Delineating cathodic extracellular electron transfer pathways in microbial electrosynthesis: Modulation of polarized potential and Pt@C addition.

Microbial electrosynthesis (MES) is an innovative technology that employs microbes to synthesize chemicals by reducing CO2. A comprehensive understanding of cathodic extracellular electron transfer (CEET) is essential for the advancement of this technology. This study explores the impact of different cathodic potentials on CEET and its response to introduction of hydrogen evolution materials (Pt@C). Without the addition of Pt@C, H2-mediated CEET contributed up to 94.4 % at -1.05 V. With the addition of Pt@C, H2-mediated CEET contributions were 76.6 % (-1.05 V) and 19.9 % (-0.85 V), respectively. BRH-c20a was enriched as the dominated microbe (>80 %), and its relative abundance was largely affected by the addition of Pt@C NPs. This study highlights the tunability of MES performance through cathodic potential control and the addition of metal nanoparticles.

Efficient production of medium chain fatty acids in microbial electrosynthesis with simultaneous bio-utilization of carbon dioxide and ethanol.

Microbial electrosynthesis (MES) is a promising technology for chemicals production driven by renewable energy. However, how the medium chain fatty acids (MCFAs) production in MES is affected by the method of chain elongation is not clear, and no direct evidence is provided yet for a simultaneous bio-utilization of CO2 and ethanol. In this study, different methods of chain elongation in MES reactors were investigated. During in-situ chain elongation, a maximum caproate concentration of 11.9 ± 0.6 g L-1 was achieved, while the C6 specificity (56.4% ± 0.5%) was much lower than that of ex-situ chain elongation (78.7% ± 1.5%). Carbon distribution and reduction degree balance indicated a simultaneous bio-utilization of CO2 and ethanol, and it was validated by the isotope tracer technique. MCFAs-forming microbes, acetogens, and electrochemically active microorganisms were enriched. This study provides fundamental insights relevant to the carbon and electron fluxes driven by electricity.

Caproate production from xylose via the fatty acid biosynthesis pathway by genus Caproiciproducens dominated mixed culture fermentation.

Caproate production from organic wastes is deemed as a novel strategy in mixed culture fermtation (MCF). However, producing caproate from natural sugar of xylose by MCF is seldom reported and the metabolic pathway is still unclear. Thus, the caproate production from xylose was investigated in this study by mesophilic MCF. The results showed that the caproate concentration from xylose (10 g/L) was 1.2 ± 0.17 g/L (equal to 2.7 gCOD/L) under pH 5.0. Dosing extra ethanol of 5 g/L could slightly increase the caproate production by âˆ¼ 30% (i.e., 1.6 g/L). While dosing extra acetate of 5 g/L negatively affected the caproate production, which was just 0.2 g/L. The microbial analysis illustrated that genus Caproiciproducens was a main identified caproate producer, occupying over 80% of enriched mixed culture. The fatty acid biosynthesis pathway was identified via metagenomic analysis. These unexpected differences extended the understanding of caproate production from organic wastes.

Microbial electrochemical stimulation of caproate production from ethanol and carbon dioxide.

The production of value added chemicals from CO2 is of critical importance for the practical application of microbial electrosynthesis (MES). Here, a binary electron donor (ED) design (using electrode and ethanol) was introduced to provide an efficient caproate production with the bioconversion of both CO2 and ethanol. A maximum caproate production rate of 2.41 ±â€¯0.69 g L-1 d-1, and a final concentration of 7.66 ±â€¯1.38 g L-1 was achieved. Caproate production selectivity based on the substrate increased to 91.47 ±â€¯0.58% (Binary EDs) from 32.22 ±â€¯32.58% (open circuit Electrode ED). An observed amount of 23.43 ±â€¯0.69% of carbon within the final binary ED products originated from the CO2. This work proves for the first time the potential of caproate production from CO2 utilization and ethanol upgrading using solid electrodes to regulate the chain elongation process.

Enrichment of hydrogen-oxidizing bacteria with nitrate recovery as biofertilizers in the mixed culture.

Hydrogen-oxidizing bacteria (HOB) can utilize hydrogen and oxygen to produce valuable products in biomass, including polyhydroxyalkanoates, microbial proteins, and biofertilizers. However, the method of enriching HOB as biofertilizers from mixed culture remains unknown. In this study HOB were enriched with nitrate as nitrogen source at a hydraulic retention time of 10 h. The nitrate consumption rate was 120 mgN/L/d or 16 mg N/g VSS/h, which was comparable to those of denitrification using organic carbon or hydrogen. The percentage of Azospirillum (dominated genus, reported biofertilizing HOB) was 84.89% and the dominated species was Azospirillum lipoferum strain DSM 1691. Furthermore, the enriched HOB had the abilities of 1-aminocyclopropane-1-carboxylate conversion and phosphate solubilization, the functions of biofertilizers. This is the first report on the enrichment of biofertilizing HOB from mixed culture. Meanwhile, the enriched HOB can recover nitrate from wastewater without any secondary nitrogen pollution, extending HOB application for resource recovery from wastewater.

