Overlooked ammonium assimilation during sulfur autotrophic dissimilatory nitrate reduction to ammonium

Dissimilatory nitrate reduction to ammonium is an important nitrogen cycling pathway for nature and wastewater treatment, but its contribution has been underestimated because the produced ammonium can be rapidly assimilated by microorganisms. Here we operated a laboratory-scale sulfide driven continuous flow reactor under different sulfur-to-nitrogen ratios, and combined isotope tracing, batch test, pure culture experiment, metagenomics, and transcriptional analyses were used to investigate nitrogen and sulfur transformation. At a sulfur-to-nitrogen molar ratio of 4, the ammonium conversion efficiency reached to 46.8%, with a potential dissimilatory nitrate reduction to ammonium rate of 30.75 ± 3.81 μmol/(L·h). Electron microscopy and elemental analysis indicated the accumulation of elemental sulfur. Metagenomic and pure-culture analyses with Thiobacillus thioparus ATCC158 suggested that ammonium produced during nitrate reduction was rapidly assimilated into biomass. These findings reveal coupling between sulfur-driven nitrate reduction and ammonium assimilation during nitrogen and sulfur transformation. Ammonium assimilation provides a reliable theoretical support for cell proliferation in sulfur-autotrophic dissimilatory nitrate reduction to ammonium in nature and wastewater treatment plants, which was overlooked earlier, based on metagenomic and incubation experiments.