Polarized potential and electrode materials implication on electro-fermentative di-hydrogen production: Microbial assemblages and hydrogenase gene copy variation

Prospects and trends in bioelectrochemical systems: Transitioning from CO2 towards a low-carbon circular bioeconomy

Resource scarcity and climate change are the most quested topics in view of environmental sustainability. CO2 sequestration through bioelectrochemical systems is an attractive option for fostering bioeconomy development upon several value-added products generation. This review details the state-of-the-art of bioelectrochemical systems for resource recovery from CO2 along with various biocatalysts capable of utilizing CO2. Two bioprocesses (photo-electrosynthesis and chemolithoelectrosynthesis) were discussed projecting their potential for biobased economy development from CO2. Significance of adopting circular strategies for efficient resource recycling, intensifying product value, integrations/interlinking of multiple process chains for the development of circular bioeconomy were discussed. Existing constrains as well as outlook for near establishment of circular bioeconomy from CO2 is presented by weighing fore-sighted plans with current actions. Need for developing CO2-based circular bioeconomy via innovative business models by analyzing social, technical, environmental and product related aspects are also discussed providing a roadmap of gaps to pursue for attaining practicality.

Metabolic flux of Bacillus subtilis under poised potential in electrofermentation system: Gene expression vs product formation

The role of poised (negative/positive) potential (0.2/0.4/0.6/0.8 V vs Ag/AgCl at anode) was studied in electrofermentation system (EF) to understand the metabolic flux of Bacillus subtilis with pyruvate as a carbon source. The relative expression of genes encoding pyruvate dehydrogenase (pdhA), lactate dehydrogenase (lctE), acetate kinase (ackA), pyruvate carboxylase (pycA), adenylosuccinate lyase (purB), acylCoA dehydrogenase (acdA) and NADH dehydrogenase (ndh) allowed evaluation of metabolic changes in correlation to product formation and bioelectrochemical analysis. In comparison to control, poised circumstances showed marked influence on product profile with up-regulation of key enzymes involved in pyruvate metabolism. EF poised with - 0.8 V and -0.6 V enhanced bio-hydrogen production by 6 folds and 4 folds respectively. Concomitantly, -0.8 V resulted in maximum ethanol and acetic acid production whilst, -0.6 V and + 0.6 V resulted in maximum lactic acid and succinic acid production respectively. The transcripts for genes associated synthesis were upregulated in the respected poised reactors.

Emerging trends in microbial fuel cell diversification-Critical analysis

Global need for transformation from fossil-based to bio-based economy is constantly emerging for the production of low-carbon/renewable energy/products. Microbial fuel cell (MFC) catalysed by bio-electrochemical process gained significant attention initially for its unique potential to generate energy. Diversification of MFC is an emerging trend in the context of prioritising/enhancing product output while exploring the mechanism specificity of individual processes. Bioelectrochemical treatment system (BET), microbial electrosynthesis system (MES), bioelectrochemical system (BES), electro-fermentation (EF), microbial desalination cell (MDC), microbial electrolysis cell (MEC) and electro-methanogenesis (EM) are the diversified MFC systems that are being researched actively. Owing to its broad diversification, MFC domain is increasing its potential credibility as a platform technology. Microbial catalyzed electrochemical reactions are the key which directly/indirectly are proportionally linked to electrometabolic activity of microorganisms towards final anticipated output. This review intends to holistically document the mechanisms, applications and current trends of MFC diversifications towards multi-faced applications.