Caproate production from xylose by mesophilic mixed culture fermentation.

Caproate production by mixed culture fermentation (MCF) is economically attractive. Xylose is known as the second most abundant sugar in nature, however, producing caproate from xylose is never reported. In this study, caproate production from xylose by mesophilic MCF was firstly investigated. The results showed that as pH decreasing to 5.0, the caproate concentration was 2.06 g/L in a batch reactor and was between 0.45 and 1.07 g/L in a continuously stirred reactor. Microbial analysis illustrated that Caproiciproducens and Clostridium_sensu_stricto_12, as two main identified caproate producers, occupied over 50% and around 10% of mixed culture, respectively. Thus, caproate production from xylose was proposed via the fatty acid biosynthesis pathway, not the well-known reverse ß-oxidation pathway. These unexpected differences from literatures gains more understanding about caproate production from organic substrates via MCF.

Nitrification plays a key role in N2O emission in electric-field assisted aerobic composting.

Nitrous oxide (N2O) emission is a serious environmental problem in composting. Previous studies have indicated that electric field assistance results in lower N2O emissions in aerobic composting; however, the exact mechanisms involved in electric-field assisted aerobic composting (EAAC) are not clear. In this study, the biological N transformation processes and the N-associated genes were investigated. The results demonstrated that electric field application inhibited nitrification, weakened the nitrifying functional genes (the hao and nxrA genes declined maximally by 86% and 86.8%, respectively), and increased the N2O consumption-related gene (nosZ) by a maximum factor of 2.76 compared with that in CAC. The correlation analysis demonstrated that nitrification was the main source of N2O emission in EAAC. The findings imply that EAAC is a promising process for mitigating N2O emission at the source during aerobic composting.

Electricity production and microbial community in psychrophilic microbial fuel cells at 10 °C.

Psychrophilic microbial fuel cell (PMFC) offers an alternative method for low temperature wastewater treatment, but is seldom reported. In this study, the two-chamber PMFC was constructed at 10 °C using acetate as an electron donor. The maximum voltage under external resistance of 1000 Ω was around 550 mV. The columbic efficiency (CE) was 82.4% under external resistance of 100 Ω and the max power density was 582.4 mW/m2. After temperature decreasing to 4 °C, the maximum voltage also reached 530 mV and CE was 38.4%. The direct electron transfer was proposed in PMFC according to cyclic voltammetry curves. The short enriching time (~30 days) of biofilm in the anodic electrode may be due to the high activity of enriched novel exoelectrogens of M. fermentans (46.2%) and E. lemanii (15.4%). The development of PMFC involved biotechnologies in low temperature regions shall benefit for valuable chemicals production and energy generation in the future.

Inhibitory effects of free propionic and butyric acids on the activities of hydrogenotrophic methanogens in mesophilic mixed culture fermentation.

The aim of this work was to study the inhibitory of free propionic acid (FPA) and free butyric acid (FBA) on enriched hydrogenotrophic methanogens. It demonstrated that concentrations of FPA and FBA were correlated well with the specific methanogenic activity. Coenzyme M concentrations also agreed well with the trends of FPA and FBA. Two fators of C50% (concentration at 50% inhibition) and CRC (recoverable concentration from inhibition) were used to quantitively analyze the inhibitory order using the former result of free acetic acid (FAA) and the results of FBA and FPA. The order according to C50% was FAA (5.2 mM) > FBA (8.3 mM) > FPA (8.5 mM), while for CRC it was FPA (9.3 mM) > FAA = FBA (13.5 mM). After comparing with literatue, it suggests that the toxicities of these three organic acids are similar. Thus, accumulating free organic acid offers a cost-effective method to inhibit methanogenesis.

Expanding the product spectrum of value added chemicals in microbial electrosynthesis through integrated process design-A review.

Microbial electrosynthesis (MES) is a novel microbial electrochemical technology proposed for chemicals production with the storage of sustainable energy. However, the practical application of MES is currently restricted by the limited low market value of products in one-step conversion process, mostly acetate. A theme that is pervasive throughout this review is the challenges associated with the expanded product spectrum. Several recent research efforts to improve acetate production, using novel reactor configuration, renewable power supply, and various 3-D cathode are summarized. The importance of genetic modification, two-step hybrid process, as well as input substrates other than CO2 are highlighted in this review as the future research paths for higher value chemicals production. At last, how to integrate MES with existing biochemicals processes is proposed. Definitely, more studies are encouraged to evaluate the overall performances and economic efficiency of these integrated process designs to make MES more competitive.