Direct ammonium oxidation to nitrogen gas (Dirammox) in Alcaligenes strain HO-1: The electrode role

Advancement in Anaerobic Ammonia Oxidation Technologies for Industrial Wastewater Treatment and Resource Recovery: A Comprehensive Review and Perspectives

Anaerobic ammonium oxidation (anammox) technologies have attracted substantial interest due to their advantages over traditional biological nitrogen removal processes, including high efficiency and low energy demand. Currently, multiple side-stream applications of the anammox coupling process have been developed, including one-stage, two-stage, and three-stage systems such as completely autotrophic nitrogen removal over nitrite, denitrifying ammonium oxidation, simultaneous nitrogen and phosphorus removal, partial denitrification-anammox, and partial nitrification and integrated fermentation denitritation. The one-stage system includes completely autotrophic nitrogen removal over nitrite, oxygen-limited autotrophic nitrification/denitrification, aerobic de-ammonification, single-stage nitrogen removal using anammox, and partial nitritation. Two-stage systems, such as the single reactor system for high-activity ammonium removal over nitrite, integrated fixed-film activated sludge, and simultaneous nitrogen and phosphorus removal, have also been developed. Three-stage systems comprise partial nitrification anammox, partial denitrification anammox, simultaneous ammonium oxidation denitrification, and partial nitrification and integrated fermentation denitritation. The performance of these systems is highly dependent on interactions between functional microbial communities, physiochemical parameters, and environmental factors. Mainstream applications are not well developed and require further research and development. Mainstream applications demand a high carbon/nitrogen ratio to maintain levels of nitrite-oxidizing bacteria, high concentrations of ammonium and nitrite in wastewater, and retention of anammox bacteria biomass. To summarize various aspects of the anammox processes, this review provides information regarding the microbial diversity of different genera of anammox bacteria and the engineering aspects of various side streams and mainstream anammox processes for wastewater treatment. Additionally, this review offers detailed insights into the challenges related to anammox technology and delivers solutions for future sustainable research.

New Insight Into the Mechanism of Nitrite Enhancement on Heterotrophic Nitrification and Aerobic Denitrification Bacterium in Gene Expression

The growth and nitrogen metabolism of heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria are affected by nitrite, but the mechanisms underlying this for strain Acinetobacter johnsonii EN-J1 are unclear. In this study, the addition of 10 mg/L nitrite increased the reduction rate of ammonium by 1.0 mg/L/h, and 20 mg/L nitrite increased the reduction rate of nitrate by 3.9 mg/L/h. Compared with the control, the nitrate reductase activity, electron transfer activity, and adenosine triphosphate content of EN-J1 were enhanced by 142.0%, 278.0% and 279.0%, respectively, in the nitrate removal process after the addition of 20 mg/L nitrite. The whole genome was annotated with nitrogen removal genes such as narGHI, narK, nsrR, nirBD, nasA, glnA, gltB, gdhA and amt. Transcriptome analysis showed that nitrite triggered significant upregulation of several key pathways, including nitrogen metabolism, the tricarboxylic acid cycle, and amino acid metabolism for enhancing denitrification. The expression of key denitrification genes (narG, narK, hmp, nirBD, glnA and nasA) was detected by real-time quantitative polymerase chain reaction. These results suggested that nitrite enhances denitrification by increasing the expression of denitrification genes, electron transfer and adenosine triphosphate levels, which is important for elucidating the mechanism of nitrite promotion of biological nitrogen removal efficiency.

A powerful but frequently overlooked role of thermodynamics in environmental microbiology: inspirations from anammox

Thermodynamics has long been applied in predicting undiscovered microorganisms or analyzing energy flows in microbial metabolism, as well as evaluating microbial impacts on global element distributions. However, further development and refinement in this interdisciplinary field are still needed. This work endeavors to develop a whole-cycle framework integrating thermodynamics with microbiological studies, focusing on representative nitrogen-transforming microorganisms. Three crucial concepts (reaction favorability, energy balance, and reaction directionality) are discussed in relation to nitrogen-transforming reactions. Specifically, reaction favorability, which sheds lights on understanding the diversity of nitrogen-transforming microorganisms, has also provided guidance for novel bioprocess development. Energy balance, enabling the quantitative comparison of microbial energy efficiency, unravels the competitiveness of nitrogen-transforming microorganisms under substrate-limiting conditions. Reaction directionality, revealing the niche-differentiating patterns of nitrogen-transforming microorganisms, provides a foundation for predicting biogeochemical reactions under various environmental conditions. This review highlights the need for a more comprehensive integration of thermodynamics in environmental microbiology, aiming to comprehensively understand microbial impacts on the global environment from micro to macro scales.

