Dissimilatory nitrate/nitrite reduction to ammonium (DNRA) has recently regained attention as a nitrogen retention pathway that may potentially be harnessed to alleviate nitrogen loss resulting from denitrification. Until recently, the ecophysiology of DNRA bacteria inhabiting agricultural soils has remained largely unexplored, due to the difficulty in targeted enrichment and isolation of DNRA microorganisms. In this study, >100 DNRA bacteria were isolated from NO₃⁻-reducing anoxic enrichment cultures established with rice paddy soils using a newly developed colorimetric screening method. Six of these isolates, each assigned to a different genus, were characterized to improve the understanding of DNRA physiology. All the isolates carried nrfA and/or nirB, and the Bacillus sp. strain possessed a clade II nosZ gene conferring the capacity for N₂O reduction. A common prominent physiological feature observed in the isolates was NO₂⁻ accumulation before NH₄⁺ production, which was further examined with Citrobacter sp. strain DNRA3 (possessing nrfA and nirB) and Enterobacter sp. strain DNRA5 (possessing only nirB). Both isolates showed inhibition of NO₂⁻-to-NH₄⁺ reduction at submillimolar NO₃⁻ concentrations and downregulation of nrfA or nirB transcription when NO₃⁻ was being reduced to NO₂⁻. In batch and chemostat experiments, both isolates produced NH₄⁺ from NO₃⁻ reduction when incubated with excess organic electron donors, while incubation with excess NO₃⁻ resulted in NO₂⁻ buildup but no substantial NH₄⁺ production, presumably due to inhibitory NO₃⁻ concentrations. This previously overlooked link between NO₃⁻ repression of NO₂⁻-to-NH₄⁺ reduction and the C-to-N ratio regulation of DNRA activity may be a key mechanism underpinning denitrification-versus-DNRA competition in soil.

中文翻译：

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通过比色筛选方法分离的土壤DNRA细菌的生理特性暗示了NO3-参与硝化硝酸盐/亚硝酸盐还原为铵盐（DNRA）的生态生理调节。

硝酸盐/亚硝酸盐异化还原为铵盐（DNRA）作为一种氮保留途径而重新受到关注，该途径可潜在地用于缓解因反硝化作用而导致的氮损失。直到最近，由于难以有针对性地富集和分离DNRA微生物，居住在农业土壤中的DNRA细菌的生态生理学仍未得到充分研究。在这项研究中，> 100个DNRA细菌从没有孤立₃ ^- -还原使用了新开发的比色筛选法稻田土壤缺氧建立富集培养物。这些分离物中的六个，每个都分配给不同的属，其特征在于提高对DNRA生理的理解。所有分离株均携带nrfA和/或nirB和芽孢杆菌。该菌株具有II型进化枝nosZ基因，赋予N ₂ O还原能力。在菌株观察到的常见突出生理特征是NO ₂ ^-累积NH之前₄ ⁺生产，这进一步用检查柠檬酸杆菌属。菌株DNRA3（拥有nrfA和nirB）和肠杆菌。菌株DNRA5（仅具有nirB）。两个分离株表现出NO的抑制₂ ^- -to-NH ₄ ⁺还原在亚毫摩尔NO ₃ ^-浓度和下调nrfA或nirB转录当NO ₃ ^-被还原成NO ₂ ^- 。在分批和恒化实验中，两个分离物产生的NH ₄ ⁺从NO ₃ ^-还原时用过量的有机给电子体一起温育，同时温育与过量的NO ₃ ^-导致NO ₂ ^-积聚，但没有实质性的NH ₄ ⁺的生产，这可能是由于抑制NO ₃ ^-的浓度。以前被忽略的NO ₃ ^- NO抑制之间的联系₂ ^-- NH ₄ ⁺还原和DNRA活性的C-N比调节可能是土壤中反硝化与DNRA竞争的基础。