Propionate Degradation Research Articles

Deciphering the role of biochar pseudocapacitance strengthening in promoting syntrophic methanogenesis: Microbial capacitance-tropism in propionate metabolism

The redox functional groups of biochar can act as pseudocapacitance, alleviating acid accumulation and facilitating electron transfer in anaerobic digestion. Fe modification further enhances its pseudocapacitance properties. However, the specific role of pseudocapacitance strengthening in the electron transfer of volatile fatty acid syntrophic methanogenesis, as well as the factors inducing the enrichment behavior and metabolic mechanisms of microbes on its surface, remain unclear. In this study, experiments using acetate and propionate as substrates were conducted with the addition of biochar and electro-enhanced biochar, while 16S rRNA gene amplicon sequencing and metagenomic analysis were employed to investigate the underlying mechanisms. The results demonstrate that strengthening biochar’s pseudocapacitance significantly facilitates the electron shunt by accepting electrons, accelerating NADH/NAD+ conversion and ATP synthesis. This further promotes the oxidative degradation of propionate and acetate through the methylmalonyl-CoA (MMC) and syntrophic acetate oxidation (SAO) pathways. During methanogenesis, biochar directly transfers electrons to methanogens, with the strengthened pseudocapacitance further accelerating electron transfer through the CO2 reduction pathway and specifically promoting the methanation of propionate rather than acetate. Moreover, during propionate metabolism, Coprothermobacter and Methanosarcina exhibit capacitance-tropism in their enrichment on biochar, serving as key functional contributors to the biochar-microbial aggregates. These findings provide valuable insights for selecting exogenous materials in engineering applications to alleviate acid accumulation in anaerobic digestion systems.

Deciphering the function of Fe3O4 in alleviating propionate inhibition during high-solids anaerobic digestion: Insights of physiological response and energy conservation

Fe3O4 is a recognized addictive to enhance low solid anaerobic digestion (AD), while for high solid AD challenged by acidity inhibition, its feasibility and mechanism remain unclear. In this study, the positive effect of Fe3O4 on high-solids AD of food waste by regulating microbial physiology and energy conservation to enhance mutualistic propionate methanation was documented. The methane yield was increased by 36.7 % with Fe3O4, which because Fe3O4 alleviated propionate stress on methane generation, along with improved propionate degradation and methanogenic metabolism. Fe3O4 facilitated the production of extracellular polymeric substances and the formation of tightly bio-aggregates, fostering an enriched microbial population (e.g., Smithella and Methanosaeta) to resist propionate stress. Also, Fe3O4 up-regulated the genes in stress defense system, cytomembrane biosynthesis/function, metal irons transporter, cell division and enzyme synthesis, verifying its superiority on cellular physiology. In addition, energy-conservation strategies related to intracellular and extracellular electron transfer were enhanced by Fe3O4. Specifically, the enzyme expressions involved in reversed electron transfer and electron bifurcation coupled with direct interspecies electron transfer (DIET) were upregulated by at least 2.2 times with Fe3O4, providing sufficient energy to drive thermodynamic adverse methanogenesis from propionate-stressed condition. Consequently, the reinforced enzyme expression in the dismutation and DIET pathway make it to be the predominant drivers for enhanced methanogenic propionate metabolism. This study fills the knowledge gaps of Fe3O4-induced microbial physiological and energetic strategies to resist environmental stress, and has remarkable practical implicated for restoring inhibited bioactivities.

External voltage promoting electro-fermentation of propionate wastewater in high concentration

The performance of anaerobic digestion (AD) could be destabilized by the accumulation of intermediate metabolites at high concentrations. Emerging electro-fermentation provides a new and suitable way to address the decomposition of intermediate metabolites in AD. This study aims to examine the effect of applied voltages on propionate degradation at high concentrations (5−20 g/L) in an electro-fermentation system. Compared with the control, the experimental group with an applied voltage of 1.2 V exhibited a 45% increase in methane peak production rate and a 20% rise in methane yield. Simultaneously, the degradation rate of propionate in the experimental group was also accelerated compared to the control group. In the experimental group with an applied voltage of 1.2 V, the abundance of Syntrophobacter increased to 6−7%, representing an increase of 2.3−6.1 times higher than the control. The applied voltage facilitated the formation of electroactive biofilms, which exhibited high electrochemical activity. This study not only provides a new approach for conventional wastewater treatment but also generates additional revenue from biogas production.

