Probing extracellular reduction mechanisms of Bacillus subtilis and Escherichia coli with nitroaromatic compounds

Growth of electroautotrophic microorganisms using hydrovoltaic energy through natural water evaporation

It has been previously shown that devices based on microbial biofilms can generate hydrovoltaic energy from water evaporation. However, the potential of hydrovoltaic energy as an energy source for microbial growth has remained unexplored. Here, we show that the electroautotrophic bacterium Rhodopseudomonas palustris can directly utilize evaporation-induced hydrovoltaic electrons for growth within biofilms through extracellular electron uptake, with a strong reliance on carbon fixation coupled with nitrate reduction. We obtained similar results with two other electroautotrophic bacterial species. Although the energy conversion efficiency for microbial growth based on hydrovoltaic energy is low compared to other processes such as photosynthesis, we hypothesize that hydrovoltaic energy may potentially contribute to microbial survival and growth in energy-limited environments, given the ubiquity of microbial biofilms and water evaporation conditions.

Extracellular electron transfer and the conductivity in microbial aggregates during biochemical wastewater treatment: A bottom-up analysis of existing knowledge

Microbial extracellular electron transfer (EET) plays a crucial role in bioenergy production and resource recovery from wastewater. Interdisciplinary efforts have been made to unveil EET processes at various spatial scales, from nanowires to microbial aggregates. Electrical conductivity has been frequently measured as an indicator of EET efficiency. In this review, the conductivity of nanowires, biofilms, and granular sludge was summarized, and factors including subjects, measurement methods, and conducting conditions that affect the conductivity difference were discussed in detail. The high conductivity of nanowires does not necessarily result in efficient EET in microbial aggregates due to the existence of non-conductive substances and contact resistance. Improving the conductivity measurement of microbial aggregates is important because it enables the calculation of an EET flux from conductivity and a comparison of the flux with mass transfer coefficients. This review provides new insight into the significance, characterization, and optimization of EET in microbial aggregates during a wastewater treatment process.

Detoxification mechanisms of electroactive microorganisms under toxicity stress: A review

Remediation of environmental toxic pollutants has attracted extensive attention in recent years. Microbial bioremediation has been an important technology for removing toxic pollutants. However, microbial activity is also susceptible to toxicity stress in the process of intracellular detoxification, which significantly reduces microbial activity. Electroactive microorganisms (EAMs) can detoxify toxic pollutants extracellularly to a certain extent, which is related to their unique extracellular electron transfer (EET) function. In this review, the extracellular and intracellular aspects of the EAMs’ detoxification mechanisms are explored separately. Additionally, various strategies for enhancing the effect of extracellular detoxification are discussed. Finally, future research directions are proposed based on the bottlenecks encountered in the current studies. This review can contribute to the development of toxic pollutants remediation technologies based on EAMs, and provide theoretical and technical support for future practical engineering applications.

Deciphering the role of extracellular polymeric substances in the regulation of microbial extracellular electron transfer under low concentrations of tetracycline exposure: Insights from transcriptomic analysis

Low concentrations of antibiotics can regulate the formation of electroactive biofilms, however, the underlying mechanisms, especially the composition and spatial distribution of extracellular polymeric substances (EPS) and their effects on extracellular electron transfer (EET) process, have not been fully deciphered. Here, the response of EPS of Geobacter sulfurreducens biofilm to low concentrations of tetracycline (μg L^-1 to mg L^-1) was explored, and the impact of such EPS variations on EET efficiency was further elucidated by transcriptomic analysis. Results showed that 0.05 mg L^-1 of tetracycline achieved both beneficial quantitative and spatial regulation of redox-active proteins and non-conducting exopolysaccharides in EPS, while higher concentrations induced negative effects. Moreover, 1 mg L^-1 of tetracycline upregulated multiple exopolysaccharide biosynthesis-related genes, indicating a stress response for cell-protection, while 0.05 mg L^-1 of tetracycline upregulated most direct EET-related gene expressions, resulting in the promoted EET efficiency. Furthermore, 0.05 mg L^-1 of tetracycline selectively enriched Geobacter (45.55% vs 19.55% in control, respectively) from mixed inoculum. This research provides a new insight of how antibiotics at low concentrations regulated EET process through modulation of EPS.

