In situ role of extracellular polymeric substances in microbial electron transfer by Methylomonas sp. LW13

Pressure-centric regulation for efficient anaerobic digestion: State-of-the-art, challenges and prospects

Anaerobic digestion (AD) is an environmentally friendly technology that simultaneously stabilizes biowaste and produces biogas. Conventional AD faces challenges such as inadequate substrate degradation and low methane purity. Pressure-centric regulation serves as an AD optimization strategy that can enhance the digestion efficiency and generate higher-energy-value biogas. However, limited reviews have been undertaken to focus on this technology. This review is designed to discuss innovations in ex-situ high-pressure pretreatment and in-situ high-pressure anaerobic digestion (HPAD) processes. Moreover, comprehensive understandings on the intrinsic mechanisms of HPAD are critically examined, including physicochemical reaction principles and microbial responses. The constraints currently curtailing these technologies and potential mitigation strategies are also scrutinized. Additionally, current knowledge gaps and future research directions on mechanisms, model fitting, and engineering practices are presented. Overall, this work highlights the feasibility of pressure-centric regulated AD and provides novel insights to overcome existing technical barriers in its application.

Regulatory roles of extracellular polymeric substances in uranium reduction via extracellular electron transfer by Desulfovibrio vulgaris UR1

The pathway of reducing U(VI) to insoluble U(IV) using electroactive bacteria has become an effective and promising approach to address uranium-contaminated water caused by human activities. However, knowledge regarding the roles of extracellular polymeric substances (EPS) in the uranium reduction process involving in extracellular electron transfer (EET) mechanisms is limited. Here, this study isolated a novel U(VI)-reducing strain, Desulfovibrio vulgaris UR1, with a high uranium removal capacity of 2.75 mM/(g dry cell). Based on a reliable EPS extraction method (45 °C heating), manipulation of EPS in D. vulgaris UR1 suspensions (removal or addition of EPS) highlighted its critical role in facilitating uranium reduction efficiency. On the second day, U(VI) removal rates varied significantly across systems with different EPS contents: 60.8% in the EPS-added system, 48.5% in the pristine system, and 22.2% in the EPS-removed system. Characterization of biogenic solids confirmed the reduction of U(VI) by D. vulgaris UR1, and the main products were uraninite and UO2 (2.88-4.32 nm in diameter). As EPS formed a permeable barrier, these nanoparticles were primarily immobilized within the EPS in EPS-retained/EPS-added cells, and within the periplasm in EPS-removed cells. Multiple electroactive substances, such as tyrosine/tryptophan aromatic compounds, flavins, and quinone-like substances, were identified in EPS, which might be the reason for enhancement of uranium reduction via providing more electron shuttles. Furthermore, proteomics revealed that a large number of proteins in EPS were enriched in the subcategories of catalytic activity and electron transfer activity. Among these, iron-sulfur proteins, such as hydroxylamine reductase (P31101), pyruvate: ferredoxin oxidoreductase (A0A0H3A501), and sulfite reductase (P45574), played the most critical role in regulating EET in D. vulgaris UR1. This work highlighted the importance of EPS in the uranium reduction by D. vulgaris UR1, indicating that EPS functioned as both a reducing agent and a permeation barrier for access to heavy metal uranium.

The role of extracellular polymeric substances (EPS) in chemical-degradation of persistent organic pollutants in soil: A review

Persistent organic pollutants (POPs) in soil show high environmental risk due to their high toxicity and low biodegradability. Studies have demonstrated the degradation function of microbial extracellular polymeric substances (EPS) on POPs in various matrices. However, the degradation mechanisms and the factors that influence the process in soil have not been clearly illustrated. In this review, the characteristics of EPS were introduced and the possible mechanisms of EPS on degradation of organic pollutants (e.g., external electron transfer, photodegradation, and enzyme catalysis) were comprehensively discussed. In addition, the environmental conditions (e.g., UV, nutrients, and redox potential) that could influence the production and degradation-related active components of EPS were addressed. Moreover, the current approaches on the application of EPS in biotechnology were summarized. Further, the future perspectives of enhancement on degradation of POPs by regulating EPS were discussed. Overall, this review could provide a new thought on remediation of POPs by widely-existing EPS in soil with low-cost and minimized eco-disturbance.

