Search

Phylogenetically diverse species of bacteria can mediate anaerobic oxidation of ferrous iron [Fe(II)] and/or arsenite [As(III)] coupled to nitrate reduction, impacting the biogeochemical cycles of Fe and As. However, the mechanisms for nitrate-dependent anaerobic oxidation of Fe(II) and As(III) remain unclear. In this study, we isolated two bacterial strains from arsenic-contaminated paddy soils, Ensifer sp. ST2 and Paracoccus sp. QY30. Both strains were capable of anaerobic As(III) oxidation, but only QY30 could oxidize Fe(II) under nitrate-reducing conditions. Both strains contain the As(III) oxidase gene aioA, whose expression was induced greatly by As(III) exposure. Both strains contain the whole suite of genes for complete denitrification, but the nitrite reductase gene nirK was not expressed in QY30 and nitrite accumulated under nitrate-reducing conditions. When the functional nirK gene was knocked out in strain ST2, its nitrite reduction ability was completely abolished and nitrite accumulated in the medium. Moreover, the ST2ΔnirK mutant gained the ability to oxidize Fe(II). When nirK gene from ST2 was introduced to QY30, the recombinant strain QY30::nirK gained the ability to reduce nitrite but lost the ability to oxidize Fe(II). These genetic manipulations did not affect the ability of both strains to oxidize As(III). Our results indicate that nitrite accumulation is required for anaerobic oxidation of Fe(II) but not for As(III) oxidation in these strains. The results suggest that anaerobic Fe(II) oxidation in the two bacterial strains is primarily driven by abiotic reaction of Fe(II) with nitrite, while reduction of nitrate to nitrite is sufficient for redox coupling with anaerobic As(III) oxidation catalyzed by Aio. Deletion of nirK gene in denitrifiers can enhance anaerobic Fe(II) oxidation.

An aerobic oxidative cyanation for the synthesis of α-aminonitriles was reported. The formation of C(sp3)-CN bonds was achieved under a metal-free condition by utilizing azobis(isobutyronitrile) as a sole organic cyanide source with the combination of pivalic acid and sodium acetate as additives.

Arsenic (As) bioavailability to rice plants is elevated in flooded paddy soils due to reductive mobilization of arsenite [As(III)]. However, some microorganisms are able to mediate anaerobic As(III) oxidation by coupling to nitrate reduction, thus attenuating As mobility. In this study, we investigated the impact of nitrate additions on As species dynamics in the porewater of four As-contaminated paddy soils. The effects of nitrate on microbial community structure and the abundance and diversity of the As(III) oxidase (aioA) genes were quantified using 16S rRNA sequencing, quantitative PCR, and aioA gene clone libraries. Nitrate additions greatly stimulated anaerobic oxidation of As(III) to As(V) and decreased total soluble As in the porewater in flooded paddy soils. Nitrate additions significantly enhanced the abundance of aioA genes and changed the microbial community structure by increasing the relative abundance of the operational taxonomic units (OTUs) from the genera Acidovorax and Azoarcus. The aioA gene sequences from the Acidovorax related OTU were also stimulated by nitrate. A bacterial strain (ST3) belonging to Acidovorax was isolated from nitrate-amended paddy soil. The strain was able to oxidize As(III) and Fe(II) under anoxic conditions using nitrate as the electron acceptor. Abiotic experiments showed that Fe(II), but not As(III), could be oxidized by nitrite. These results show that nitrate additions can stimulate As(III) oxidation in flooded paddy soils by enhancing the population of anaerobic As(III) oxidizers, offering a potential strategy to decrease As mobility in As-contaminated paddy soils.