Microbial Interactions in Electroactive Biofilms for Environmental Engineering Applications: A Role for Nonexoelectrogens

Electrically Driven Biocatalysis for Sustainable CO2-to-Chemicals Transformation

The catalytic transformation of CO2 into value-added chemicals has become a critical strategy for mitigating environmental issues and generating economic benefits. Although substantial progress has been made in renewable electricity-driven CO2 conversion into C1/C2 products, the efficient synthesis of high-value, and long-chain compounds remains a significant challenge. Biosynthesis offers a feasible route for producing long-chain value-added products at mild conditions. Consequently, the integration of electrocatalysis with bioconversion has emerged as a promising approach for sustainable CO2 conversion. This short review outlines recent advances in the sustainable synthesis of long-chain compounds from CO2 via electrically driven biocatalysis, highlighting innovative coupling strategies that combine electrochemical and biochemical processes. Furthermore, the remaining challenges and prospects are tentatively discussed for further advancing CO2-based sustainable synthesis.

Revealing the underestimated role of Gram-positive bacteria in iron reduction within paddy soils

Iron reduction in paddy fields is crucial for heavy metal migration and nutrient transformation. Gram-negative iron-reducing bacteria like Geobacter and Shewanella have been widely studied due to their strong extracellular electron transfer capabilities. In contrast, Gram-positive bacteria such as Bacillus and Clostridium are often labeled as weak exoelectrogens, leading to their contributions being overlooked. But could their role in iron reduction have been underestimated? To explore this, we enriched paddy soils with glucose, resulting in a predominance of Gram-positive bacteria (e.g., Clostridiaceae and Bacillaceae) and a small presence of Gram-negative reducers (e.g., Geobacteraceae). Remarkably, despite the abundance of Gram-positive iron reducers being 880 times higher in the glucose system, their iron reduction rate was comparable to that of acetate and lactate systems with more Gram-negative bacteria. This suggests a significant contribution by Gram-positive bacteria to iron reduction. Pure culture experiments further revealed that the iron reduction rates of Bacillus and Clostridium are 15.4 and 4.0 times slower, respectively, than a Gram-negative reducer, Geobacter. However, even in natural paddy soil of this study, Gram-positive iron reducers are 9 times more abundant than Gram-negative ones. This gives us an unexpected insight: Considering both abundance and rate, Gram-positive weak exoelectrogens may actually contribute significantly to iron reduction. This study reveals the underestimated role of Gram-positive bacteria in paddy soils, highlighting the unique characteristics of paddy habitats and the need for further research on these weak exoelectrogens.

Evaluating advancements and opportunities in electro-assisted biodehalogenation of emerging halogenated contaminants

Electro-assisted biodehalogenation (EASB) as a biostimulation strategy can accelerate the slow attenuation of emerging halogenated contaminants (EHCs) in anaerobic aqueous environments. A timely review is urgent to evaluate the knowledge gaps and potential opportunities, further facilitating its design and application. Till now, EASB achieves promising progress in accelerating biohalogenation rates, promoting the detoxification of EHCs to cope with unfavourable environments and mitigating greenhouse gas emissions. However, EASB of EHCs still faces several knowledge gaps. Exploring crucial microbes and deciphering insights into dehalogenase characteristics and extracellular electron transfer (EET) pathways remain the prominent task for EASB of EHCs. Moreover, microbial ecological relationships and intricate environmental factors affecting performances and applications are largely underexplored. The emergence of emerging tools holds promises for sorting the intricate changes and addressing these knowledge gaps. Judicious use of emerging tools will rejuvenate EASB strategy, from EET to scale-up, to purposefully and effectively address cascading EHCs.

Electrode functional microorganisms in bioelectrochemical systems and its regulation: A review

Bioelectrochemical systems (BES) as environmental remediation biotechnologies have boomed in the last two decades. Although BESs combined technologies with electro-chemistry, -biology, and -physics, microorganisms and biofilms remain at their core. In this review, various functional microorganisms in BESs for CO2 reduction, dehalogenation, nitrate, phosphate, and sulfate reduction, metal removal, and volatile organic compound oxidation are summarized and compared in detail. Moreover, interrelationship regulation approaches for functional microorganisms and methods for electroactive biofilm development, such as targeted electrode surface modification, chemical treatment, physical revealing, biological optimization, and genetic programming are pointed out. This review provides promising guidance and suggestions for the selection of microbial inoculants and provides an analysis of the role of individual microorganisms in mixed microbial communities and its metabolisms.

