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In natural and engineered anaerobic ecosystems, syntrophic oxidation of short-chain fatty acid is one of the key factors and interspecies electron transfer mechanisms between bacteria and archaea play a pivotal role. Growing evidence suggests that microorganisms have the capability to promote interspecies electrons transfer in a more direct manner. In the current study, three different types of iron oxides were used to thermophilic anaerobic reactors (at 50 °C), quantitative polymerase chain reaction and 16S rRNA sequencing technique were applied to investigate the relation between iron reduction bacteria and syntrophic acetate oxidation bacteria. Experiment results showed that goethite and hematite promoted the methane production and generation rate and partially relieved the negative effects of H 2. The syntrophic relation between Syntrophaceticus and Methanobacterium worked with a direct interspecies electron transfer model rather than interspecies hydrogen transfer in goethite and hematite dosed reactors. The gene copies of Syntrophaceticus predominated over that of Geobacter. Theoretical calculations revealed that the rates of iron oxide mediated interspecies electron transfer among syntrophic partners were 105-106 times higher than those attainable via interspecies H 2 transfer. • Iron oxide promoted the growth of Syntrophaceticu s rather than Geobacter. • DIET happened between acetate oxidation bacteria and methanogenic archaea. • Theoretical iron oxides-mediated electron transfer rate was higher than H 2. • Maximum electron carrier flux was 9.0 × 10−6 A in goethite reactor. [ABSTRACT FROM AUTHOR]

BACKGROUND: Microbial electrochemical technologies (METs) represent a novel platform to harvest the energy trapped in municipal wastewater. At the anode of METs, electro‐active bacteria (EAB) anaerobically oxidize wastewater constituents using the electrode as the terminal electron acceptor and, by so doing, generate an electric current. To convert complex wastewater constituents into electricity, EAB must not only establish syntrophic relationships with other members of the microbial community, but also compete with methanogens for consumption of hydrogen and acetate. Here, we examined the addition of magnetite nanoparticles (NPs) (250 mg Fe L−1) as a novel strategy to manipulate such metabolic interactions and in turn maximize the efficiency of wastewater treatment and the yield of electric current generation. RESULTS: Batch experiments carried out either in the presence of a mixture of volatile fatty acids or of a synthetic sewage demonstrated that magnetite addition accelerate the rate of electrogenic oxidation of specific compounds, particularly propionate (up to 120%), an intermediate which frequently accumulates during anaerobic treatment processes, while correspondingly enhancing electric current generation (up to 90%), and diminishing the rate of competing methane generation (up to 50%). Notably, the composition of the microbial community was not substantially affected by the presence of magnetite nanoparticles, possibly suggesting that these latter facilitated extracellular electron transfer mechanisms (among microbes and with the electrode), rather than enriching conditions for specific microorganisms. CONCLUSION: The addition of magnetite NPs may represent a practical strategy to kick‐start a bioelectrochemical system designed for wastewater treatment and improve the effectiveness of electrogenic substrate oxidation processes. © 2019 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

pH influences the occurrence and distribution of microorganisms. Microbes typically live over a range of 3–4 pH units and are described as acidophiles, neutrophiles, and alkaliphiles, depending on the optimal pH for growth. Their growth rates vary with pH along bell- or triangle-shaped curves, which reflect pH limits of cell structural integrity and the interference of pH with cell metabolism. We propose that pH can also affect the thermodynamics and kinetics of microbial respiration, which then help shape the composition and function of microbial communities. Here we use geochemical reaction modeling to examine how environmental pH controls the energy yields of common redox reactions in anoxic environments, including syntrophic oxidation, iron reduction, sulfate reduction, and methanogenesis. The results reveal that environmental pH changes energy yields both directly and indirectly. The direct change applies to reactions that consume or produce protons whereas the indirect effect, which applies to all redox reactions, comes from the regulation of chemical speciation by pH. The results also show that energy yields respond strongly to pH variation, which may modulate microbial interactions and help give rise to the pH limits of microbial metabolisms. These results underscore the importance of pH as a control on microbial metabolisms and provide insight into potential impacts of pH variation on the composition and activity of microbial communities. In a companion paper, we continue to explore how the kinetics of microbial metabolisms responds to pH variations, and how these responses control the outcome of microbial interactions, including the activity and membership of microbial consortia.