Concomitant use of Azolla derived bioelectrode as anode and hydrolysate as substrate for microbial fuel cell and electro-fermentation applications

The potential of deoiled Azolla pinnata biomass (DAB) as electrode and substrate was evaluated for microbial fuel cell (MFC) operation. The anode electrode was fabricated using biochar obtained by subjecting DAB to pyrolysis at 600 °C, while the reducing sugars after hydrolysis of DAB by acid pretreatment was used as substrate. The post pyrolyzed biochar (P-DAB) was characterized for structural and elemental functionalities using SEM, XRD and Raman spectroscopy, whereas the reducing sugar obtained from hydrolyzed DAB (H-DAB) was analyzed for its composition. Experimental results indicated that at a given 3 g COD/L resulted in a voltage of 382 mV with 65.6% of COD reduction in closed circuit (CC) mode of operation. Cyclic voltammetric analysis depicted maximum oxidative and reductive peak currents of 3.42 mA and -4.0 mA. Noticeable peaks were also identified in CC (-0.2 V to +0.2 V and -0.19 V to -0.3 V) and OC (+0.2 V to +0.4 V and -0.1 V to -0.3 V) corresponding to complex IV cytochrome c couples (cytochrome C_ox (Cyt C_ox)/cytochrome C_rd (Cyt C_rd)), signifying the participation of electron carriers during electron transfer. The microbiome diversity showed dominance of Proteobacteria, a phylum known for exo-electrogenic bacterial species. The DAB-derived products account to environmental sustainability and support circular bioeconomy in a biorefinery mode.

From an extremophilic community to an electroautotrophic production strain: identifying a novel Knallgas bacterium as cathodic biofilm biocatalyst

Coupling microbial electrosynthesis to renewable energy sources can provide a promising future technology for carbon dioxide conversion. However, this technology suffers from a limited number of suitable biocatalysts, resulting in a narrow product range. Here, we present the characterization of the first thermoacidophilic electroautotrophic community using chronoamperometric, metagenomic, and ¹³C-labeling analyses. The cathodic biofilm showed current consumption of up to -80 µA cm^-2 over a period of 90 days (-350 mV vs. SHE). Metagenomic analyses identified members of the genera Moorella, Desulfofundulus, Thermodesulfitimonas, Sulfolobus, and Acidianus as potential primary producers of the biofilm, potentially thriving via an interspecies sulfur cycle. Hydrogenases seem to be key for cathodic electron uptake. An isolation campaign led to a pure culture of a Knallgas bacterium from this community. Growth of this organism on cathodes led to increasing reductive currents over time. Transcriptomic analyses revealed a distinct gene expression profile of cells grown at a cathode. Moreover, pressurizable flow cells combined with optical coherence tomography allowed an in situ observation of cathodic biofilm growth. Autotrophic growth was confirmed via isotope analysis. As a natural polyhydroxybutyrate (PHB) producer, this novel species, Kyrpidia spormannii, coupled the production of PHB to CO₂ fixation on cathode surfaces.

Capacitive biocathodes driving electrotrophy towards enhanced CO2 reduction for microbial electrosynthesis of fatty acids

Electron transfer towards biocathode is a rate limiting step for CO₂ reduction during microbial electrosynthesis (MES). Current study is designed to offer an understanding on electrotrophy using four different electrode materials viz., carbon cloth (CC), stainless-steel mesh (SS), combination of both (CC-SS) and a hybrid material (CC-SS-AC with activated carbon (AC)) as capacitive biocathodes for MES. Non turn-over and turn-over electrochemical investigations revealed electrode properties in terms of electron transfer, capacitance and redox catalytic currents relatively higher with CC-SS-AC and CC-SS. Acetic acid production was higher in CC-SS-AC (4.31 g/l) than CC-SS (4.21 g/l), CC (3.5 g/l) and SS (2.83 g/l) along with noticeable ethanol production with all the biocathodes except SS. Interestingly, long-term operation of all biocathodes witnessed reduction in resistance visualized through Nyquist impedance spectra relatively efficient with CC-SS-AC. Biocompatible property of CC-SS-AC with increased surface area was presumed to be a critical factor for enhancing electrotrophy linked with capacitive nature of biocathode towards enhanced bioelectrochemical CO₂ reduction.

