Enhancing the microbial advanced oxidation of P-nitrophenol in sediment through accelerating extracellular respiration with electrical stimulation.

Microbial advanced oxidation, a fundamental process for pollutant degradation in nature, is limited in efficiency by the weak respiration of indigenous microorganisms. In this study, an electric field was employed to enhance microbial respiration and facilitate the microbial advanced oxidation of p-nitrophenol (PNP) in simulated wetlands with alternation of anaerobic and aerobic conditions. With intermittent air aeration, an electric field of 0.8 V promoted extracellular electron transfer to increase Fe2+ generation through dissimilatory iron reduction and the production of hydroxyl radicals (•OH) through Fenton-like reactions. As a result, the PNP removal rate of the electrically-stimulated group was higher than that of the control (72.15 % vs 46.88 %). Multiple lines of evidence demonstrated that the electrically-induced polarization of respiratory enzymes expedited proton-coupled electron transfer within the respiratory chain to accelerate microbial advanced oxidation of PNP. The polarization of respiratory enzymes with the electric field hastened proton outflow to increase cell membrane potential for adenosine triphosphate (ATP) generation, which enhanced intracellular electron transportation to benefit reactive oxygen species generation. This study provided a new method to enhance microelectrochemical remediation of the contaminant in wetlands via the combination of intermittent air aeration.

Ferroheme/Ferriheme Directly Involved in the Synthesis and Decomposition of Hydrazine as an Electron Carrier during Anammox.

Anammox bacteria performed the reaction of NH4+ and NO with hydrazine synthase to produce N2H4, followed by the decomposition of N2H4 with hydrazine dehydrogenase to generate N2. Ferroheme/ferriheme, which serves as the active center of both hydrazine synthase and hydrazine dehydrogenase, is thought to play a crucial role in the synthesis and decomposition of N2H4 during Anammox due to its high redox activity. However, this has yet to be proven and the exact mechanisms by which ferroheme/ferriheme is involved in the Anammox process remain unclear. In this study, abiotic and biological assays confirmed that ferroheme participated in NH4+ and NO reactions to generate N2H4 and ferriheme, and the produced N2H4 reacted with ferriheme to generate N2 and ferroheme. In other words, the ferroheme/ferriheme cycle drove the continuous reaction between NH4+ and NO. Raman, ultraviolet-visible spectroscopy, and X-ray absorption fine structure spectroscopy confirmed that ferroheme/ferriheme is involved in the synthesis and decomposition of N2H4 via the core FeII/FeIII cycle. The mechanism of ferroheme/ferriheme participation in the synthesis and decomposition of N2H4 was proposed by density functional theory calculations. These findings revealed for the first time the heme electron transfer mechanisms, which are of great significance for deepening the understanding of Anammox.

Stress Granule Core Protein-Derived Peptides Inhibit Assembly of Stress Granules and Improve Sorafenib Sensitivity in Cancer Cells.

Upon a variety of environmental stresses, eukaryotic cells usually recruit translational stalled mRNAs and RNA-binding proteins to form cytoplasmic condensates known as stress granules (SGs), which minimize stress-induced damage and promote stress adaptation and cell survival. SGs are hijacked by cancer cells to promote cell survival and are consequently involved in the development of anticancer drug resistance. However, the design and application of chemical compounds targeting SGs to improve anticancer drug efficacy have rarely been studied. Here, we developed two types of SG inhibitory peptides (SIPs) derived from SG core proteins Caprin1 and USP10 and fused with cell-penetrating peptides to generate TAT-SIP-C1/2 and SIP-U1-Antp, respectively. We obtained 11 SG-inducing anticancer compounds from cell-based screens and explored the potential application of SIPs in overcoming resistance to the SG-inducing anticancer drug sorafenib. We found that SIPs increased the sensitivity of HeLa cells to sorafenib via the disruption of SGs. Therefore, anticancer drugs which are competent to induce SGs could be combined with SIPs to sensitize cancer cells, which might provide a novel therapeutic strategy to alleviate anticancer drug resistance.

