Enabling anoxic acetate assimilation by electrode-driven respiration in the obligate aerobe, Pseudomonas putida

Harnessing Pseudomonas putida in bioelectrochemical systems

Bioelectrochemical systems (BESs), a group of promising integrated systems that combine the advantages of biotechnology and electrochemical techniques, offer new opportunities to address environmental and energy challenges. Exoelectrogens capable of extracellular electron transfer (EET) are the critical factor enabling electrocatalytic activity in BESs. Pseudomonas putida, an aerobe widely used in environmental bioremediation, the biosynthesis of valuable chemicals, and energy bioproduction, has attracted much attention due to its unique application potential in BESs. This review provides a comprehensive understanding of the working principles, key factors, and applications of BESs using P. putida as the exoelectrogen. The challenges and perspectives for the development of BESs with P. putida as the exoelectrogen are also proposed and discussed.

Electricity generation and real oily wastewater treatment by Pseudomonas citronellolis 620C in a microbial fuel cell: pyocyanin production as electron shuttle

In the present study, the potential of Pseudomonas citronellolis 620C strain was evaluated, for the first time, to generate electricity in a standard, double chamber microbial fuel cell (MFC), with oily wastewater (OW) being the fuel at 43.625 mg/L initial chemical oxygen demand (COD). Both electrochemical and physicochemical results suggested that this P. citronellolis strain utilized efficiently the OW substrate and generated electricity in the MFC setup reaching 0.05 mW/m2 maximum power. COD removal was remarkable reaching 83.6 ± 0.1%, while qualitative and quantitative gas chromatography/mass spectrometry (GC/MS) analysis of the OW total petroleum and polycyclic aromatic hydrocarbons, and fatty acids revealed high degradation capacity. It was also determined that P. citronellolis 620C produced pyocyanin as electron shuttle in the anodic MFC chamber. To the authors' best knowledge, this is the first study showing (phenazine-based) pyocyanin production from a species other than P. aeruginosa and, also, the first time that P. citronellolis 620C has been shown to produce electricity in a MFC. The production of pyocyanin, in combination with the formation of biofilm in the MFC anode, as observed with scanning electron microscopy (SEM) analysis, makes this P. citronellolis strain an attractive and promising candidate for wider MFC applications.

A double-edged sword: Constructed wetland-microbial fuel cells promote organics removal via entrapment

Recently, constructed wetland-microbial fuel cells (CW-MFCs) are found to enhance the organics removal via the connection of the external circuit. Yet, it is unclear why the energy output is unmatched with the enhancement of the organics removal. This study compared the dynamic changes of the organics in a CW-MFC microcosm operated under the close circuit and open circuit. As a result, the close circuit facilitated the organics removal by 9 % before the proportional discharge of carbon metabolites. This suggested that organics entrapment should account for the huge loss of carbon recovery; and closing the external circuit could further promote the organics entrapment. Besides, polyhydroxybutyrate was found accumulated in the MFC culture experiment, evidencing that the fed-batch mode of operation could result in a feast-famine pattern of microbial metabolism. Despite the fast organics entrapment during the first hours, prolonging the operation time would lead to continuous carbon gas emission, along with the substantially elevated coulombic efficiency. Together, these results explained the substantial COD removal enhancement with low electricity yield, and cautioned the safe use of the MFC integration to spare the system from overaccumulation of organics.

Electrodeposited copper enhanced removal of 2,4-dichlorophenol in batch and flow reaction in Cu@CC-PS-MFC system

