sulfide input favors chemoautotrophic denitrification instead of DNRA. • Thiobacillus was the key genus driving sulfide-oxidizing denitrification. • Inherent C and S in the samples influenced microbial interactions under S2− input. • Internal SO 4 2− release increased cooperation between N and S cycling microorganisms. Long-term sulfide input favors chemoautotrophic denitrifiers instead of DNRA bacteria in freshwater sediments. [ABSTRACT FROM AUTHOR]

Dissimilatory nitrate reduction to ammonia (DNRA) is an important nitrate reduction pathway in lake sediments; however, little is known about the biotic factors driving the DNRA potential rates and contributions to the fate of nitrate. This study reports the first investigation of DNRA potential rates and contributions in lake sediments linked to DNRA community structures. The results of 15N isotope-tracing incubation experiments showed that 12 lakes had distinct DNRA potentials, which could be clustered into 2 groups, one with higher DNRA potentials (rates varied from 2.7 to 5.0 nmol N g−1 h−1 and contributions varied from 27.5% to 35.4%) and another with lower potentials (rates varied from 0.6 to 2.3 nmol N g−1 h−1 and contributions varied from 8.1% to 22.8%). Sediment C/N and the abundance of the nrfA gene were the key abiotic and biotic factors accounting for the distinct DNRA potential rates, respectively. A high-throughput sequencing analysis of the nrfA gene revealed that the sediment C/N could also affect the DNRA potential rates by altering the ecological patterns of the DNRA community composition. In addition, the interactions between the DNRA community and the denitrifying community were found to be obviously different in the two groups. In the higher DNRA potential group, the DNRA community mainly interacted with heterotrophic denitrifiers, while in the lower DNRA potential group, both heterotrophic and sulfur-driven autotrophic denitrifiers might cooperate with the DNRA community. The present study highlighted the role of the sulfur-driven nitrate reduction pathway in C-limited sediments, which has always been overlooked in freshwater environments, and gave new insights into the molecular mechanism influencing the fate of nitrate. Image 1 • Distinct DNRA potential rates and contributions were found among 12 shallow lakes. • C/N was the key factor driving the distinction of DNRA potentials. • The ecological patterns of the nrfA community composition could reflect the distinction of DNRA potentials. • Sulfur-driven autotrophic denitrifiers might cooperate with the DNRA community in C-limited sediments. [ABSTRACT FROM AUTHOR]

Nitrate pollution in fresh water is becoming increasingly serious. In this study, the effects of temperature and graphene oxide materials on the potential functions of denitrification communities in lake sediments were investigated by metagenome. The addition of graphene oxide significantly affected the abundance of denitrification genes such as Nap , Nos , and enhanced the contribution of Pseudomonas , making low temperature and material addition conducive to the denitrification process. Module network implied that low temperature increased the centrality of denitrification in community functions. At low temperatures, graphene oxide enhanced community anabolism by stimulation organic carbon consumption and regulating the gene abundance in the citric acid cycle and the semi-phosphorylation Entner-Doudoroff, thus possibly stimulating extracellular polymeric substances (EPS) synthesis and secretion. In addition, graphene oxide may also regulate the transfer of reducing electrons from NADH to denitrifying enzymes by affecting the gene abundances of complex I and complex IV. [Display omitted] • Graphene oxide led to an increase in the proportion of denitrification genes at 8 °C. • Graphene oxide enhanced community anabolism at 8 °C. • Graphene oxide stimulated EPS synthesis and secretion at 8 °C. • Graphene oxide regulated the transfer of electrons to denitrifying enzymes. [ABSTRACT FROM AUTHOR]