Carbon chain elongation characterizations of electrode-biofilm microbes in electro-fermentation

The higher efficiency of electro-fermentation in synthesizing medium-chain fatty acids (MCFAs) compared to traditional fermentation has been acknowledged. However, the functional mechanisms of electrode-biofilm enhancing MCFAs synthesis remain research gaps. To address this, this study proposed a continuous flow electrode-biofilm reactor for chain elongation (CE). After 225 days of operation, stable electrode-biofilms formed and notably improved caproate yield by more than 38 %. The electrode-biofilm was enriched with more CE microorganisms and electroactive bacteria compared to the suspended sludge microorganisms, including Caproicibacterium, Oscillibacter and Pseudoramibacter. Besides, the upregulated CE pathways were evaluated by metagenomic analysis, and the results indicated that the pathways such as acetyl-CoA and malonyl-[acp] formation, reverse beta-oxidation, and fatty acid biosynthesis pathway were all markedly enhanced in cathodic biofilm, more than anodic biofilm and suspended microorganisms. Moreover, microbial community regulated processes like bacterial chemotaxis, flagellar assembly and quorum sensing, crucial for electrode-biofilm formation. Electron transfer, energy metabolism, and microbial interactions were found to be prominently upregulated in the cathodic biofilm, surpassing levels observed in anodic biofilm and suspended sludge microorganisms, which further enhanced CE efficiency. In addition, the statistical analyses further highlighted key microbial functions and interactions within the cathodic biofilm. Oscillospiraceae_bacterium was identified to be the most active microbe, alongside pivotal roles played by Caproiciproducens_sp._NJN-50, Clostridiales_bacterium, Prevotella_sp. and Pseudoclavibacter_caeni. Eventually, the proposed microbial collaboration mechanisms of cathodic biofilm were ascertained. Overall, this study uncovered the biological effects of the electrode-biofilm on MCFAs electrosynthesis, thereby advancing biochemicals production and filling the knowledge gaps in CE electroactive biofilm reactors.

Linking proteomic function and structure to electroactive biofilms development across electrode orientations

The functionality of electroactive biofilms (EABs) is profoundly influenced by the proteomic dynamics within microbial communities, particularly through the participation of proteins in electron transfer. This study explored the impact of electrode surface orientation, measured by varying oblique angles, on the performance of EABs in bioelectrochemical systems (BES). Utilizing quantitative proteomics, results indicated that a slightly oblique angle (45°) optimized the spatial arrangement of microbial cells, enhancing electron transport efficiency compared to other angles tested. Specifically, the 45° orientation resulted in a 2.36-fold increase in the abundance of c-type cytochromes compared to the 90°. Additionally, Geobacter, showed a relative abundance of 83.25 % at 45°, correlating with a peak current density of 1.87 ± 0.04 A/m2. These microbial and proteomic adaptations highlighted the intricate balance between microbial behavior and the physical environment, which could be tuned to optimize operations. The findings provided new insights into the design and enhancement of BES.

Non-electroactive bacteria behave variously in AnMBR biofilm control using electric field