Linking methanotroph phenotypes to genotypes using a simple spatially resolved model ecosystem

Connecting genes to phenotypic traits in bacteria is often challenging because of a lack of environmental context in laboratory settings. Laboratory-based model ecosystems offer a means to better account for environmental conditions compared with standard planktonic cultures and can help link genotypes and phenotypes. Here, we present a simple, cost-effective, laboratory-based model ecosystem to study aerobic methane-oxidizing bacteria (methanotrophs) within the methane-oxygen counter gradient typically found in the natural environment of these organisms. Culturing the methanotroph Methylomonas sp. strain LW13 in this system resulted in the formation of a distinct horizontal band at the intersection of the counter gradient, which we discovered was not due to increased numbers of bacteria at this location but instead to an increased amount of polysaccharides. We also discovered that different methanotrophic taxa form polysaccharide bands with distinct locations and morphologies when grown in the methane-oxygen counter gradient. By comparing transcriptomic data from LW13 growing within and surrounding this band, we identified genes upregulated within the band and validated their involvement in growth and band formation within the model ecosystem using knockout strains. Notably, deletion of these genes did not negatively affect growth using standard planktonic culturing methods. This work highlights the use of a laboratory-based model ecosystem that more closely mimics the natural environment to uncover bacterial phenotypes missing from standard laboratory conditions, and to link these phenotypes with their genetic determinants.

Bubbleless aerated-biological activated carbon as a superior process for drinking water treatment in rural areas

Drinking water supply in rural areas remains a substantial challenge due to complex natural, technical and economic conditions. To provide safe and affordable drinking water to all, as targeted in the UN Sustainable Development Goals (2030 Agenda), low-cost, efficient water treatment processes suitable for rural areas need to be developed. In this study, a bubbleless aeration BAC (termed ABAC) process is proposed and evaluated, involving the incorporation of a hollow fiber membrane (HFM) assembly within a slow-rate BAC filter, to provide dissolved oxygen (DO) throughout the BAC filter and an increased DOM removal efficiency. The results showed that after a 210-day period of operation, the ABAC increased the DOC removal by 54%, and decreased the disinfection byproduct formation potential (DBPFP) by 41%, compared to a comparable BAC filter without aeration (termed NBAC). The elevated DO (> 4 mg/L) not only reduced secreted extracellular polymer, but also modified the microbial community with a stronger degradation ability. The HFM-based aeration showed comparable performance to 3 mg/L pre-ozonation, and the DOC removal efficiency was four times greater than that of a conventional coagulation process. The proposed ABAC treatment, with its various advantages (e.g., high stability, avoidance of chemicals, ease of operation and maintenance), is well-suited to be integrated as a prefabricated device, for decentralized drinking water systems in rural areas.

Bubbleless Air Shapes Biofilms and Facilitates Natural Organic Matter Transformation in Biological Activated Carbon

The biodegradation in the middle and downstream of slow-rate biological activated carbon (BAC) is limited by insufficient dissolved oxygen (DO) concentrations. In this study, a bubbleless aerated BAC (termed ABAC) process was developed by installing a hollow fiber membrane (HFM) module within a BAC filter to continuously provide aeration throughout the BAC system. The BAC filter without an HFM was termed NBAC. The laboratory-scale ABAC and NBAC systems operated continuously for 426 days using secondary sewage effluent as an influent. The DO concentrations for NBAC and ABAC were 0.78 ± 0.27 and 4.31 ± 0.44 mg/L, respectively, with the latter providing the ABAC with greater electron acceptors for biodegradation and a microbial community with better biodegradation and metabolism capacity. The biofilms in ABAC secreted 47.3% less EPS and exhibited greater electron transfer capacity than those in NBAC, resulting in enhanced contaminant degradation efficiency and long-term stability. The extra organic matter removed by ABAC included refractory substances with a low elemental ratio of oxygen to carbon (O/C) and a high elemental ratio of hydrogen to carbon (H/C). The proposed ABAC filter provides a valuable, practical example of how to modify the BAC technology to shape the microbial community, and its activity, by optimizing the ambient atmosphere.