Dirammox (direct ammonia oxidation) to nitrogen (N2): discovery, current status, and perspectives

Microbial ammonia oxidation plays an important role in nitrogen (N2) cycling in natural and man-made systems. Heterotrophic microorganisms that oxidize ammonia were observed more than a century ago; however, the underlying molecular mechanism of ammonia oxidation is still mysterious. Dirammox (direct ammonia oxidation to N2) is a newly described heterotrophic ammonia oxidation process in which ammonia or its organic amine is oxidized into hydroxylamine and then directly converted to N2 gas without the involvement of nitrite and nitrate. As demonstrated with Alcaligenes species, the conversion of ammonia to hydroxylamine is mediated by the dnf genes, and hydroxylamine conversion to N2 is considered both a biotic and abiotic process. Dirammox is different from the N2-producing processes of nitrification-denitrification and anaerobic ammonia oxidation (anammox), in which nitrite or nitrate is involved. Here, we review the discovery of dirammox, progress toward understanding its genetics, biochemistry, physiology, and ecology, and future perspectives and directions.

Electro-bioremediation of wastewater: Transitioning the focus on pure cultures to elucidate the missing mechanistic insights upon electro-assisted biodegradation of exemplary pollutants

Electro-bioremediation of exemplary water pollutants such as nitrogenous, phosphorous, and sulphurous compounds, hydrocarbons, metals and azo dyes has already been studied at a macro-scale level using mixed cultures. The technology has been generally established as a proof of concept at the technology readiness level (TRL) of 3, and there are already specific cases where the technology reached TRL 5. However, this technology is less utilized compared to traditional approaches. Although, mixed cultures result in high electro-biodegradation efficiency, their use hinders process' mechanistic insights which are better determined through pure cultures studies. This knowledge can lead to improved technologies. Therefore, this manuscript focuses on the specific pollutants' electro-biodegradation by pure cultures, assessing the availability of information regarding genes, enzymes, proteins and metabolites involved. Furthermore, studies characterizing the dominant genera or species are assessed, in which the available information at molecular level is evaluated. In total, less than 40 studies were found which were predominantly focused on the electro-biodegradation potential rather than the mechanistic insights. This highlights a gap in the field featuring a motivation to transitioning the focus on the study of pure cultures to unravel the mechanistic insights. Therefore, specific actions are suggested. Characterization of the mixed cultures followed by microorganisms' isolation is crucial. Thus, electroactive and biodegradation characteristics will be revealed using omics, genome annotation and transcriptional kinetics. This can lead to optimization at the microbiological level through genetic engineering, synthetic biology, mathematical modelling and strategic building of co-cultures. This research focus offers new avenues for sustainable wastewater treatment.

A critical review of comammox and synergistic nitrogen removal coupling anammox: Mechanisms and regulatory strategies

Nitrification is highly crucial for both anammox systems and the global nitrogen cycle. The discovery of complete ammonia oxidation (comammox) challenges the inherent concept of nitrification as a two-step process. Its wide distribution, adaptability to low substrate environments, low sludge production, and low greenhouse gas emissions may make it a promising new nitrogen removal treatment process. Meanwhile, anammox technology is considered the most suitable process for future wastewater treatment. The diverse metabolic capabilities and similar ecological niches of comammox bacteria and anammox bacteria are expected to achieve synergistic nitrogen removal within a single system. However, previous studies have overlooked the existence of comammox, and it is necessary to re-evaluate the conclusions drawn. This paper outlined the ecophysiological characteristics of comammox bacteria and summarized the environmental factors affecting their growth. Furthermore, it focused on the enrichment, regulatory strategies, and nitrogen removal mechanisms of comammox and anammox, with a comparative analysis of hydroxylamine, a particular intermediate product. Overall, this is the first critical overview of the conclusions drawn from the last few years of research on comammox-anammox, highlighting possible next steps for research.