Nitrification, a process carried out by aerobic microorganisms that oxidizes ammonia to nitrate via nitrite, is an indispensable step in wastewater nitrogen removal. To facilitate energy and carbon savings, applying low dissolved oxygen (DO) is suggested to shortcut the conventional biological nitrogen removal pathway, however, the impact of low DO on nitrifying communities within activated sludge is not fully understood. This study used genome-resolved metagenomics to compare nitrifying communities under extremely low- and high-DO. Two bioreactors were parallelly operated to perform nitrification and DO was respectively provided by limited gas-liquid mass transfer from the atmosphere (AN reactor, DO < 0.1 mg/L) and by sufficient aeration (AE reactor, DO > 5.0 mg/L). Low DO was thought to limit nitrifiers growth; however, we demonstrated that complete nitrification could still be achieved under the extremely low-DO conditions, but with no nitrite accumulation observed. Kinetic analysis showed that after long-term exposure to low DO, nitrifiers had a higher oxygen affinity constant and could maintain a relatively high nitrification rate, particularly at low levels of DO (<0.2 mg/L). Community-level gene analysis indicated that low DO promoted enrichment of nitrifiers (the genera Nitrosomonas and Nitrospira, increased by 2.3- to 4.3-fold), and also harbored with 2.3 to 5.3 times higher of nitrification functional genes. Moreover, 46 high-quality (>90 % completeness and <5 % contamination) with 3 most abundant medium-quality metagenome-assembled genomes (MAGs) were retrieved using binning methods. Genome-level phylogenetic analysis revealed the species succession within nitrifying populations. Surprisingly, compared to DO-rich conditions, low-DO conditions were found to efficiently suppressed the ordinary heterotrophic microorganisms (e.g., the families Anaerolineales, Phycisphaerales, and Chitinophagales), but selected for the specific candidate denitrifiers (within phylum Bacteroidota). This study provides new microbial insights to demonstrate that low-DO favors the enrichment of autotrophic nitrifiers over heterotrophs with species-level successions, which would facilitate the optimization of energy and carbon management in wastewater treatment.

中文翻译：

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基因组分辨的宏基因组学揭示了极低溶解氧下活性污泥中的硝化剂富集和物种演替


硝化作用是好氧微生物通过亚硝酸盐将氨氧化成硝酸盐的过程，是废水脱氮不可或缺的步骤。为了促进能源和碳的节约，建议使用低溶解氧 （DO） 来简化传统的生物脱氮途径，但是，低溶解氧对活性污泥内硝化群落的影响尚不完全清楚。本研究使用基因组分辨宏基因组学来比较极低和高 DO 下的硝化群落。两个生物反应器并联运行进行硝化反应，DO 分别由大气中的有限气液传质（AN 反应器，DO < 0.1 mg/L）和充分曝气（AE 反应器，DO > 5.0 mg/L）提供。低 DO 被认为会限制硝化剂的生长;然而，我们证明在极低的 DO 条件下仍然可以实现完全硝化，但没有观察到亚硝酸盐积累。动力学分析表明，长期暴露于低 DO 后，硝化剂具有较高的氧亲和常数，并且可以保持相对较高的硝化速率，尤其是在低水平 DO （<0.2 mg/L） 下。群落水平基因分析表明，低 DO 促进了硝化物（亚硝化单胞菌属和亚硝化螺旋菌属，增加了 2.3 至 4.3 倍）的富集，并且硝化功能基因也高出 2.3 至 5.3 倍。此外，使用分箱方法检索到 46 个高质量 （>90% 完整性和 <5 % 污染） 和 3 个最丰富的中等质量宏基因组组装基因组 （MAG）。基因组水平的系统发育分析揭示了硝化种群内的物种演替。 令人惊讶的是，与富含 DO 的条件相比，发现低 DO 条件可以有效抑制普通的异养微生物（例如，拟氧菌科、Phycisphaerales 和 Chitinophagales 科），但被选择用于特定的候选反氮化剂（在拟杆菌门内）。这项研究提供了新的微生物见解，以证明低 DO 有利于自养硝化剂的富集，而不是具有物种水平演替的异养生物，这将有助于优化废水处理中的能源和碳管理。