Electroactive bacteria are often regarded as key players responding to electric fields that are used to control biofilm development during AnMBR (anaerobic membrane bioreactor) operation. Consequently, little attention has been given to non-electroactive bacteria in the same systems because of their incapability to acquire and transfer electrons directly. However, in this study, we identified some functionally important non-electroactive bacteria from biofilm established under low-voltage (0, 0.3, 0.5 and 1 V) electric fields in AnMBRs, designated as E-AnMBRs in this study. During the whole experiment, non-electroactive bacteria, mainly belonging to Proteobacteria, Bacteroidetes, and Chloroflexi, were found in all biofilm samples taken from each E-AnMBR. Under 0.3 V and 1 V conditions, non-electroactive bacteria did not seem to contribute to the development of biofilm significantly. Whereas under 0.5 V conditions, the growth of non-electroactive bacteria contributed up to 0.61 kPa/day biofilm formation. Therefore, 0.5 V was identified as a critical voltage, leading to the most severe biofilm formation. The microbial community structure in the reactor with a 0.5 V electric field was distinctly unique, caused by the increase of non-electroactive bacterial activity and the upregulation of their metabolic pathways. Notably, functional genes involved in carbon metabolism and oxidative phosphorylation pathway were upregulated. Furthermore, the 0.5 V electric field enhanced the protein/polysaccharide ratio and increased zeta potential to 31.6 mV (p < 0.01) of the biofilm samples. This was because upregulating quorum sensing genes accelerated the coordinated gene regulations and functional activities among non-electroactive bacteria.

Metabolic cross-feeding interactions modulate the dynamic community structure in microbial fuel cell under variable organic loading wastewaters

The efficiency of microbial fuel cells (MFCs) in industrial wastewater treatment is profoundly influenced by the microbial community, which can be disrupted by variable industrial operations. Although microbial guilds linked to MFC performance under specific conditions have been identified, comprehensive knowledge of the convergent community structure and pathways of adaptation is lacking. Here, we developed a microbe-microbe interaction genome-scale metabolic model (mmGEM) based on metabolic cross-feeding to study the adaptation of microbial communities in MFCs treating sulfide-containing wastewater from a canned-pineapple factory. The metabolic model encompassed three major microbial guilds: sulfate-reducing bacteria (SRB), methanogens (MET), and sulfide-oxidizing bacteria (SOB). Our findings revealed a shift from an SOB-dominant to MET-dominant community as organic loading rates (OLRs) increased, along with a decline in MFC performance. The mmGEM accurately predicted microbial relative abundance at low OLRs (L-OLRs) and adaptation to high OLRs (H-OLRs). The simulations revealed constraints on SOB growth under H-OLRs due to reduced sulfate-sulfide (S) cycling and acetate cross-feeding with SRB. More cross-fed metabolites from SRB were diverted to MET, facilitating their competitive dominance. Assessing cross-feeding dynamics under varying OLRs enabled the execution of practical scenario-based simulations to explore the potential impact of elevated acidity levels on SOB growth and MFC performance. This work highlights the role of metabolic cross-feeding in shaping microbial community structure in response to high OLRs. The insights gained will inform the development of effective strategies for implementing MFC technology in real-world industrial environments.

Intermittent polarization: A promising strategy for microbial electricity driven reduction of DOM toxicity in actual industrial wastewater

Dissolved organic matter (DOM) in actual industrial wastewater comprises various compounds that trigger toxicity in aquatic organisms; thus, advanced treatment for reducing DOM toxicity is urgently needed to ensure safe effluent discharge. Herein, we successfully reduced the toxicity of DOM in actual industrial wastewater without external chemical addition by applying intermittent polarization to electrochemical bioreactors. The bioreactor operated under intermittent polarization effectively reduced the toxicity of DOM by 76.7 %, resulting in the toxicity of effluent DOM (determined by malformation rate of zebrafish larvae) reaching less than 3.5 %. Notably, DOM compounds with high double-bond equivalence (DBE ≥ 8) were identified as the key components responsible for the toxicity of DOM through ultrahigh-resolution mass spectrometry analysis. Insight into microbe-DOM interactions revealed that intermittent polarization promoted the microbial consumption of high-DBE components of DOM by both affecting microbial composition (β = -0.5421, p < 0.01) and function (β = -0.4831, p < 0.01), thus regulating effluent DOM toxicity. The study findings demonstrate that intermittent polarization is a promising strategy for microbial electricity-driven reduction of DOM toxicity in actual industrial wastewater to meet the increasing safety requirements of receiving waters.