Combination of microbial fuel cell (MFC) and advanced oxidation process (AOP) is promising for pollutant removal. In this paper, Cu0-loaded carbon cloth cathode by electrodeposition (Cu@CC-PS-MFC) was applied to enhance 2,4-dichlorophenol (2,4-DCP) degradation based on persulfate (PS) activation in microbial fuel cell. Cu0 exhibited a typical structure of face-centered cubic metal polyhedron on carbon cloth. The removal of 2,4-DCP by Cu@CC-PS-MFC (75.6%) was enhanced by more than 50% compared to CC-PS-MFC (49.2%) after 1 h of reaction. 30 mg/L 2,4-DCP in Cu@CC-PS-MFC was completely removed and achieved a high mineralization (80.6%) after 9 h of reaction under optimized condition with low dissolved copper ion concentration (0.615 mg/L). Meanwhile, more than 90% removal of 2,4-DCP was stably achieved with flow operation condition (hydraulic residence time of 7.2 h). The change of copper valent state Cu0/Cu2O/CuO was the main mechanism of PS activation with main reactive species of O•H and O21. The bioanode of MFC enhanced the in-situ regeneration of ≡Cu+ and ≡Cu0 on the catalyst surface by transporting electrons, which was believed to contribute to good catalyst lifetime and excellent 2,4-DCP removal. Electrodeposited copper contributes to the enhanced degradation of 2,4-DCP with energy recovery at the same time which can further broaden the application MFC.

Citrate Synthase Overexpression of Pseudomonas putida Increases Succinate Production from Acetate in Microaerobic Cultivation

Acetate is an end-product of anaerobic biodegradation and one of the major metabolites of microbial fermentation and lingo-cellulosic hydrolysate. Recently, acetate has been highlighted as a feedstock to produce value-added chemicals. This study examined acetate conversion to succinate by citrate synthase (gltA)-overexpressed Pseudomonas putida under microaerobic conditions. The acetate metabolism is initiated with the gltA enzyme, which converts acetyl-CoA to citrate. gltA-overexpressing P. putida (gltA-KT) showed an ∼50% improvement in succinate production compared to the wild type. Under the optimal pH of 7.5, the accumulation of succinate (4.73 ± 0.6 mM in 36 h) was ∼400% higher than that of the wild type. Overall, gltA overexpression alone resulted in 9.5% of the maximum theoretical yield in a minimal medium with acetate as the sole carbon source. This result shows that citrate synthase is important in acetate conversion to succinate by P. putida under microaerobic conditions.

Bioconversion of Glycerol to 1,3-Propanediol Using Klebsiella pneumoniae L17 with the Microbially Influenced Corrosion of Zero-Valent Iron

The bacterial redox state is essential for controlling the titer and yield of the final metabolites in most bioconversion processes. Glycerol conversion to 1,3-propanediol (PDO) requires a large amount of reducing equivalent and the expression of reductive pathways. Zero-valent iron (ZVI) was used in the glycerol bioconversion of Klebsiella pneumoniae L17. The level of 1,3-PDO production increased with the oxidation of ZVI (31.8 ± 1.2 vs. 25.7 ± 0.5, ZVI vs. no ZVI) while the cellular NADH/NAD+ level increased (0.6 vs. 0.3, ZVI vs. no ZVI). X-ray diffraction showed that the iron oxide (Fe2O3) was formed during glycerol fermentation. L17 obtained electrons from ZVI and dissolved the iron continuously to form cracks on the surface, suggesting microbially influenced corrosion (MIC) was involved on the surface of ZVI. The ZVI-implemented fermentation shifted bioconversion to a more glycerol-reductive pathway. The qPCR-presented glycerol dehydratase (DhaB) with ZVI implementation was strongly expressed compared to the control. These results suggest that ZVI can contribute to the biotransformation of PDO by inducing intracellular metabolic shifts. This study could also suggest a novel microbial fermentation strategy with the application of MIC.

Polydopamine/polypyrrole-modified graphite felt enhances biocompatibility for electroactive bacteria and power density of microbial fuel cell

The interactions between the microbes and the surface of an anode play an important role in capturing the respiratory electrons from bacteria in a microbial fuel cell (MFC). The chemical and electrochemical characteristics of the carbon material affect biofilm growth and direct electron transfer in MFCs. This study examined the electrodeposition of polydopamine (PDA) and polypyrrole (PPY) on graphite felt electrode (GF). The MFC with the modified PDA/PPY-GF reached 920 mW/m², which was 1.5, 1.17, and 1.18 times higher than those of the GF, PDA-GF, and PPY-GF, respectively. PDA has superior hydrophilicity and adhesive force biofilm formation, while PPY provides electrochemically active sites for microbial electron transfer. Raman spectroscopy, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller surface area measurements, and contact angle analysis revealed the enhanced physicochemical properties of the carbon electrode. These results show that co-doped PDA/PPY provides a strategy for electroactive biofilm development and improves the bioelectrochemical performance in realistic MFC reactors.