Food waste biorefinery: Sustainable strategy for circular bioeconomy

Enormous quantity of food waste (FW) is becoming a global concern. To address this persistent problem, sustainable interventions with green technologies are essential. FW can be used as potential feedstock in biological processes for the generation of various biobased products along with its remediation. Enabling bioprocesses like acidogenesis, fermentation, methanogenesis, solventogenesis, photosynthesis, oleaginous process, bio-electrogenesis, etc., that yields various products like biofuels, platform chemicals, bioelectricity, biomaterial, biofertilizers, animal feed, etc can be utilized for FW valorisation. Integrating these bioprocesses further enhances the process efficiency and resource recovery sustainably. Adapting biorefinery strategy with integrated approach can lead to the development of circular bioeconomy. The present review highlights the various enabling bioprocesses that can be employed for the generation of energy and various commodity chemicals in an integrated approach addressing sustainability. The waste biorefinery approach for FW needs optimization of the cascade of the individual bioprocesses for the transformation of linear economy to circular bioeconomy.

Microbiome involved in microbial electrochemical systems (MESs): A review

Microbial electrochemical systems (MESs) are an attracting technology for the disposal of wastewater treatment and simultaneous energy production. In MESs, at the anode microorganisms through the catalytic activity generates electrons that can be converted into electricity or other valuable chemical compounds. Microorganisms those having ability to donate and accept electrons to and from anode and cathode electrodes, respectively are recognized as 'exoelectrogens'. In the MESs, it renders an important function for its performance. In the present mini-review, we have discussed the role of microbiome including pure culture, enriched culture and mixed culture in different BESs application. The effects of operational and biological factors on microbiome development have been discussed. Further discussion about the molecular techniques for the evaluation of microbial community analysis is addressed. In addition different electrochemical techniques for extracellular electron transfer (EET) mechanism of electroactive biofilms have been discussed. This review highlights the importance of microbiome in the development of MESs, effective operational factors for exo-electrogens activities as well their key challenges and future technological aspects are also briefly discussed.

Strategy and design of Innovation Policy Road Mapping for a waste biorefinery

Looming energy crisis, climate change concerns coupled with decreasing fossil fuel resources has garnered significant global attention toward development of alternative, renewable, carbon-neutral and eco-friendly fuels to fulfil burgeoning energy demands. Waste utilization and its management are being pursued with renewed interest due to the gamut of biobased products it can offer apart from providing enough energy to meet a major fraction of the world's energy demand. Biorefining is the sustainable processing of biomass into a spectrum of marketable products and energy. Integrating all components of waste treatment culminating into biobased products and energy recovery in a single integrated waste biorefinery is self sufficient, highly sustainable and is very beneficial. Designing systematic innovation policies are essential for development and commercialization of new technologies in this important futuristic research area. This communication explores Innovation Policy Road Mapping (IPRM) methodology available in the literature and applies it to design integrated waste biorefinery.

Selective enrichment of electrogenic bacteria for fuel cell application: Enumerating microbial dynamics using MiSeq platform

This study is intended to examine the effect of pretreatment on selective enrichment of electrogenic bacteria from mixed culture. It has been observed that the iodopropane and heat-shock pretreatments suppress the growth of non-exoelectrons, while selecting only a limited number of strains belonging to genera Xanthomonas, Pseudomonas and Prevotella while untreated control inoculum showed more diverse community comprising of both exoelectrogens and non-exoelectrogens. High power output was observed in iodopropane (180mW/m(2)) pretreated microbial fuel cell (MFC) compared to heat-shock pretreated MFC (128mW/m(2)) and untreated control (92mW/m(2)). Coulombic efficiency of iodopropane and heat-shock pretreated MFC was higher compared to untreated control MFC, while drop in pH and volatile fatty acids (VFA) production was less in iodopropane pretreated MFC signifying the shifts in bacterial community structure toward electrogenesis instead of fermentation. These results signify the role of iodopropane and heat pretreatments on enrichment of electrogenic bacteria for fuel cell application.