Insights into feasibility and microbial characterizations on simultaneous elimination of dissolved methane from anaerobic effluents and nitrate/nitrite reduction in a conventional anoxic reactor with magnetite.

Discovery of nitrate/nitrite-dependent anaerobic methane oxidation (DAMO) challenges the conventional biological treatment processes, since it provides a possibility of simultaneously mitigating dissolved methane emissions from anaerobic effluents and reducing additional carbon sources for denitrification. Due to the slow growth of specialized DAMO microbes, this possibility has been just practiced with biofilms in membrane biofilm reactors or granular sludge in membrane bioreactors. In this study, simultaneous elimination of dissolved methane from anaerobic effluents and nitrate/nitrite reduction was achieved in a conventional anoxic reactor with magnetite. Calculations of electron flow balance showed that, with magnetite the eliminated dissolved methane was almost entirely used for nitrate/nitrite reduction, while without magnetite approximately 52 % of eliminated dissolved methane was converted to unknown organics. Metagenomic sequencing showed that, when dissolved methane served as an electron donor, the abundance of genes for reverse methanogenesis and denitrification dramatically increased, indicating that anaerobic oxidation of methane (AOM) coupled to nitrate/nitrite reduction occurred. Magnetite increased the abundance of genes encoding the key enzymes involved in whole reverse methanogenesis and Nir and Nor involved in denitrification, compared to that without magnetite. Analysis of microbial communities showed that, AOM coupled to nitrate/nitrite reduction was proceeded by syntrophic consortia comprised of methane oxidizers, Methanolinea and Methanobacterium, and nitrate/nitrite reducers, Armatimonadetes_gp5 and Thauera. With magnetite syntrophic consortia exchanged electrons more effectively than that without magnetite, further supporting the microbial growth.

New insights into inhibition of high Fe(III) content on anaerobic digestion of waste-activated sludge.

The impacts of the increased iron in the waste-activated sludge (WAS) on its anaerobic digestion were investigated. It was found that low Fe(III) content (< 750 mg/L) promoted WAS anaerobic digestion, while the continual increase of Fe(III) inhibited CH4 production and total chemical oxygen demand (TCOD) removal. As the Fe(III) content increased to 1470 mg/L, methane production has been slightly inhibited about 5 % compared with the group containing 35 mg/L Fe(III). Particularly, as Fe(III) concentration was up to 2900 mg/L, CH4 production, and TCOD removal decreased by 43.6 % and 37.5 %, respectively, compared with the group with 35 mg/L Fe(III). Furthermore, the percentage of CO2 of the group with 2900 mg/L Fe(III) decreased by 52.8 % compared with the group containing 35 mg/L Fe(III). It indicated that Fe(II) generated by the dissimilatory iron reduction might cause CO2 consumption, which was confirmed by X-ray diffraction that siderite (FeCO3) was generated in the group with 2900 mg/L Fe(III). Further study revealed that Fe(III) promoted the WAS solubilization and hydrolysis, but inhibited acidification and methane production. The methanogenesis test with H2/CO2 as a substrate showed that CO2 consumption weakened hydrogenotrophic methanogenesis and then increased H2 partial pressure, further causing VFA accumulation. Microbial community analysis indicated that the abundance of hydrogen-utilizing methanogens decreased with the high Fe(III) content. Our study suggested that the increase of Fe(III) in sludge might inhibit methanogenesis by consuming or precipitating CO2. To achieve maximum bioenergy conversion, the iron content should be controlled to lower than 750 mg/L. The study may provide new insights into the mechanistic understanding of the inhibition of high Fe(III) content on the anaerobic digestion of WAS.

Focus on Co-digestion of waste activated sludge and food waste via yeast pre-fermentation and biochar supplementation: The optimization and mechanism.