Graphical abstract Highlights • The VFAs were main inhibition factors on methane production. • The threshold VFA inhibition concentration on methane production was 16.5–18.0 g/L. • The possible functions of some bacteria were first estimated. • Syntrophic oxidation with hydrogenotrophic methanogenesis was the dominant pathway. • An inoculum rate of 50% & FW/PM ratio of 50:50 was preferable operation conditions. Abstract Compared with wet digestion, dry digestion of organic wastes reduces reactor volume and requires less energy for heating, but it is easily inhibited by high volatile fatty acid (VFA) or ammonia concentration. The inhibition on methane production kinetics during dry co-digestion of food waste and pig manure is rarely reported. The aim of this study was to explore the inhibition mechanisms and the microbial interactions in food waste and pig manure dry co-digestion systems at different inoculum rates (25% and 50% based on volatile solids) and food waste/pig manure ratios (0:100, 25:75, 50:50, 75:25 and 100:0 based on volatile solids). The results showed that the preferable operation conditions were obtained at the inoculum rate of 50% and food waste/pig manure ratio of 50:50, with a specific methane yield of 263 mL/g VS added. High VFA concentration was the main inhibition factor on methane production, and the threshold VFA inhibition concentrations ranged 16.5–18.0 g/L. Syntrophic oxidation with hydrogenotrophic methanogenesis might be the main methane production pathway in dry co-digestion systems due to the dominance of hydrogenotrophic methanogens in the archaeal community. In conclusion, dry co-digestion of food waste and pig manure is feasible for methane production without pH adjustment and can be operated stably by choosing proper operation conditions. [ABSTRACT FROM AUTHOR]

A novel type of anaerobic bacteria was previously isolated from profundal lake sediment by direct dilution of the sediment in mineral agar medium containing glucose and a background lawn of Methanospirillum hungatei as a syntrophic partner. The isolated bacteria grouped with aerobic Bacillus spp. according to their 16S rRNA gene sequence, and the most closely related species is Bacillus thioparans . Fermentative growth of the novel strain with glucose was possible only in the presence of syntrophic partners, and cocultures produced acetate and methane, in some cases also lactate and traces of succinate as fermentation products. In contrast, the closely related strains Bacillus jeotgali and Bacillus sp. strain PeC11 are able to grow with glucose axenically by mixed acid fermentation yielding lactate, acetate, formate, succinate, and ethanol as fermentation products. Alternatively, the isolated strain grew anaerobically in pure culture if pyruvate was added to glucose-containing media, and lactate, acetate and formate were the major fermentation products, but the strain never produced ethanol. Aerobic growth was found with a variety of organic substrates in the presence of partly reduced sulfur compounds. In the absence of sulfide and oxygen, nitrate served as an electron acceptor. Strain BoGlc83 was characterized as the type strain of a new species for which the name Bacillus stamsii sp. nov. (DSM 19598 = JCM 30025) is proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The abundant microbial populations were determined in thermophilic THP-AD reactors. • Propionate-oxidizing activity could be severely prohibited. • Syntrophic acetate-oxidizing bacteria likely contribute to acetate consumption. • Hydrogenotrophic methanogenesis was prevalent in thermophilic THP-AD reactors. The ecological roles of microbial communities and how they interact with each other in thermal hydrolysis process (THP) assisted thermophilic anaerobic digestion (THP-AD) reactors remain largely unknown, especially under propionate stress. Two thermophilic THP-AD reactors had methane yield of 240–248 mL/g VS added , but accumulated approximately 2000 mg/L propionate. Genome-centric metagenomics analysis showed that 68 metagenome-assembled genomes (MAGs) were recovered, 32 MAGs of which were substantially enriched. Firmicutes spp. dominated the enriched microbial community, including hydrolytic/fermentative bacteria and syntrophs. Methanogenic activities were mainly mediated by Methanosarcina sp. and Methanothermobacter spp. In addition to hydrogenotrophic methanogens, Thermodesulfovibrio sp. could also be a vital H 2 scavenger, contributing to maintaining low H 2 partial pressure in the bioreactors. The remarkable accumulation of propionate could be likely attributed to the weak syntrophic propionate-oxidizing activity or its absence. These findings advanced our knowledge about the mutualistic symbiosis of carbon metabolism in thermophilic THP-AD reactors. [ABSTRACT FROM AUTHOR]

