Nitrite Accumulation Is Required for Microbial Anaerobic Iron Oxidation, but Not for Arsenite Oxidation, in Two Heterotrophic Denitrifiers

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 ST2^ΔnirK 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.