Regulating microbial redox reactions towards enhanced removal of refractory organic nitrogen from wastewater

Biotechnology for wastewater treatment is mainstream and effective depending upon microbial redox reactions to eliminate diverse contaminants and ensure aquatic ecological health. However, refractory organic nitrogen compounds (RONCs, e.g., nitro-, azo-, amide-, and N-heterocyclic compounds) with complex structures and high toxicity inhibit microbial metabolic activity and limit the transformation of organic nitrogen to inorganic nitrogen. This will eventually result in non-compliance with nitrogen discharge standards. Numerous efforts suggested that applying exogenous electron donors or acceptors, such as solid electrodes (electrostimulation) and limited oxygen (micro-aeration), could potentially regulate microbial redox reactions and catabolic pathways, and facilitate the biotransformation of RONCs. This review provides comprehensive insights into the microbial regulation mechanisms and applications of electrostimulation and micro-aeration strategies to accelerate the biotransformation of RONCs to organic amine (amination) and inorganic ammonia (ammonification), respectively. Furthermore, a promising approach involving in-situ hybrid anaerobic biological units, coupled with electrostimulation and micro-aeration, is proposed towards engineering applications. Finally, employing cutting-edge methods including multi-omics analysis, data science driven machine learning, technology-economic analysis, and life-cycle assessment would contribute to optimizing the process design and engineering implementation. This review offers a fundamental understanding and inspiration for novel research in the enhanced biotechnology towards RONCs elimination.

Insights into the response of electroactive biofilm with petroleum hydrocarbons degradation ability to quorum sensing signals

Bioelectrochemical technologies based on electroactive biofilms (EAB) are promising for petroleum hydrocarbons (PHs) remediation as anode can serve as inexhaustible electron acceptor. However, the toxicity of PHs might inhibit the formation and function of EABs. Quorum sensing (QS) is ideal for boosting the performance of EABs, but its potential effects on reshaping microbial composition of EABs in treating PHs are poorly understood. Herein, two AHL signals, C4-HSL and C12-HSL, were employed to promote EABs for PHs degradation. The start-times of AHL-mediated EABs decreased by 18-26%, and maximum current densities increased by 28-63%. Meanwhile, the removal of total PHs increased to over 90%. AHLs facilitate thicker and more compact biofilm as well as higher viability. AHLs enhanced the electroactivity and direct electron transfer capability. The total abundance of PH-degrading bacteria increased from 52.05% to 75.33% and 72.02%, and the proportion of electroactive bacteria increased from 26.14% to 62.72% and 63.30% for MFC-C4 and MFC-C12. Microbial networks became more complex, aggregated, and stable with addition of AHLs. Furthermore, AHL-stimulated EABs showed higher abundance of genes related to PHs degradation. This work advanced our understanding of AHL-mediated QS in maintaining the stable function of microbial communities in the biodegradation process of petroleum hydrocarbons.

Biofilm metagenomic characteristics behind high coulombic efficiency for propanethiol deodorization in two-phase partitioning microbial fuel cell

Hydrophobic volatile organic sulfur compounds (VOSCs) are frequently found during sewage treatment, and their effective management is crucial for reducing malodorous complaints. Microbial fuel cells (MFC) are effective for both VOSCs abatement and energy recovery. However, the performance of MFC on VOSCs remains limited by the mass transfer efficiency of MFC in aqueous media. Inspired by two-phase partitioning biotechnology, silicone oil was introduced for the first time into MFC as a non-aqueous phase (NAP) medium to construct two-phase partitioning microbial fuel cell (TPPMFC) and augment the mass transfer of target VOSCs of propanethiol (PT) in the liquid phase. The PT removal efficiency within 32 h increased by 11-20% compared with that of single-phase MFC, and the coulombic efficiency of TPPMFC (11.01%) was 4.32-2.68 times that of single-phase MFC owing to the fact that highly active desulfurization and thiol-degrading bacteria (e.g., Pseudomonas, Achromobacter) were attached to the silicone oil surface, whereas sulfur-oxidizing bacteria (e.g., Thiobacillus, Commonas, Ottowia) were dominant on the anodic biofilm. The outer membrane cytochrome-c content and NADH dehydrogenase activity improved by 4.15 and 3.36 times in the TPPMFC, respectively. The results of metagenomics by KEGG and COG confirmed that the metabolism of PT in TPPMFC was comprehensive, and that the addition of a NAP upregulates the expression of genes related to sulfur metabolism, energy generation, and amino acid synthesis. This finding indicates that the NAP assisted bioelectrochemical systems would be promising to solve mass-transfer restrictions in low solubility contaminates removal.