Anaerobic digestion is a promising method to recover energy from waste, but the slow rate of fermentation hinders its application. Yeast pre-fermentation has been reported to enhance organic matter solubilization and ethanol production to promote syntrophic metabolism and methanogenesis. However, the pre-fermentation with yeast has not been optimized so far. In this study, the lab-scale experiment was conducted to optimize operational conditions, and a pilot-scale study was conducted to evaluate the combined strategy of yeast pre-fermentation and biochar supplementation. Results demonstrated that at a fermentation time of 6 h, temperature of 30 °C, and dry yeast dosage of 2‰, the highest ethanol production was achieved, which accounted for 6.2% of the total COD of pre-fermentation effluent of a mixture of waste-activated sludge and food waste. The methane yield of the pre-fermented waste averaged 161.3 mL/g VS/d, which was 18.7% higher than that of the control group without the yeast inoculation (135.8 mL/g VS/d). With supplementing biochar of 0.5 and 1 g/L, the average methane production was 27.8% and 36.4% higher than the control group, respectively. The volatile solid removal rate was over 10% higher than the control (58.2 ± 3.12%). Consistently, the electrochemical properties of sludge with biochar were significantly improved. A pilot-scale experiment further showed that the methane production with the yeast pre-fermentation and biochar supplementation reached 227 mL/g VS/d, 54.3% higher than that without yeast pre-fermentation and biochar. This study provided a feasible method to combine yeast pre-fermentation and biochar supplementation under optimal conditions, which effectively increased methane production during anaerobic digestion of organic waste.

Facilitating direct interspecies electron transfer in anaerobic digestion via speeding up transmembrane transport of electrons and CO2 reduction in methanogens by Na+ adjustment.

The possibility of facilitating direct interspecies electron transfer (DIET) in anaerobic digestion with different concentrations of NaCl was explored. Additional NaCl at 2 or 4 g/L strengthened anaerobic digestion to resist the high-organic loading rate impacts, whereas the higher concentrations of NaCl (6 or 8 g/L) suppressed methanogenesis. Additional MgCl2 with the same ion strength as NaCl at 2 g/L had no effect on performances. Additional NaCl at 2 or 4 g/L dramatically increased the abundance of Methanosarcina species (20.7%/23.4% vs 8.6%) and stimulated the growth of Sphaerochaeta and Petrimonas species that could transfer electrons to the soluble Fe(III) or elemental sulfur. Electrochemical evidences showed that, additional NaCl at 2 or 4 g/L increased capacitances and decreased charge transfer resistances of Methanosarcina-dominant communities. Metagenomic evidences showed that, additional NaCl at 2 or 4 g/L increased the abundance of genes that encoded the type IV pilus assembly proteins (1.98E-04/1.87E-04 vs 1.85E-04) and cytochrome c-like proteins (5.51E-04/5.60E-04 vs 5.31E-04). In addition, additional NaCl at 2 or 4 g/L increased the abundance of genes for methanophenazine (MP)/MPH2 transformation (1.04E-05/1.24E-05 vs 8.06E-06) and CO2 reduction (1.64E-03/1.86E-03 vs 1.06E-03), suggesting a rapid transmembrane transport of electrons and CO2 reduction in methanogens. Both processes were closely associated with F420/F420H2 transformation that required ATP. Additional NaCl at 2 or 4 g/L increased the yield of ATP (256.0/249.3 vs 231.8 nmol/L) that might promote F420/F420H2 transformation in methanogens, which overcame the thermodynamic limitations of combining electrons with protons for the reduction of CO2 to methane and facilitated DIET.

Source apportionment and migration characteristics of heavy metal(loid)s in soil and groundwater of contaminated site.

