Redox cycling of manganese by Bacillus horikoshii biET1 via oxygen switch

Unlocking anaerobic digestion potential via extracellular electron transfer by exogenous materials: Current status and perspectives

The efficiency of energy transfer among microorganisms presents a substantial hurdle for the widespread implementation of anaerobic digestion techniques. Nonetheless, recent studies have demonstrated that enhancing the extracellular electron transfer (EET) can markedly enhance this efficiency. This review highlights recent advancements in EET for anaerobic digestion and examines the contribution of external additives to fostering enhanced efficiency within this context. Diverse mechanisms through which additives are employed to improve EET in anaerobic environments are delineated. Furthermore, specific strategies for effectively regulating EET are proposed, aiming to augment methane production from anaerobic digestion. This review thus offers a perspective on future research directions aimed at optimizing waste resources, enhancing methane production efficiency, and improving process predictability in anaerobic digestion.

A critical review of Mnammox coupled with the NDMO for innovative nitrogen removal

In the context of increasing global nitrogen pollution, traditional biological nitrogen removal technologies like nitrification and denitrification are hindered by high energy consumption. Additionally, the deployment of anaerobic ammonium oxidation (Anammox) technology is constrained due to the slow growth rate of Anammox bacteria and there is a bottleneck in nitrogen removal efficiency. To overcome these technical bottlenecks, researchers have discovered a revolutionary nitrogen removal technology that cleverly combines the redox cycling of manganese with nitrification and denitrification reactions. In this new process, manganese dependent anaerobic ammonium oxidation (Mnammox) bacteria can convert NH4+ to N2 under anaerobic conditions, while nitrate/nitrite dependent manganese oxidation (NDMO) bacteria use NO3-/NO2- as electron acceptors to oxidize Mn2+ to Mn4+. Mn4+ acts as an electron acceptor in Mnammox reaction, thereby realizing the autotrophic nitrogen removal process. This innovative method not only simplifies the steps of biological denitrification, but also significantly reduces the consumption of oxygen and organic carbon, providing a more efficient and environmentally friendly solution to the problem of nitrogen pollution. The article initially provides a concise overview of prevalent nitrogen removal technologies and the application of manganese in these processes, and discusses the role of manganese in biogeochemical cycles, including its discovery, mechanism of action, microbial communities involved, and its impact on these key factors in the process. Subsequently, metabolic principles, benefits, advantages, and environmental considerations of Mnammox coupled with the NDMO process are analyzed in detail. Finally, this article summarizes the shortcomings of current research and looks forward to future research directions. The goal of this article is to provide a valuable reference for researchers to fully understand the application of manganese in nitrogen removal processes.

Dual role of birnessite on the modulation of acid production and reinforcement of interspecific electron transfer in anaerobic digestion

Achieving efficient anaerobic digestion of highly loaded substrates is one of the most challenging problems in the field of waste resourcing. Here, the addition of birnessite (2.0 g/L) to kitchen wastewater increased the acetate and final methane yields by 40.53 and 99.18 %, respectively, while reducing the yields of propionate and butyrate by 38.17 and 48.86 %, respectively. There were two main pathways for birnessite to enhance anaerobic digestion, one of which is to act as an electron acceptor, by inducing an alteration in the ratio of reduced-state coenzyme I in the microorganism, allowing the acid production process to proceed towards deeper oxidation. Another pathway enhances the interspecific electron transfer between bacteria and archaea and improves methane yield by optimizing the metabolic relationship. The Kyoto Encyclopedia of Genes and Genomes (KEGG) functional predictions suggest that the extracellular electron transport pathway of the microorganism is enhanced with the addition of birnessite and that its intracellular metabolic pathway is biased towards the nicotinamide adenine dinucleotide (NADH) generation pathway. This work demonstrated that anaerobic digestion facilitation by metallic minerals was not monolithic; that is, different properties of the minerals were employed to intensify the different stages of anaerobic digestion and obtain an amplification cascade.

Towards BioMnOx-mediated intra/extracellular electron shuttling for doxycycline hydrochloride metabolism in Bacillus thuringiensis

Doxycycline hydrochloride (DCH) could be continuously removed by Bacillus thuringiensis S622 with the in-situ biogenic manganese oxide (BioMnOx) via oxidizing/regenerating. The DCH removal rate was significantly increased by 3.01-fold/1.47-fold at high/low Mn loaded via the integration of biological (intracellular/extracellular electron transfer (IET/EET)) and abiotic process (BioMnOx, Mn(III) and •OH). BioMnOx accelerated IET via activating coenzyme Q to enhance electrons transfer (ET) from complex I to complex III, and as an alternative electron acceptor for respiration and provide another electron transfer transmission channel. Additionally, EET was also accelerated by stimulating to secrete flavins, cytochrome c (c-Cyt) and flavin bounded with c-Cyt (Flavins & Cyts). To our best knowledge, this is the first report about the role of BioMnOx on IET/EET during antibiotic biodegradation. These results suggested that Bacillus thuringiensis S622 incorporated with BioMnOx could adopt an alternative strategy to enhance DCH degradation, which may be of biogeochemical and technological significance.

Redox reaction between solid-phase humins and Fe(III) compounds: Toward a further understanding of the redox properties of humin and its possible environmental effects

Redox reactions between humic substances and Fe(III) compounds play a critical role in the biogeochemical cycle of pollutants. Most humic substances in soils and sediments are in a solid form (i.e. humin (HM)). In order to assess the contribution of electron shuttling via HM within the electron transfer network in natural environments and to predict environmental fate of pollutants associated with iron oxides, it is necessary to understand the electron transfer processes from HM to the environmentally relevant Fe(III) minerals, and to examine the redox reversibility of HM. The results of this study demonstrated that non-reduced HMs could only donate electrons to dissolved ferric citrate and poorly crystalline ferrihydrite, but reduced HMs could also reduce hematite and magnetite that had high crystallinity. The degree of reduction depended on the difference in redox potential and the crystallinity of the Fe(III) compounds. The electron-accepting capacities of different HMs correlated well with their organic carbon content, and quinones and Fe-bound organic component were important electron-accepting groups in HMs. Furthermore, the redox reversibility experiments demonstrated that HMs could maintain stable electron transfer capacities over three reduction-oxidation cycles, indicating that the HM could be an environmentally sustainable electron shuttle. Our results suggest that (1) HM may play an unrecognized and important role in biogeochemical cycles of pollutants in Fe(III) mineral-rich environments; (2) electron shuttling through HM to ferric citrate and ferrihydrite can occur even in the presence of O₂; and (3) HM would be a promising material for environmental remediation.

Capturing the signal of weak electricigens: a worthy endeavour

Recently several non-traditional electroactive microorganisms have been discovered. These can be considered weak electricigens; microorganisms that typically rely on soluble electron acceptors and donors in their lifecycle but are also capable of extracellular electron transfer (EET), resulting in either a low, unreliable, or otherwise unexpected current. These unanticipated electroactive microorganisms represent a new chapter in electromicrobiology and have important medical, environmental, and biotechnological relevance. As such, it is essential to continue the momentum of their discovery. However, their study poses unique challenges due to their low current output. Capturing their signal necessitates novel approaches including unconventional electrode choice, the use of sensitive electrochemical techniques, and modifications of conventional experiments that use bioelectrochemical systems (BES).