Electron transfer routes in nitrate-pentavalent vanadium co-contaminated system of oligotrophic microbiology niche

Microbial techniques have been extensively used for the remediation of nitrate and V(V) co-contaminations, but the mechanisms of electron and substances transport and metabolism of co-contaminations under oligotrophic niche have been largely overlooked. This study quantified the electron transfer and consumption, substance transfer, and metabolic pathways in the nitrate and V(V) co-contamination system under oligotrophic condition to explore the underlying mechanisms by characterizing the products and elucidating conventional cognitive pathways. This study compared the composition of the precipitates under the conditions of sufficient and insufficient carbon sources using energy-dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy, and discovered the re-oxidation process of the already reduced V(IV). Electronic evidence for the re-oxidation process of V(IV) was also provided by electron transfer and quantitative analysis. Besides, this study found that the electron contribution ratio of NO₃^--N → NO₂^--N and V(V) → V(IV) reduction was 40.2:1. In addition, based on the functional prediction of PICRUSt 2, it was found that the utilization of intracellular reserve carbon source and enzymes in the transport chain were enhanced in oligotrophic microbiology niche. These results provide new insights into the stability of co-contamination reduction in oligotrophic microbiology niche and demonstrate a new mobilization pathway for V(V) in oligotrophic systems.

A new insight to explore toxic Cd(II) affecting denitrification: Reaction kinetic, electron behavior and microbial community

Denitrification process is a crucial step in nitrogen removal and is more vulnerable to external shocks due to the fact that anoxic process is always located before aerobic process in conventional sewage treatment. This study aims to elaborate the nitrogen conversion characteristics by investigating denitrification kinetics, electron behavior and microbial community under Cd(II) shock. Reaction kinetics showed that 10 mg/L of Cd(II) accelerated nitrate reduction rate by 52.29% but 80 mg/L of Cd(II) severely decelerated it by 95.41% with the accumulation of nitrite. High concentration of COD (C/N = 10.4) in the system caused by Cd(II) disrupting the integrity of cell membrane (lactate dehydrogenase increased by 328.7%) was proved to induce occurrence of Dissimilatory Nitrate Reduction to Ammonia (DNRA). The electron transport system activity (ETSA), electron consumption and electron distribution were combined to reveal the electron behavior regulated by Cd(II). The electron ratio of nitrate reductase to nitrite reductase increased from 1.48 (control) to 3.91 and 3.52 (40 and 80 mg/L of Cd(II)) indicated the electrons allocating tendency and further explained the nitrite accumulation. High concentration of Cd(II) also decreased ETSA by weakening the physiological activities of flavin adenine dinucleotide, flavin mononucleotide and cytochrome c or hindered the microbes to secrete these electron carriers. Furthermore, Cd(II) inhibited dominant bacteria genera containing napA gene (Azospirillum and Thauera) and nirS gene (unclassified_c_Betaproteobacteria). Enterobacteriaceae family was found to dominate the DNRA process.

Research on efficient denitrification system based on banana peel waste in sequencing batch reactors: Performance, microbial behavior and dissolved organic matter evolution

Nitrate pollution presents a serious threat to the environment and public health. As an excellent heterotrophic denitrification carbon source, banana peel (a kind of agricultural waste) provides a feasible alternative to deal with the persistent high concentrations of nitrate pollution. Although the feasibility and economy of banana peel for denitrification have already been reported, the long-term stability and mechanism were still unclear. The coupling mechanism of organic matters and microorganism in the denitrification process was systematically investigated through a 17-cycle experiment. The results showed that significant NO₃^--N removal load and rate of 164.42 mg/g and 4.69 mg/(L·h) after long-term tests could be obtained. Organic matter analysis and 16S rRNA sequencing showed that the evolution of organic matter was dominated by Anaerolineaceae (fermenting bacteria), and, in the final step, the humification of organic matter was realized. Moreover, the presence of Lentimicrobium (denitrifying bacteria) was indispensable for the continuous removal of high concentrations of nitrate. The main functional gene of nitrogen transformation in this reaction system was NirS (haem-containing). This lab-scale heterotrophic denitrification process could contribute to a better understanding of the carbon and nitrogen cycles in the biogeochemical cycles to some extent, and it also provides a reference for the construction of highly efficient nitrate degradation reactors, based on agricultural wastes.

Survey of dissimilatory nitrate reduction to ammonium microbial community at national wetland of Shanghai, China

Dissimilatory nitrate reduction to ammonia (DNRA) process is an important nitrate reduction pathway in the environment. Numerous studies focused on the DNRA, especially in various natural habitats. However, little is known about the envrionmental parameters driving the DNRA process in anthropogenic ecosystem. Human activities put forward significant influence on nitrogen cycle and bacterial communities of sediment. This study aimed to assess the DNRA potential rates, nrfA gene abundance, DNRA bacterial community's diversity and influencing factors in a national wetland park near the Yangtze River estuary, Shanghai. The results of ¹⁵N isotope tracer experiments showed that DNRA potential rates from 0.13 to 0.44 μmol N/kg/h and contribution of nitrate reduction varied from 1.56% to 7.47%. The quantitative real-time PCR results showed that DNRA functional gene nrfA abundances ranged from 9.87E+10 to 1.98E+11 copies/g dry weight. The results of nrfA gene pyrosequencing analysis showed that Lacunisphaera (10.4-13.4%), Sorangium (7.1-10.7%), Aeromonas (4.2-6.8%), Corallococcus (1.8-6.9%), and Geobacter (3.3-6.6%) showed higher relative abundances in their genus levels. Combined with environmental parameters of sediments, redundancy analysis indicated that the nrfA functional gene was positively correlated with moisture content, the concentration of NO₂^--N and NO₃^-N; the DNRA rates was positively correlated with sediment organic carbon (SOC), C/NO₃^- ratio and salinity (ranked by explains %). This study is the first simultaneous determination of nitrate reduction pathways including denitrification, anammox and DNRA rates to assess the role of DNRA in a national wetland park and revealed the community abundance, diversity of DNRA bacteria and its relationship with environmental factors.