The rapid industrial growth has generated heavy metal(loid)s contamination in the soil, which poses a serious threat to the ecology and human health. In this study, 580 samples were collected in Henan Province, China, for source apportionment, migration characterization and health risk evaluation using self-organizing map, positive matrix factorization and multivariate risk assessment methods. The results showed that samples were classified into four groups and pollution sources included chromium slag dump, soil parent rock and abandoned factory. The contents of Cr, Pb, As and Hg were low in Group 1. Group 2 was characterized by total Cr, Cr(Ⅵ) and pH. The enrichment of total Cr and Cr(Ⅵ) in soil was mainly attributed to chromium slag dump, accounting for more than 84.0%. Group 3 was dominated by Hg and Pb. Hg and Pb were primarily attributed to abandoned factory, accounting for 84.7% and 70.0%, respectively. Group 4 was characterized by As. The occurrence of As was not limited to one individual region. The contribution of soil parent rock reached 83.0%. Furthermore, the vertical migration of As, Hg, Pb and Cr(Ⅵ) in soil was mainly influenced by medium permeability, pH and organic matter content. The trends of As, Pb, and Hg with depth were basically consistent with the trends of organic matter with depth, and were negatively correlated with the change in pH with depth. The trends of Cr(Ⅵ) with depth were basically consistent with the changes in pH with the depth. The content of Cr(Ⅵ) in the deep soil did not exceed the detection limits and Cr(Ⅵ) contamination occurred in the deep aquifer, suggesting that Cr(Ⅵ) in the deep groundwater originated from the leakage of shallow groundwater. The assessment indicated that the non-carcinogenic and carcinogenic risks for children and adults could not be neglected. Moreover, children were more susceptible than adults.

Sustainable disposal of Fenton sludge and enhanced organics degradation based on dissimilatory iron reduction in the hydrolytic acidification process.

Fenton sludge generated in the flocculation stage of the Fenton oxidation process contains significant amounts of ferric iron and organic pollutants, which require proper treatment. Previous studies have demonstrated that adding Fenton sludge to an anaerobic digester can decompose some of the organic pollutants in the Fenton sludge to lower its environmental risk, but iron gradually accumulates in the reactor, which weakens the sustainability of the method. In this study, Fenton sludge was introduced into a hydrolytic acidification reactor with a weak acid environment to relieve the iron accumulation as well as improve the degradation of organic matter. The results showed that the added Fenton sludge acted as an extracellular electron acceptor to induce dissimilatory iron reduction, which increased chemical oxygen demand (COD) removal and acidification efficiency by 16.1% and 19.8%, respectively, compared to the group without Fenton sludge. Along with the operation, more than 90% of the Fe(III) in Fenton sludge was reduced to Fe(II), and part of them was released to the effluent. Moreover, the Fe(II) in the effluent could be used as flocculants and Fenton reagents to further decrease the effluent COD by 29.8% and 44.5%, respectively. It provided a sustainable strategy to reuse Fenton sludge to enhance organic degradation based on the iron cycle.

Electromotive force induced by dynamic magnetic field electrically polarized sediment to aggravate methane emission.

As a primary driving force of global methane production, methanogens like other living organisms are exposed to an environment filled with dynamic electromagnetic waves, which might induce electromotive force (EMF) to potentially influence the metabolism of methanogens. However, no reports have been found on the effects of the induced electromotive force on methane production. In this study, we found that exposure to a dynamic magnetic field enhanced bio-methanogenesis via the induced electromotive force. When exposed to a dynamic magnetic field with 0.20 to 0.40 mT of intensity, the methane emission of the sediments increased by 41.71%. The respiration of methanogens and bacteria was accelerated by the EMF, as the ratios of F420H2/F420 and NAD+/NADH of the sediment increased by 44.12% and 55.56%, respectively. The respiratory enzymes in respiration chains might be polarized with the EMF to accelerate the proton-coupled electron transfer to enhance microbial metabolism. Together with the enriched exoelectrogens and electrotrophic methanogens, as well as the increased sediment electro-activities, this study indicated that the EMF could enhance the electron exchange among extracellular respiratory microorganisms to increase the methane emission from sediments.

Reason for the increased electroactivity of extracellular polymeric substances with electrical stimulation: Structural change of α-helix peptide of protein.