Enhanced performance and electron transfer of sulfur-mediated biological process under polyethylene terephthalate microplastics exposure

Microplastics are ubiquitous in estuaries, coasts, sewage and wastewater treatment plants (WWTPs), which could arouse unexpected effects on critical microbial processes in wastewater treatment. In this study, polyethylene terephthalate microplastics (PET-MPs) were selected to investigate the mechanism of its influence on the performance of sulfur-mediated biological process from the perspective of microbial metabolic activity, electron transfer capacity and microbial community. The results indicated that the exposure of 50 particles/L PET-MPs improved the chemical oxygen demand (COD) and sulfate removal efficiencies by 6.6 ± 0.5% and 4.5 ± 0.3%, respectively, due to the stimulation of microbial metabolic activity and the enrichment of sulfate-reducing bacteria (SRB) species, such as Desulfobacter. In addition, we found that the PET-MPs promoted Cytochrome C (Cyt C) production and improved the direct electron transfer (DET) capacity mediated by Cyt C. The long-term presence of PET-MPs stimulated the secretion of extracellular polymeric substance (EPS), especially the proteins and humic substances, which have been verified to be electroactive polymers to act as electron shuttles to promote the interspecies electron transfer pathway in sulfur-mediated biological process. Meanwhile, the transformation products (bis-(2-hydroxyethyl) terephthalate (BHET) and Mono (2-hydroxyethyl) terephthalic acid (MHET) of PET-MPs were detected in sulfur-mediated biological process. These findings indicate that the sulfur-mediated biological process has good adaptability to the toxicity of PET-MPs, which strengthens a deeper understanding of the dual function of microplastics in WWTPs.

Enhanced anaerobic reduction of nitrobenzene at high salinity by betaine acting as osmoprotectant and regulator of metabolism

Anaerobic technology is extensively applied in the treatment of industrial organic wastewater, but high salinity always triggers microbial cell dehydration, causing the failure of the anaerobic process. In this work, betaine, one kind of compatible solutes which could balance the osmotic pressure of anaerobic biomass, was exogenously added for enhancing the anaerobic reduction of nitrobenzene (NB) at high salinity. Only 100 mg L^-1 betaine dosing could significantly promote the removal efficiency of NB within 35 h at 9% salinity (36.92 ± 4.02% without betaine and 72.94 ± 6.57% with betaine). The relieving effects on salt stress could be observed in the promotion of more extracellular polymeric substances (EPS) secretion with betaine addition. Additionally, the oxidation-reduction potential (ORP), as well as the electron transfer system (ETS) value, was increased with betaine addition, which was reflected in the improvement of system removal efficiency and enzyme activity. Microbial community analysis demonstrated that Bacillus and Clostridiisalibacter which were positively correlated with the stability of the anaerobic process were enriched with betaine addition at high salinity. Metagenomic analysis speculated that the encoding genes for salt tolerance (kdpB/oadA/betA/opuD/epsP/epsH) and NB degradation (nfsA/wrbA/ccdA/menC) obtained higher relative abundance with betaine addition under high salt environment, which might be the key to improving salt tolerance of anaerobic biomass. The long-term assessment demonstrated that exogenous addition betaine played an important role in maintaining the stability of the anaerobic system, which would be a potential strategy to achieve a high-efficiency anaerobic process under high salinity conditions.

Recent advances in biological removal of nitroaromatics from wastewater

Various nitroaromatic compounds (NACs) released into the environment cause potential threats to humans and animals. Biological treatment is valued for cost-effectiveness, environmental friendliness, and availability when treating wastewater containing NACs. Considering the significance and wide use of NACs, this review focuses on recent advances in biological treatment systems for NACs removal from wastewater. Meanwhile, factors affecting biodegradation and methods to enhance removal efficiency of NACs are discussed. The selection of biological treatment system needs to consider NACs loading and cost, and its performance is affected by configuration and operation strategy. Generally, sequential anaerobic-aerobic biological treatment systems perform better in mineralizing NACs and removing co-pollutants. Future research on mechanism exploration of NACs biotransformation and performance optimization will facilitate the large-scale application of biological treatment systems.

Extracellular proteins of Desulfovibrio vulgaris as adsorbents and redox shuttles promote biomineralization of antimony

Biomineralization is the key process governing the biogeochemical cycling of multivalent metals in the environment. Although some sulfate-reducing bacteria (SRB) are recently recognized to respire metal ions, the role of their extracellular proteins in the immobilization and redox transformation of antimony (Sb) remains elusive. Here, a model strain Desulfovibrio vulgaris Hildenborough (DvH) was used to study microbial extracellular proteins of functions and possible mechanisms in Sb(V) biomineralization. We found that the functional groups (N-H, CO, O-CO, NH₂-R and RCOH/RCNH₂) of extracellular proteins could adsorb and fix Sb(V) through electrostatic attraction and chelation. DvH could rapidly reduce Sb(V) adsorbed on the cell surface and form amorphous nanometer-sized stibnite and/or antimony trioxide, respectively with sulfur and oxygen. Proteomic analysis indicated that some extracellular proteins involved in electron transfer increased significantly (p < 0.05) at 1.8 mM Sb(V). The upregulated flavoproteins could serve as a redox shuttle to transfer electrons from c-type cytochrome networks to reduce Sb(V). Also, the upregulated extracellular proteins involved in sulfur reduction, amino acid transport and protein synthesis processes, and the downregulated flagellar proteins would contribute to a better adaption under 1.8 mM Sb(V). This study advances our understanding of how microbial extracellular proteins promote Sb biomineralization in DvH.