The degradation of high emulsified oil wastewater in an intermittently-aerated MBBR packed with magnetic polystyrene particles

This work aims to explore the effects of the magnetic polystyrene particles (MPS) on contaminants removal of the high emulsified oil wastewater. 26 days intermittently-aerated progress illustrated that COD removal efficiency and the resistance to the shock loading was promoted in the presence of MPS. Gas chromatography (GC) results also indicated that MPS enhanced the number of organic species reduced. According to the cyclic voltammetry test, conductive MPS appeared special redox performance which was considered could to facilitate the extracellular electron transfer. Furthermore, MPS dosing accelerated the electron-transporting system (ETS) activity by 24.91% compared the control. Based on the superior performance above, the conductivity of MPS is considered to be responsible for the enhanced organic removal efficiency. Moreover, the high-throughput sequencing displayed that electroactive Cloacibacterium and Acinetobacter accounted for a higher proportion in the MPS reactor. Additionally, Porphyrobacter and Dysgonomonas which were capable of degrading organics were also enriched more by MPS. To sum up, MPS is a promising additive to enhance the organic substances removal for the high emulsified oil wastewater.

Layered Design of a Highly Repeatable Electroactive Biofilm for a Standardized Biochemical Oxygen Demand Sensor

Microbial electrochemical sensors are promising to monitor bioavailable organics in real environments, but their application is restricted by the unpredictable performance of the electroactive biofilm (EAB), which is randomly acclimated from environmental microflora. With a long-term stable EAB as a template, we successfully designed EAB (DEAB) by the sequential growth of Geobacter anodireducens and automatched microbes, achieving a reproducible high current than those naturally acclimated from wastewater (NEAB). Pre-inoculation of planktonic aerobes as oxygen bioscavengers was necessary to ensure the colonization of Geobacter in the inner layer, and the abundant Geobacter (50%) in DEAB guaranteed 4 times higher current density with a 15-fold smaller variation among 20 replicates than those of NEAB. The sensor constructed with DEAB exhibited a shorter measuring time and a precise biochemical oxygen demand (BOD) measurement with acetate, real domestic wastewater, and supernatant of anaerobic digestion. Here, we for the first time proposed an applicable strategy to standardize EABs for BOD sensors, which is also crucial to ensure a stable performance of all bioelectrochemical technologies.

Protection of electroactive biofilms against hypersaline shock by quorum sensing

Quorum sensing (QS) is an ideal strategy for boosting the operating performance of electroactive biofilms (EABs), but its potential effects on the protection of electroactive biofilms against environmental shocks (e.g., hypersaline shock) have been rarely revealed. In this study, a QS signaling molecule, the N-(3-oxo-dodecanoyl)-L-homoserine lactone, was employed to promote the anti-shock property of the EABs against extreme saline shock. The maximum current density of the QS-regulated biofilm recovered to 0.17 mA/cm² after 10% salinity exposure, which was much higher than those of its counterparts. The laser scanning confocal microscope confirmed a thicker and more compact biofilm with the presence of the QS signaling molecule. The extracellular polymeric substances (EPS) might play a crucial role in the anti-shocking behaviors, as the polysaccharides in EPS of QS-biofilm had doubled compared to the groups with acylase (the QS quencher). The microbial community analysis indicated that the QS molecule enriched the relative abundance of key species including Pseudomonas sp. and Geobacter sp., which were both beneficial to the stability and electroactivity of the biofilms. The functional genes related to the bacterial community were also up-regulated with the presence of the QS molecule. These results highlight the importance of QS effects in protecting electroactive biofilm under extreme environmental shock, which provides effective and feasible strategies for the future development of microbial electrochemical technologies.