Electroactivity is an important parameter to assess the ability of the extracellular polymeric substance (EPS) of microorganisms to participate in extracellular respiration. Many reports have found that the electroactivity of microbial sludge could be enhanced with electrical stimulation, but the reason remains unclear. The results of this study showed that the current generation of the three microbial electrolysis cells increased by 1.27-1.76 times during 49 days of electrical stimulation, but the typical electroactive microorganisms were not enriched. Meanwhile, the capacitance and conductivity of EPS of sludge after the electrical stimulation increased by 1.32-1.83 times and 1.27-1.32 times, respectively. In-situ FTIR analysis indicated that the electrical stimulation could lead to the polarization of amide groups in the protein, likely affecting the protein structure related to the electroactivity. Accordingly, the dipole moment of the α-helix peptide of protein of sludge increased from 220 D to 280 D after the electrical stimulation, which was conducive to electron transfer in the α-helix peptide. Moreover, the vertical ionization potential and ELUMO-EHOMO energy gap of the C-terminal in the α-helix peptide decreased from 4.43 eV to 4.10 eV and 0.41 eV to 0.24 eV, respectively, which indicated that the α-helix was easier to serve as the electron transfer site of electron hopping. These results meant that the enhancement of the dipole moment of the α-helix peptide unchoked the electron transfer chain of the protein, which was the main reason for the increased electroactivity of EPS protein.

Electro-polarization of protein-like substances accelerates trans-cell-wall electron transfer in microbial extracellular respiration.

Electrical stimulation has been used to strengthen microbial extracellular electron transfer (EET), however, the deep-seated reasons remain unclear. Here we reported that Bacillus subtilis, a typical gram-positive bacterium capable of extracellular respiration, obtained a higher EET capacity after the electrical domestication. After the electrical domestication, the current generated by the EET of B. subtilis was 23.4-fold that of the control group without pre-domestication. Multiple lines of evidence in bacterial cells of B. subtilis, their cell walls, and a model tripeptide indicated that the polarization of amide groups after the electrical stimulation forwarded the H-bonds recombination and radical generation of protein-like substances to develop extracellular electron transfer via the proton-coupled pattern. The improved electrochemical properties of protein-like substances benefited the trans-cell-wall electron transfer and strengthen extracellular respiration. This study was the first exploration to promote microbial extracellular respiration by improving the electrochemical properties of protein-like substances in cell envelopes.

Applying Dynamic Magnetic Field To Promote Anaerobic Digestion via Enhancing the Electron Transfer of a Microbial Respiration Chain.

Electrochemical methods have been reported to strengthen anaerobic digestion, but the continuous electrical power supply and the complicated electrode installed inside the digester have restricted it from practical use. In this study, a dynamic magnetic field (DMF) was placed outside a digester to induce an electromotive force to electrically promote anaerobic digestion. With the applied DMF, an electromotive force of 0.14 mV was generated in the anaerobic sludge, and a 65.02% methane increment was obtained from the anaerobic digestion of waste-activated sludge. Experiments on each stage of anaerobic digestion showed that acidification and methanogenesis that involve electron transfer of respiration chains were promoted with the DMF, while solubilization and hydrolysis less related to respiration chains were not enhanced. Further analysis indicated that the induced electromotive force polarized the protein-like substances in the sludge to increase the conductivity and capacitance of the sludge. Electrotrophic methanogens (Methanothrix) and exoelectrogens (Exiguobacterium) were enriched with DMF. The kinetic isotope effect test confirmed that electron transfer was accelerated with DMF. Consistently, the concentration ratio of co-enzymes (NADH/NAD+ and F420H2/F420) that reflects the electron exchange with respiration chains significantly increased. Applying the DMF seemed a more accessible strategy to electrically strengthen anaerobic digestion.

Ferrate pretreatment-anaerobic fermentation enhances medium-chain fatty acids production from waste activated sludge: Performance and mechanisms.

The rupture of cytoderm and extracellular polymeric substances (EPS), and competitive inhibition of methanogens are the main bottlenecks for medium-chain fatty acids (MCFAs) production from waste activated sludge (WAS). This study proposes a promising ferrate (Fe (VI))-based technique to enhance MCFAs production from WAS through accelerating WAS disintegration and substrates transformation, and eliminating competitive inhibition of methanogens, simultaneously. Results shows that the maximal MCFAs production attains 8106.3 mg COD/L under 85 mg Fe/g TSS, being 58.6 times that of without Fe (VI) pretreatment. Mechanism exploration reveals that Fe (VI) effectively destroys EPS and cytoderm through electron transfer, reactive oxygen species generation (i.e., OH, O2- and 1O2) and elevated alkalinity, resulting in the transfer of organics from solid to soluble phase and from macromolecules to intermediates. Generation and transformation of intermediates analyses illustrate that Fe (VI) facilitates hydrolysis, acidification and chain elongation (CE) but suppresses methanogenesis, promoting the targeted conversion of intermediates to MCFAs. Also, Fe (VI) pretreatment provides potential electron shuttles for chain elongation. Microbial community and functional genes encoding key enzymes analysis indicates that Fe (VI) screens key microorganisms and up-regulates functional genes expression involved in CE pathways. Overall, this technology avoids methanogens inhibitor addition and stimulates vivianite synthesis during MCFAs production from WAS.