Metagenomic insights into the enhanced methane production by hydrochar at varied propionate concentrations

Propionate is a crucial intermediate product in anaerobic digestion (AD), but its degradation is thermodynamically unfavorable. Its accumulation in the AD system can significantly reduce methane production efficiency and may lead to severe acidification. The present study investigated the methane production at various propionate concentrations (15, 30 and 60 mM) with and without hydrochar and also explored the microbial shifts by metagenomic analysis. The findings revealed that higher propionate concentrations reduced methane production and propionate degradation efficiency, leading to acetate accumulation. The addition of hydrochar mitigated the inhibitory effects of increased propionate concentrations, notably reduced the lag phase by 20.13 % and enhanced the methane production rate by 138.43 % under 30 mM propionate. This also resulted in faster degradation of propionate and acetate. Hydrochar impacted the microbial communities’ functional potentials, enhancing methane metabolic potential in all conditions. Genomic-centric analysis revealed that hydrochar selectively enriched syntrophic propionate-oxidizing bacteria (SPOB) candidate MAG79 Syntrophopropionicum sp., which could participate in DIET, across different concentrations of propionate, with the most pronounced effect at 30 mM propionate. This was distinct from the dominant species in the control groups, MAG63 Syntrophosphaera sp. and MAG69 Syntrophobacteraceae sp.. Hydrochar further supported the growth of another DIET candidate MAG4 Methanosarcina mazei and increased the abundance of proposed syntrophic acetate-oxidizing bacterium (SAOBs), across all propionate concentrations. These effects collectively led to more efficient propionate degradation and methane production.

Impact of additives on syntrophic propionate and acetate enrichments under high-ammonia conditions

High ammonia concentrations in anaerobic degradation systems cause volatile fatty acid accumulation and reduced methane yield, which often derive from restricted activity of syntrophic acid-oxidising bacteria and hydrogenotrophic methanogens. Inclusion of additives that facilitate the electron transfer or increase cell proximity of syntrophic species by flocculation can be a suitable strategy to counteract these problems, but its actual impact on syntrophic interactions has yet to be determined. In this study, microbial cultivation and molecular and microscopic analysis were performed to evaluate the impact of conductive (graphene, iron oxide) and non-conductive (zeolite) additives on the degradation rate of acetate and propionate to methane by highly enriched ammonia-tolerant syntrophic cultures derived from a biogas process. All additives had a low impact on the lag phase but resulted in a higher rate of acetate (except graphene) and propionate degradation. The syntrophic bacteria ‘Candidatus Syntrophopropionicum ammoniitolerans’, Syntrophaceticus schinkii and a novel hydrogenotrophic methanogen were found in higher relative abundance and higher gene copy numbers in flocculating communities than in planktonic communities in the cultures, indicating benefits to syntrophs of living in close proximity to their cooperating partner. Microscopy and element analysis showed precipitation of phosphates and biofilm formation in all batches except on the graphene batches, possibly enhancing the rate of acetate and propionate degradation. Overall, the concordance of responses observed in both acetate- and propionate-fed cultures highlight the suitability of the addition of iron oxide or zeolites to enhance acid conversion to methane in high-ammonia biogas processes.Key points• All additives promoted acetate (except graphene) and propionate degradation.• A preference for floc formation by ammonia-tolerant syntrophs was revealed.• Microbes colonised the surfaces of iron oxide and zeolite, but not graphene.Graphical

Anaerobic treatment of fruit and vegetable wastewater using EGSB: From strategies for regulating over-acidification to microbial community

Fruit and vegetable waste (FVW) are characterized by high-water content. Solid-liquid separation of FVW by crushing-extrusion physical pre-treatment provides fruit and vegetable wastewater (FVWW), and then for anaerobic biological treatment to recover methane, which is considered a cost-effective approach. However, anaerobic treatment of FVWW faces difficulties with low methane productivity and over-accumulation of volatile fatty acids (VFAs). In this study, the expanded granular sludge bed (EGSB) reactor was used for the anaerobic treatment of FVWW and the regulatory strategy to alleviate over-acidification was proposed. When the influent chemical oxygen demand (COD) concentration was 10000 mg/L, the hydraulic retention time (HRT) was 2 days, the organic loading rate (OLR) reached 5 g COD/L/d, the maximum methane productivity reached 301.14 ± 2.32 mL/g COD with a COD removal rate of 96 % ± 2 %. Lower VFAs accumulation was observed when decreasing the influent COD concentration to the same OLR compared with extending HRT, resulting in 38.5 % higher methane productivity. Decreasing the influent COD concentration not only benefited the enrichment of acetogens and hydrogenotrophic methanogens but also specifically enriched syntrophic bacteria. The enhanced syntrophic acetogenesis and hydrogenotrophic methanogens maybe the main reasons for promoting the degradation of propionate and butyrate and improving the methane productivity.