Background Biogas is a renewable energy carrier which is used for heat and power production or, in the form of purified methane, as a vehicle fuel. The formation of methane from organic materials is carried out by a mixed microbial community under anaerobic conditions. However, details about the microbes involved and their function are limited. In this study we compare the metagenomes of four parallel biogas reactors digesting a protein-rich substrate, relate microbiology to biogas performance, and observe differences in these reactors' microbial communities compared to the original inoculum culture. Results The biogas process performance during the startup phase of four parallel continuous stirred tank reactors (designated R1, R2, R3, and R4) co-digesting fish waste and cow manure was studied. The microbial composition of the inoculum (day 0) and the four reactors at day 59 was studied and compared using 454 FLX Titanium pyrosequencing. In the inoculum and the reactor samples, the Bacteria Clostridium and Syntrophomonas were highly abundant, and the dominating methanogen was the hydrogenotrophic Methanoculleus. Syntrophic prokaryotes frequently found in biogas reactors with high concentrations of ammonium and volatile fatty acids were detected in all samples. The species Candidatus Cloacimonas acidaminovorans of the candidate phylum Cloacimonetes (WWE1) increased in all reactors and was the dominating bacterium at day 59. In particular, this bacterium showed a very high abundance in R1, which distinguished this reactor significantly from the other reactors in terms of microbial composition. Methane production and the reactor slurry characteristics were monitored in the digestion period. Generally all four reactors operated stably and showed rather similar characteristics. The average methane production in the reactors varied between 0.278 and 0.296 L gVS-1, with the lowest production in R1. Conclusions This study showed that four parallel reactors co-digesting manure and fish waste silage operated stably during a startup phase. Several important Archaea and Bacteria degrading the protein-rich substrate were identified. In particular, microorganisms involved in syntrophic methane production seemed to be important. The detailed characterization of the microbial communities presented in this work may be useful for the operation of biogas plants degrading substrates with high concentrations of proteins. [ABSTRACT FROM AUTHOR]

A new strain of syntrophically propionate-oxidizing fermenting bacteria, strain KoProp1, was isolated from anoxic sludge of a municipal sewage plant. It oxidized propionate or lactate in cooperation with the hydrogen- and formate-utilizing Methanospirillum hungatei and grew as well in pure culture without a syntrophic partner with propionate or lactate plus sulfate as energy source. In all cases, the substrates were oxidized stoichiometrically to acetate and CO, with concomitant formation of methane or sulfide. Cells formed gas vesicles in the late growth phase and contained cytochromes b and c, a menaquinone-7, and desulforubidin, but no desulfoviridin. Enzyme measurements in cell-free extracts indicated that propionate was oxidized through the methylmalonyl CoA pathway. Protein pattern analysis by SDS-PAGE of cell-free extracts showed that strain KoProp1 differs significantly from Syntrophobacter wolinii and from the propionate-oxidizing sulfate reducer Desulfobulbus propionicus. 16S rRNA sequence analysis revealed a significant resemblance to S. wolinii allowing the assignment of strain KoProp1 to the genus Syntrophobacter as a new species, S. pfennigii. [ABSTRACT FROM AUTHOR]