Anaerobic ammonium oxidation driven by dissimilatory iron reduction in autotrophic Anammox consortia.

Feammox has been applied to wastewater biological nitrogen removal. However, few studies have reported that Fe(III)(hydr)oxides induced Anammox consortia to remove NH4+ via the Feammox pathway. In this study, Fe(OH)3 was added to Anammox systems to investigate its effect on nitrogen removal via Feammox. The specific Anammox activity increased by 39 % by Fe(OH)3. Ammonia oxidation was observed to occur along with Fe(III) reduction and Fe(II) generation, which was further confirmed by the isotope test with feeding 15NH4+-N to detect 30N2. The cyclic voltammetry test showed that electron-storage capacity of Anammox sludge increased with Fe(OH)3. In situ Fourier transform infrared spectroscopy suggested that Fe(OH)3 enhanced the polarization of functional groups of outer membrane cytochrome of Anammox consortia to benefit extracellular electron transfer. This study demonstrated that Fe(OH)3 could induce Anammox consortia to perform extracellular respiration to enhance NH4+-N removal in the Anammox sludge system.

Continuously feeding fenton sludge into anaerobic digesters: Iron species change and operating stability.

Fenton sludge generated from the Fenton process contains a large number of ferric species and organic pollutants, which need to be properly treated before discharge. In this study, Fenton sludge as an Fe(III) source for dissimilatory iron reduction (DIR) was continuously added with increasing dosage into an anaerobic digester to enhance the treatment. Results showed continuously feeding Fenton sludge to the anaerobic digester did not deteriorate the performance and increased methane production and COD removal rate by 2.2 folds and 14.0%, respectively. The Fe content of sludge in the digester increased from 40.25 mg/g (dry weight) to 131.53 mg/g after continuously feeding for 77days, and then declined to 109.17 mg/g when the feeding was stopped. Mass balance analysis showed that 20.5 to 48.4% of Fe in the Fenton sludge was released to the effluent. After experiment, the ratio of reducible Fe species to the total Fe was 75.1%, which maintained the high activity in DIR. Microbial community analysis showed that iron-reducing bacteria were enriched with the addition of Fenton sludge and the sludge in the digester had a higher conductivity and capacitance to strengthen the electron transfer of DIR. All results suggested that feeding Fenton sludge into anaerobic digesters was a feasible method to dispose of Fenton sludge as well as to enhance the performance of anaerobic digestion.

Intermittent voltage induced sludge polarization to enhance anaerobic digestion.

Intermittent voltage supply has been reported to improve the performance of electro-assisted anaerobic digestion but has not been well understood. In this study, an intermittent voltage of 0.6 V (1 day on-1 day off) was applied in an electro-assisted anaerobic digester to explore its effects. Compared to those without the voltage, the methane yield increased nearly by 20.0%, and organic decomposition increased by 9.5% with the intermittent voltage, which was similar to those with the continuous voltage. The amide groups of the sludge protein after the electro-treatment were polarized to enhance electron transfer and electron storage of protein-like substances of the sludge. Although the voltage was supplied intermittently, the increased conductivity and capacitance of the sludge and EPS could effectively transport electrons between exoelectrogens and electrotrophs (such as Firmicutes and Methanothrix) to promote the anaerobic digestion. This study explained the essence of electrochemical enhancement of anaerobic digestion from the perspective of molecular structure, that is, the polarization of functional groups by voltage could improve the sludge electro-activity to maintain effective interspecies electron transfer in the periodic voltage supply.