Different performance of pyrene biodegradation on metal-modified montmorillonite: Role of surface metal ions from a bioelectrochemical perspective

Microbial extracellular electron transfer (EET) at microbe-mineral interface has been reported to play a significant role in pollutant biotransformation. Different metals often co-exist with organic pollutants and are immobilized on mineral surfaces. However, little is known about the influence of mineral surface metal ions on organic pollutant biodegradation and the involved electron transfer mechanism. To address this knowledge gap, pyrene was used as a model compound to investigate the biodegradation of polycyclic aromatic hydrocarbon on montmorillonite mineral saturated with metal ions (Na(I), Ni(II), Co(II), Cu(II) and Fe(III)) by Mycobacteria strain NJS-1. Further, the possible underlying electron transfer mechanism by electrochemical approaches was investigated. The results show that pyrene biodegradation on montmorillonite was markedly influenced by surface metal ions, with degradation efficiency following the order Fe(III) > Na(I) ≈ Co(II) > Ni(II) ≈ Cu(II). Bioelectrochemical analysis showed that electron transfer activities (i.e., electron donating capacity and electron transport system activity) varied in different metal-modified montmorillonites and were closely related to pyrene biodegradation. Fe(III) modification greatly stimulated degrading enzyme activities (i.e., peroxidase and dioxygenase) and electron transfer activities resulting in enhanced pyrene biodegradation, which highlights its potential as a technique for pollutant bioremediation. The bacterial extracellular protein and humic substances played important roles in EET processes. Membrane-bound cytochrome C protein and extracellular riboflavin were identified as the electron shuttles responsible for transmembrane and cross extracellular matrix electron transfer, respectively. Additions of exogenetic electron mediators of riboflavin, humic acid and potassium ferricyanide accelerated pyrene biodegradation which further verified the critical role of EET in PAH transformation at bacteria-mineral interfaces. These results support the development of clay mineral based advanced bioremediation techniques through regulating the electron transfer processes at the microbe-mineral interfaces by mineral surface modification.

Biochar-mediated reduction of m-nitrotoluene: Interaction between reduction of m-nitrotoluene and sequestration of contaminants

Biochar is a highly effective adsorbent for nitroaromatic compounds (NACs), and acts as an electron shuttle that mediates the reduction of NACs. Hence, when biochar is used to mediate NAC reduction, adsorption and reduction will occur simultaneously and affect each other. However, the effect of biochar-mediated NAC reduction on sorption remains unknown. Eight biochars with different physicochemical properties were used to adsorb m-nitrotoluene and mediate its reduction. The results showed that the adsorption of m-nitrotoluene onto the various biochars facilitated its reduction, whereas biochar-mediated reduction retarded and weakened contaminant adsorption, which increased the environmental risk posed by m-nitrotoluene. Nevertheless, biochars with a high graphitization degree and developed porosity not only had a great catalytic ability, but also significantly alleviated the negative effect of reduction on adsorption. This was ascribed to the π-π interaction and pore-filling effect, which played more important roles than the hydrophobic effect in adsorbing the reduction product (m-toluidine) onto the studied biochars during reduction. Furthermore, the methanol extraction results indicated that the eight biochars presented significantly stronger sequestration abilities for adsorbed m-toluidine than for adsorbed m-nitrotoluene. This resulted from the hydrogen bonding and the Lewis acid-base effect between m-toluidine and each biochar, which were absent for m-nitrotoluene. These results suggest that biochars with a high graphitization degree and developed porosity are applicable for mediating reduction-enhancing sequestration of NACs, which could be a novel strategy for NAC remediation.

Bacteria Make a Living Breathing the Nitroheterocyclic Insensitive Munitions Compound 3-Nitro-1,2,4-triazol-5-one (NTO)

The nitroheterocyclic 3-nitro-1,2,4-triazol-5-one (NTO) is an ingredient of insensitive explosives increasingly used by the military, becoming an emergent environmental pollutant. Cometabolic biotransformation of NTO occurs in mixed microbial cultures in soils and sludges with excess electron-donating substrates. Herein, we present the unusual energy-yielding metabolic process of NTO respiration, in which the NTO reduction to 3-amino-1,2,4-triazol-5-one (ATO) is linked to the anoxic acetate oxidation to CO₂ by a culture enriched from municipal anaerobic digester sludge. Cell growth was observed simultaneously with NTO reduction, whereas the culture was unable to grow in the presence of acetate only. Extremely low concentrations (0.06 mg L^-1) of the uncoupler carbonyl cyanide m-chlorophenyl hydrazone inhibited NTO reduction, indicating that the process was linked to respiration. The ultimate evidence of NTO respiration was adenosine triphosphate production due to simultaneous exposure to NTO and acetate. Metagenome sequencing revealed that the main microorganisms (and relative abundances) were Geobacter anodireducens (89.3%) and Thauera sp. (5.5%). This study is the first description of a nitroheterocyclic compound being reduced by anaerobic respiration, shedding light on creative microbial processes that enable bacteria to make a living reducing NTO.