Syntrophomonas wolfei and Syntrophus buswellii were grown with butyrate or benzoate in a defined binary coculture with Methanospirillum hungatei. Both strains also grew independent of the partner bacteria with crotonate as substrate. Localization of enzymes involved in butyrate oxidation by S. wolfei revealed that ATP synthase, hydrogenase, and butyryl-CoA dehydrogenase were at least partially membrane-associated whereas 3-hydroxybutyryl-CoA dehydrogenase and crotonase were entirely cytoplasmic. Inhibition experiments with copper chloride indicated that hydrogenase faced the outer surface of the cytoplasmic membrane. Suspensions of butyrate-or benzoate-grown cells of either strain accumulated hydrogen during oxidation of butyrate or benzoate to a low concentration that was thermodynamically in equilibrium with calculated reaction energetics. The protonophore carbonylcyanide m-chlorophenyl-hydrazone (CCCP) and the proton-translocating ATPase inhibitor N,N′dicyclohexylcarbodiimide (DCCD) both specifically inhibited hydrogen formation from butyrate or benzoate at low concentrations, whereas hydrogen formation from crotonate was not affected. A menaquinone was extracted from cells of S. wolfei and S. buswellii grown syntrophically in a binary methanogenic culture. The results indicate that a proton-potential-driven process is involved in hydrogen release from butyrate or benzoate oxidation. [ABSTRACT FROM AUTHOR]

This work investigated the impact of the addition of different biochar types on mitigation of volatile fatty acid (VFA) accumulation, methane recovery and digestate quality in mesophilic food waste-sludge co-digestion. Four biochars derived from agricultural and sludge residues under different pyrolysis temperatures were compared. Specific biochar properties such as pH, surface area, chemical properties and presence of surface functional groups likely influenced biochar reactions during digestion, thereby resulting in a varying performance of different biochars. Miscanthus straw biochar addition led to the highest specific methane yield of 307 ± 0.3 mL CH 4 /g VS added versus 241.87 ± 5.9 mL CH 4 /g VS added from control with no biochar addition over 30 days of the co-digestion period. Biochar supplementation led to enhanced process stability which likely resulted from improved syntrophic VFA oxidation facilitated by specific biochar properties. Overall, a 21.4% increase in the overall methane production was obtained with biochar addition as compared to control. The resulting digestate quality was also investigated. Biochar-amended digester generated a digestate rich in macro- and micro-nutrients including K, Mg, Ca, Fe making biochar-amended digestate a potential replacement of agricultural lime fertilizer. This work demonstrated that the addition of specific biochars with desirable properties alleviated VFA accumulation and facilitated enhanced methane recovery, thereby providing a means to achieve process stability even under high organic loading conditions in co-digestions. Moreover, the availability of biochar-enriched digestate with superior characteristics than biochar-free digestate adds further merit to this process. [Display omitted] • Effect of various biochar on mesophilic food waste/sludge co-digestion was compared. • Biochar properties influenced their reactions during digestion. • Miscanthus straw biochar resulted in 21.4% increase in specific methane yield. • Biochar-amended (co)digestate was rich in macro- and micro-nutrients. [ABSTRACT FROM AUTHOR]

• Effects of biochar on mesophilic food waste/sludge co-digestion were investigated. • Wheat straw biochar resulted in 24% increase in specific methane yield. • Improved VFA degradation was correlated with enhanced methane generation. • Enhanced propionic acid and long-chain VFA degradation was observed. This work investigated the effect of biochar addition to mitigate VFA accumulation and enhance methane production in mesophilic food waste/sludge co-digestion. Different types of biochar derived from agricultural and forestry residues at two pyrolysis temperatures were tested. Results showed that wheat straw biochar 550 °C supported the highest specific methane yield of 381.9 LCH 4 /kg VS added and VS removal efficiency of 41.62% among all treatments. Degradation of propionic acid and long-chain fatty acids such as valeric, caproic and isovaleric acids was observed. This also corresponded to an increase in methanogenic favorable substrates including acetic acid (>40%) and butyric acid (~20%) over the control. Consequently, a 24% increase in overall methane production was obtained as compared to control. This demonstrated that biochar addition had positive effects on VFA degradation and methane production which could be a useful strategy to increase the organic loading in co-digestions without the fear of process failure. [ABSTRACT FROM AUTHOR]