Anodic electron shuttle mechanism based on 1-hydroxy-4-aminoanthraquinone in microbial fuel cells

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.

Efficient photocatalytic reduction of nitrate byproducts during anammox process by novel extracellular polymeric substances-embedded NH2-MIL-101(Fe) photocatalysts

Anaerobic ammonium oxidation (anammox) is an efficient nitrogen removal process; however, nitrate byproducts hampered its development. In this study, extracellular polymeric substances (EPS) were embedded into NH2-MIL-101(Fe), creating NH2-MIL-101(Fe)@EPS to reduce nitrate. Results revealed that chemical nitrate reduction efficiency of NH2-MIL-101(Fe)@EPS surpassed that of NH2-MIL-101(Fe) by 17.3 %. After adding 0.5 g/L NH2-MIL-101(Fe)@EPS within the anammox process, nitrate removal efficiency reached63.9 %, consequently elevating the total nitrogen removal efficiency to 92.4 %. 16S rRNA sequencing results elucidated the predominant role of Candidatus Brocadia within NH2-MIL-101(Fe)@EPS-anammox system. Concurrently, sufficient photogenerated electrons were transferred to microorganisms, promoting the growth of Desnitratisoma and OLB17. Additionally, photogenerated electrons activated flavin and Complex III, thereby up-regulating crucial genes involved in intra/extracellular electron transfer. Subsequently, denitrification and dissimilatory nitrate reduction to ammonium were activated to reduce nitrate. In summary, this study achieved a notable rate of photocatalytic nitrate reduction within anammox process through the NH2-MIL-101(Fe)@EPS photocatalysts.

Enhanced performance of microbial fuel cell with electron mediators from tetracycline hydrochloride degradation

Tetracycline hydrochloride (TCH) is a typical antibiotic pollutant with high toxicity and persistence. The degradation of TCH and the generation of the associated electron mediator in a dual chamber microbial fuel cells (MFCs) were studied. The results of liquid chromatography revealed that TCH could be effectively removed (>93%) in MFCs mode. The maximum COD removal was 88.14 ± 1.47% in MFCs while it was 69.57 ± 1.36% in open circuit MFCs. According to cyclic voltammetry, the presence of the relevant redox peaks clearly suggested that the intermediates from TCH degradation could act as endogenous electron mediator. The highest power density of 120.02 ± 2.76 mW/m² and the lowest internal resistance of 18.68 Ω were achieved in MFC with 2 mg/L of TCH. Microbial community analysis illustrated that Bacteroides, Comamonas, Clostridium_sensu_stricto, Desulfovibrio and Geobacter were enriched and played a dominant role in TCH degradation and power generation. Electrochemical active bacteria had certain tolerance to TCH and the inhibiting threshold value of TCH was below 5 mg/L. This study provided a new thinking that low concentration of TCH could produce electron mediators to improve the performance of MFC system.

Bio-energy generation in an affordable, single-chamber microbial fuel cell integrated with adsorption hybrid system: effects of temperature and comparison study

A microbial fuel cell (MFC) integrated with adsorption system (MFC-AHS) is tested under various operating temperatures with palm oil mill effluent as the substrate. The optimum operating temperature for such system is found to be at ∼35°C with current, power density, internal resistance (R_in), Coulombic efficiency (CE) and maximum chemical oxygen demand (COD) removal of 2.51 ± 0.2 mA, 74 ± 6 mW m^-3, 25.4 Ω, 10.65 ± 0.5% and 93.57 ± 1.2%, respectively. Maximum current density increases linearly with temperature at a rate of 0.1772 mA m^-2 °C^-1, whereas maximum power density was in a polynomial function. The temperature coefficient (Q₁₀) is found to be 1.20 between 15°C and 35°C. Present studies have demonstrated better CE performance when compared to other MFC-AHSs. Generally, MFC-AHS has demonstrated higher COD removals when compared to standalone MFC regardless of operating temperatures.

ABBREVIATIONS

ACFF: activated carbon fiber felt; APHA: American Public Health Association; CE: Coulombic efficiency; COD: chemical oxygen demand; ECG: electrocardiogram; GAC: granular activated carbon; GFB: graphite fiber brush; MFC: microbial fuel cell; MFC-AHS: microbial fuel cell integrated with adsorption hybrid system; MFC-GG: microbial fuel cell integrated with graphite granules; POME: palm oil mill effluent; PTFE: polytetrafluoroethylene; SEM: scanning electron microscope.

All ecosystems potentially host electrogenic bacteria

Instead of requiring metal catalysts, MFCs utilize bacteria that oxidize organic matter and either transfer electrons to the anode or take electrons from the cathode. These devices are thus based on a wide microbial diversity that can convert a large array of organic matter components into sustainable and renewable energy. A wide variety of explored environments were found to host electrogenic bacteria, including extreme environments. In the present review, we describe how different ecosystems host electrogenic bacteria, as well as the physicochemical, electrochemical and biological parameters that control the currents from MFCs. We also report how using new molecular techniques allowed characterization of electrochemical biofilms and identification of potentially new electrogenic species. Finally we discuss these findings in the context of future research directions.

Combination of a novel electrode material and artificial mediators to enhance power generation in an MFC

This study focuses on two main aspects: developing a novel cost-effective electrode material and power production from domestic wastewater using three different mediators. Methylene blue (MB), neutral red (NR) and 2-hydroxy-1,4-naphthoquinone (HNQ) were selected as electrode mediators with different concentrations. A tin-coated copper mesh electrode was tested as anode electrode. Maximum power density of the microbial fuel cell (MFC) with 300 μM MB was 636 mW/m². Optimal mediator concentrations with respect to the achieved maximum power output for MB, NR and HNQ were 300 μM, 200 μM and 50 μM, respectively. The results demonstrate that tin-coated copper mesh showed a higher biocompatibility and electrical conductivity.

Humus-reducing microorganisms and their valuable contribution in environmental processes

Humus constitutes a very abundant class of organic compounds that are chemically heterogeneous and widely distributed in terrestrial and aquatic environments. Evidence accumulated during the last decades indicating that humic substances play relevant roles on the transport, fate, and redox conversion of organic and inorganic compounds both in chemically and microbially driven reactions. The present review underlines the contribution of humus-reducing microorganisms in relevant environmental processes such as biodegradation of recalcitrant pollutants and mitigation of greenhouse gases emission in anoxic ecosystems, redox conversion of industrial contaminants in anaerobic wastewater treatment systems, and on the microbial production of nanocatalysts and alternative energy sources.