Extracellular electron transfer via multiple electron shuttles in waterborne Aeromonas hydrophila for bioreduction of pollutants

Distinct influence of model electron shuttles on anaerobic mononitrophenols reduction in aquatic environments by Shewanella oneidensis MR-1

The environmental fate and risks of mononitrophenols (mono-NPs), the simplest nitrophenols (NPs) often found in aquatic environments, are profoundly influenced by anaerobic bioreduction and co-existing electron shuttles (ESs), but little is known about the underlying mechanisms. Here, we elucidate the pathways of anaerobic mono-NPs bioreduction by Shewanella oneidensis MR-1 and assess the effect of model ESs on these processes. We found that all three mono-NPs isomers could be readily reduced to their corresponding aminophenols by S. oneidensis MR-1 under anaerobic conditions. CymA, a core component of the Mtr respiratory pathway, performs a dynamic role in these bioreduction, which is highly dependent on the bioreduction kinetics. The exogenous addition of quinones was found to accelerate the mono-NPs bioreduction through interactions with key outer-membrane proteins (e.g., OmcA and MtrC), and all these processes matched well to linear free energy relationships (LFERs). Surprisingly, adding riboflavin did not influence the bioreduction of all three mono-NPs isomers, which may be due to the contribution of OmcA and MtrC to these bioreduction processes and their downregulated expression. This study enhances our understanding of the environmental fate of mono-NPs and their bioconversion processes, providing valuable insights for the bioremediation of nitrophenol-contaminated sites.

In-situ biological biogas upgrading using upflow anaerobic polyfoam bioreactor: Operational and biological aspects

A high rate upflow anaerobic polyfoam-based bioreactor (UAPB) was developed for lab-scale in-situ biogas upgrading by H2 injection. The reactor, with a volume of 440 mL, was fed with synthetic wastewater at an organic loading rate (OLR) of 3.5 g COD/L·day and a hydraulic retention time (HRT) of 7.33 h. The use of a porous diffuser, alongside high gas recirculation, led to a higher H2 liquid mass transfer, and subsequently to a better uptake for high CH4 content of 56% (starting from 26%). Our attempts to optimize both operational parameters (H2 flow rate and gas recirculation ratio, which is the total flow rate of recirculated gas over the total outlet of gas flow rate) were not initially successful, however, at a very high recirculation ratio (32) and flow rate (54 mL/h), a significant improvement of the hydrogen consumption was achieved. These operational conditions have in turn driven the methanogenic community toward the dominance of Methanosaetaceae, which out-competed Methanosarcinaceae. Nevertheless, highly stable methane production rates of 1.4-1.9 L CH4/Lreactor.day were observed despite the methanogenic turnover. During the different applied operational conditions, the bacterial community was especially impacted, resulting in substantial shifts of taxonomic groups. Notably, Aeromonadaceae was the only bacterial group positively correlated with increasing hydrogen consumption rates. The capacity of Aeromonadaceae to extracellularly donate electrons suggests that direct interspecies electron transfer (DIET) enhanced biogas upgrading. Overall, the proposed innovative biological in-situ biogas upgrading technology using the UAPB configuration shows promising results for stable, simple, and effective biological biogas upgrading.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

Mediating the performance of anaerobic granular sludge by exogenous flavin supplementation: Flavin binding capacity and behavior, interspecies electron transfer, and methane production

Although flavins are known as effective electron mediators, the binding capacity of exogenous flavins by anaerobic granular sludge (AGS) and their role in interspecies electron transfer (IET) remains unknown. In this study, AGS was mediated by using three exogenous flavins of riboflavin (RF), flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). Results showed that the total amounts of flavins associated with extracellular polymeric substance (EPS) of AGS increased by 2.03-2.42 and 3.83-4.94 folds, after exposure to 50 and 200 μM of exogenous flavins, respectively. A large portion of FMN and FAD was transformed into RF by AGS. Exogenous flavin mediation also stimulated the production of EPS and cytochrome c (c-Cyts) as well as cytochrome-bound flavins. The increased abundance of these electron mediators led to a reduced electrochemical impedance of EPS and improved extracellular electron transfer capacity. The methane production of AGS after mediation with exogenous RF, FMN, and FAD increased by 19.03-31.71%, 22.86-26.04%, and 28.51-33.44%, respectively. This study sheds new light on the role of exogenous flavins in promoting the IET process of a complex microbial aggregate of AGS.

Recent advances in enrichment, isolation, and bio-electrochemical activity evaluation of exoelectrogenic microorganisms

Exoelectrogenic microorganisms (EEMs) catalyzed the conversion of chemical energy to electrical energy via extracellular electron transfer (EET) mechanisms, which underlay diverse bio-electrochemical systems (BES) applications in clean energy development, environment and health monitoring, wearable/implantable devices powering, and sustainable chemicals production, thereby attracting increasing attentions from academic and industrial communities in the recent decades. However, knowledge of EEMs is still in its infancy as only ~100 EEMs of bacteria, archaea, and eukaryotes have been identified, motivating the screening and capture of new EEMs. This review presents a systematic summarization on EEM screening technologies in terms of enrichment, isolation, and bio-electrochemical activity evaluation. We first generalize the distribution characteristics of known EEMs, which provide a basis for EEM screening. Then, we summarize EET mechanisms and the principles underlying various technological approaches to the enrichment, isolation, and bio-electrochemical activity of EEMs, in which a comprehensive analysis of the applicability, accuracy, and efficiency of each technology is reviewed. Finally, we provide a future perspective on EEM screening and bio-electrochemical activity evaluation by focusing on (i) novel EET mechanisms for developing the next-generation EEM screening technologies, and (ii) integration of meta-omics approaches and bioinformatics analyses to explore nonculturable EEMs. This review promotes the development of advanced technologies to capture new EEMs.

Enhancing anaerobic digestion of food waste with granular activated carbon immobilized with riboflavin

Previous studies have shown that anaerobic digestion of food waste can be enhanced by addition of conductive materials that stimulate direct interspecies electron transfer (DIET) between bacteria and methanogens. However, at extremely high organic loading rates (OLRs), volatile fatty acids (VFAs) still tend to accumulate even in the presence of conductive materials because of an imbalance between the formation of fermentation products and the rate of methanogenesis. In this study, granular activated carbon (GAC) immobilized with riboflavin (GAC-riboflavin) was added to an anaerobic digester treating food waste. The GAC-riboflavin reactor operated stably at OLRs as high as 11.5 kgCOD/ (m³·d) and kept VFA concentrations below 69.4 mM, COD removal efficiencies, methane production rates, and biogas methane concentrations were much higher in the GAC-riboflavin reactor than the GAC- and non-amended reactors. Transcripts associated with genes that code for proteins involved in DIET based metabolism were somewhat more highly expressed by Methanothrix in the GAC-riboflavin reactor. However, it is unlikely that riboflavin acted as an electron shuttle to stimulate DIET. Rather, it seemed to provide nutrients that enhanced the growth of microorganisms involved in the anaerobic digestion process, including those that are capable of DIET.

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.