Efficient bioelectricity generation and carbazole biodegradation using an electrochemically active bacterium Sphingobium yanoikuyae XLDN2-5

Carbazole and its derivatives are polycyclic aromatic heterocycles with unusual toxicity and mutagenicity. However, disposal of these polycyclic aromatic heterocycles remains a significant challenge. This study focused on efficient resource recovery from carbazole using an obligate aerobe, Sphingobium yanoikuyae XLDN2-5, in microbial fuel cells (MFCs). S. yanoikuyae XLDN2-5 successfully achieved carbazole degradation and simultaneously electricity generation in MFCs with a maximum power density of 496.8 mW m^-2 and carbazole degradation rate of 100%. It is the first time that S. yanoikuyae XLDN2-5 was discovered as an electrochemically active bacterium with high extracellular electron transfer (EET) capability. Redox mediator analysis indicated that no self-produced redox mediators were found for S. yanoikuyae XLDN2-5 under analysis conditions, and the exogenous redox mediators used in this study did not promote its EET. The nanowires produced by S. yanoikuyae XLDN2-5 cells were found in the biofilm by morphology characterization and the growth process of the nanowires was consistent with the discharge process of the MFC. Conductivity determination further verified that the nanowires produced by S. yanoikuyae XLDN2-5 cells were electrically conductive. Based on these results, it is speculated that S. yanoikuyae XLDN2-5 may mainly utilize conductive nanowires produced by itself rather than redox mediators to meet the requirements of normal energy metabolism when it grows in the low dissolved oxygen zone of the anodic biofilm. These novel findings on the EET mechanism of S. yanoikuyae XLDN2-5 lay a foundation for further exploration of polycyclic aromatic heterocyclic pollutants treatment in electrochemical devices, which may create new biotechnology processes for these pollutants control.

Spatial-type skeleton induced Geobacter enrichment and tailored bio-capacitance of electroactive bioanode for efficient electron transfer in microbial fuel cells

Microbial fuel cell (MFC) is a promising alternative to energy-intensive conventional wastewater technology. However, poor electron transfer efficiency, low coulombic recovery (CR), and high capital cost highly restricted its practical application. In this work, spatial electroactive biofilm is successfully developed on the carbonaceous skeleton derived from phenolic foam, which highly improved the bio-capacitance and Geobacter abundance of bioanode. Compared with carbon cloth (CC) anode, the optimal spatial electroactive biofilm (3DP_900) enriched the Geobacter abundance up to 56.8% from 17.2%, and obtained an extraordinary electroactive biomass loading of about 339 ± 63 μg cm^-2 and a remarkable bio-capacitance of about 3.4 F. In general, spatial biofilm highly reduces the barriers to electron transfer (R_ct) and mass transfer (R_d) in anodic substrate oxidation reaction and obtains the lowest R_ct of 2.0 ± 0.2 Ω and R_d of 35 ± 3.3 Ω in 3DP_900, which also supports the highest power density at 0.347 ± 0.027 W m^-2 and the highest CR at 69.2%. More importantly, due to its mature preparation technology, carbonized phenolic foam (2 cm thick pieces) reduces the capital cost of electrode preparation by three orders of magnitude from 1157.3 USD m^-2 of CC to 5.2 USD m^-2. Overall, this work offers an effective and scalable electrode to achieve high substrate utilization rate and energy recovery efficiency, and considers the economic cost of electrode fabrication for the further construction of pilot-scale MFCs equipment.