Extracellular electron uptake for CO2 fixation by Rhodopseudomonas palustris during electro-cultivation in darkness.

As a vital part of the global carbon cycle, photosynthesis helps in fixing CO2 to produce diverse biomass. However, over-reliance on optical density results in inadequate photosynthesis under limited light sources. The coupling of extracellular respiration and photosynthetic chain via the quinone pool provides a possibility for electrically driven photosynthesis in darkness, which is not well understood. In this study, CO2 fixation of photosynthetic bacteria Rhodopseudomonas palustris was enhanced in the dark via extracellular electron uptake from the electrode at -0.4 V. The copy number of R. palustris increased by 35 folds during 28 days of operation, accompanied by the increase of ATP content, NADH/NAD+, and NADPH/NADP+ of cells. Especially, the activity of Rubisco, the key enzyme of the Calvin cycle, increased by 28 % during electro-cultivation. Accordingly, the electrochemical activity of R. palustris was found to increase, which might be attributed to the structural modification of protein-like substances due to the enhanced proton-coupled electron transfer (PCET) process in electro-cultivation, which was further confirmed by in situ Fourier transform infrared spectroscopy and kinetic isotope effect tests. This study indicated that extracellular respiration could be electrostimulated via PCET to maintain photosynthesis in R. palustris in the dark.

New insight into mechanisms of ferroferric oxide enhancing medium-chain fatty acids production from waste activated sludge through anaerobic fermentation.

Medium chain fatty acids (MCFAs) production from waste activated sludge (WAS) is restricted by poor biodegradability of WAS and low electron transfer efficiency. Herein, a novel ferroferric oxide (Fe3O4) technique was proposed. Results indicated that the MCFAs yield and selectivity were respectively enhanced by 155.4% and 66.7% in the Fe3O4-mediated WAS. Mechanistic studies disclosed that Fe3O4 promoted substrates degradation through conducting dissimilatory iron reduction (DIR) and stimulating hydrolase activity, providing precursors for chain elongation (CE). Generally, Fe3O4 improved the key processes for MCFA production at different degrees, i.e., hydrolysis, acidification and CE. Interestingly, MCFAs yield enhancement was primarily ascribed to facilitated electron transfer rather than DIR or produced ferrous iron, which could be supported by the analyses of electrochemical properties, electron transfer system activity and morphology. Further, Fe3O4 shifted the key microorganisms in favor of MCFAs production. Overall, this strategy could improve MCFAs production, sludge dewatering and phosphorus removal, concurrently.

Control the greenhouse gas emission via mediating the dissimilatory iron reduction: Fulvic acid inhibit secondary mineralization of ferrihydrite.

Reducing methane emission is of great importance to control the global greenhouse effect. Dissimilatory iron reduction (DIR) coupling of organic matter decomposition may suppress methane production via reducing primary electron donors available for methanogenesis. However, during DIR, the amorphous iron oxides (e.g., ferrihydrite) are easy to transform into more stable crystalline iron minerals, which slowdowns the rate of DIR. Humic substance (HS) with redox activity has been extensively reported to facilitate DIR via "electron shuttles" mechanism, yet little known about the effect of HS on mediating the mineralization of iron oxides and the subsequent influences on DIR and methanogenesis. To clarify this, ferrihydrite and fulvic acid (FA) (as the model substance of HS) were supplied to anaerobic methanogenesis systems. Results showed that FA could significantly decrease the formation of crystalline iron oxides, enhance DIR rate by 13.72% and suppress methanogenesis by 25.13% compared to ferrihydrite supplemented only. By X-ray absorption spectra analysis, it was found that FA could complex with ferrihydrite via forming a Fe-C/O structure on the second shell of Fe atom. Quantum chemical calculation further confirmed that FA reduced the adsorption energy between Fe(II) and ferrihydrite. Our study suggested that rational use of HS to mediate mineralization pathway of iron oxides could efficiently improve the availability of iron oxides to drive DIR and control the conversion of organics into CH4 in natural or engineered systems.