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Although nitrogen (N) transformations have been widely studied under oxic or anoxic condition, few studies have been carried out to analyze the transformation accompanied with NO 2 −-N accumulation. Particularly, the control of mixed N species in N-transformation remains unclear in an oxic-anoxic transition zone (OATZ), a unique and ubiquitous redox environment. To bridge the gap, in this study, OATZ microcosms were simulated by surface water and sediments of a shallow lake. The N-transformation processes and rates at different NH 4 +-N/NO 3 −-N ratios, and NO 2 −-N accumulations in these processes were evaluated. N-transformation process exhibited a turning point. Simultaneous nitrification and denitrification occurred in its early stage (first 10 days, dissolved oxygen (DO) ≥ 2 mg/L) and then denitrification dominated (after 10 days, DO < 2 mg/L), which were not greatly affected by the NH 4 +-N/NO 3 −-N ratio, on the contrary, the transformation rates of NH 4 +-N and NO 3 −-N were distinctly affected. The NH 4 +-N transformation rates were positively correlated with the NH 4 +-N/NO 3 −-N ratio. The highest NO 3 −-N transformation rate was observed at an NH 4 +-N/NO 3 −-N ratio of 1:1 with organic carbon/NO 3 −-N of 3.09. The NO 2 −-N accumulation, which increased with the decrease in NH 4 +-N/NO 3 −-N ratio, was also controlled by organic carbon concentration and type. The peak concentration of NO 2 −-N accumulation occurred only when the NO 3 −-N transformation rate was particularly low. Thus, NO 2 −-N accumulation may be reduced by adjusting the control parameters related to N and organic carbon sources, which enhances the theoretical insights for N-polluted aquatic ecosystem bioremediation. • Nitrogen transformation in the oxic-anoxic transition zone was investigated. • Nitrification-denitrification occurred in early days, then denitrification dominated. • NH 4 +-N/NO 3 −-N ratio controlled their transformation rates and NO 2 −-N accumulation. • NO 2 −-N accumulation peaked when NO 3 −-N transformation rate was rather low. [ABSTRACT FROM AUTHOR]

Antibiotic resistance genes (ARGs) are emerging microbial pollutants that are regulated by many factors and pose potential threats to aquatic environments. In this study, we used network analysis, correlation analysis, and constructed models based on metagenomic sequencing results to explore the spatial patterns, impact mechanisms, transmission risks and differences in ARGs in the water and sediment of the Weihe River Basin. The findings revealed notable disparities in ARGs, mobile genetic elements (MGEs), and bacterial communities. In the sediment, the abundance of ARGs was considerably greater than that in water. Moreover, the percentage of ARGs shared by the two components reached a value of 85.8%. Through network analysis, it was determined that the presence of 16 MGEs and 20 bacterial phyla was strongly associated with ARGs (R2 > 0.7, P < 0.05). The Mantel test showed that abiotic factors including DO, pH, nutrients, and heavy metals played important roles in the distribution of ARGs (P < 0.05). A structural equation model revealed that the key factors influencing the distribution of ARGs in water were bacterial diversity and environmental parameters (standardized effects of −0.730 and −0.667), and those in sediment were bacterial diversity and MGEs (standardized effects of −0.751 and 0.851). Neutral modeling indicated that deterministic processes played an important role in the assembly of ARGs in the water of the Weihe River Basin, and stochastic processes were dominant in the sediment. There was a highly significant positive linear correlation between ARGs and pathogens, and there was more complex co-occurrence in the water than in the sediment (R2 > 0.9, P < 0.05), with stronger migration and transmission occurring. Exploring ARGs in large-scale watersheds is immensely important for elucidating their traits and transmission mechanisms and consequently paving the way for the formulation of efficient strategies to mitigate resistance threats. • ARGs exhibit different distribution patterns in water and sediment. • Deterministic processes are important in the assembly of ARGs in water, and stochastic processes are dominant in sediment. • ARGs are affected by bacterial diversity and environmental factors in water, and bacterial diversity and MGEs in sediment. • DO, pH, nutrients, and heavy metals play important roles in the distribution of ARGs. • ARGs are more intensely migrating and spreading to pathogens in water than sediment. [ABSTRACT FROM AUTHOR]

The present study investigated the nitrogen removal characteristics and metabolic pathway of bacteria in aquatic ecosystem, with a focus on heterotrophic nitrification and aerobic denitrification. The bacteria demonstrated significant heterotrophic nitrification and aerobic denitrification capacity. The highest ammonium-N, nitrate-N, and nitrite-N removal efficiencies were 95.31 ± 0.11%, 98.91 ± 0.05%, and 98.79 ± 0.09%, respectively. The Monod model was used to estimate the maximum rate of substrate utilization (R mo) and the half-saturation concentration (K s) for the two substrates, i.e., ammonium and nitrate. The kinetic coefficients were 3.34 mg/L/d (R mo) and 30.59 mg/L (K s) for ammonium-N, respectively, and 14.23 mg/L/d (R mo) and 215.24 mg/L (K s) for nitrate-N, respectively. The effects of initial nitrogen (ammonium-N or nitrate-N) concentration, temperature, and dissolved oxygen (DO) on nitrogen removal rate were investigated using response surface methodology (RSM), and the optimal conditions for nitrogen removal were determined. The principal nitrogen removal pathway of the bacteria was proposed as complete heterotrophic nitrification and aerobic denitrification, which was performed by six key genera: Arthrobacter , Pseudomonas , Rhodococcus , Bacillus , Massilia , and Rhizobium. Chryseobacterium and other denitrifying species may also reduce nitrification products (NO X −) via aerobic denitrification. • Heterotrophic nitrification and aerobic denitrification were established. • Monod kinetic coefficients indicated great application potential of the well-cultured sediment. • RSM was a reliable method for determining the optimum point for nitrogen removal. • Complete heterotrophic nitrification and aerobic denitrification was the principle nitrogen removal pathway. [ABSTRACT FROM AUTHOR]