Deciphering Fe@C amendment on long-term anaerobic digestion of sulfate and propionate rich wastewater: Driving microbial community succession and propionate metabolism

This study evaluated the reflection of long-term anaerobic system exposed to sulfate and propionate. Fe@C was found to efficiently mitigate anaerobic sulfate inhibition and enhance propionate degradation. With influent propionate of 12000mgCOD/L and COD/SO42- ratio of 3.0, methane productivity and sulfate removal were only 0.06 ± 0.02L/gCOD and 63 %, respectively. Fe@C helped recover methane productivity to 0.23 ± 0.03L/gCOD, and remove sulfate completely. After alleviating sulfate stress, less organic substrate was utilized to form extracellular polymeric substances for self-protection, which enhanced mass transfer in anaerobic sludge. Microbial community succession, especially for alteration of key sulfate-reducing bacteria and propionate-oxidizing bacteria, was driven by Fe@C, thus enhancing sulfate reduction and propionate degradation. Acetotrophic Methanothrix and hydrogenotrophic unclassified_f_Methanoregulaceae were enriched to promote methanogenesis. Regarding propionate metabolism, inhibited methylmalonyl-CoA degradation was a limiting step under sulfate stress, and was mitigated by Fe@C. Overall, this study provides perspective on Fe@C’s future application on sulfate and propionate rich wastewater treatment.

Multi-omics revealed the mechanism of feed efficiency in sheep by the combined action of the host and rumen microbiota

This study was conducted to investigate potential regulatory mechanisms of feed efficiency (FE) in sheep by linking rumen microbiota with its host by the multi-omics analysis. One hundred and ninety-eight hybrid female sheep (initial body weight = 30.88 ± 4.57 kg; 4-month-old) were selected as candidate sheep. Each test sheep was fed in an individual pen for 60 days, and the residual feed intake (RFI) was calculated. The ten candidate sheep with the highest RFI were divided into the Low-FE group, and the ten with the lowest RFI were divided into the High-FE group, all selected for sample collection. The RFI, average daily gain and average daily feed intake were highly significantly different between the two experimental groups (P < 0.05). Compared with Low-FE group, the insulin-like growth factor-1 and very low-density lipoprotein in serum and the propionate in rumen significantly increased in High-FE group (P < 0.01), but the acetate:propionate ratio in rumen significantly decreased in High-FE group (P = 0.034). Metagenomics revealed Selenomonas ruminantium, Selenomonas sp. and Faecalibacterium prausnitzii were key bacteria, and increased abundance of the genes encoding the enzymes for cellulose degradation and production of propionate in High-FE group. The results of proteomics and section showed the rumen papilla length and expression of carbonic anhydrase and Na+/K+-ATPase were significantly higher in High-FE group (P < 0.05). On the other hand, the acetyl-CoA content significantly increased in the liver of High-FE group (P = 0.002). The relative expression levels of insulin-like growth factor-1 and apolipoprotein A4 genes were significantly up-regulated in the liver of High-FE group (P < 0.05), but relative expression level of monoacylglycerol O-acyltransferase 3 gene was significantly down-regulated (P = 0.037). These findings provide the mechanism by which the collaborative interaction between rumen microbiota fermentation and host uptake and metabolism of fermentation products impacts feed efficiency traits in sheep.

Reverse electron transfer: Novel anaerobic methanogenesis pathway regulated through exogenous CO2 synergized with biochar

Acid accumulation and carbon emission are two major challenges in anaerobic digestion. Syntrophic consortia can employ reverse electron transfer (RET) to facilitate thermodynamically unfavorable redox reactions during acetogenesis. However, the potential mechanisms and regulatory methods of RET remain unclear. This study examines the regulatory mechanisms by which exogenous CO2 affects RET and demonstrates that biochar maximizes CO2 solubility at 25.8 mmol/L to enhance effects further. CO2 synergized with biochar significantly increases cumulative methane production and propionate degradation rate. From the bioenergetic perspective, CO2 decreases energy level to a maximum of −87 kJ/mol, strengthening the thermodynamic viability. The underlying mechanism can be attributed to RET promotion, as indicated by increased formate dehydrogenase and enrichment of H2/formate-producing bacteria with their partner Methanospirillum hungatei. Moreover, the 5 % 13CH4 and methane contribution result show that CO2 accomplishes directed methanogenesis. Overall, this investigation riches the roles of CO2 and biochar in AD surrounding RET.

Potential mechanisms of propionate degradation and methanogenesis in anaerobic digestion coupled with microbial electrolysis cell system: Importance of biocathode

Microbial electrolysis cells (MEC) have the potential for enhancing the efficiency of anaerobic digestion (AD). In this study, microbiological and metabolic pathways in the biocathode of anaerobic digestion coupled with microbial electrolysis cells system (AD-MEC) were revealed to separate bioanode. The biocathode efficiently degraded 90 % propionate within 48 h, leading to a methane production rate of 3222 mL·m-2·d-1. The protein and heme-rich cathodic biofilm enhanced redox capacity and facilitated interspecies electron transfer. Key acid-degrading bacteria, including Dechloromonas agitata, Ignavibacteriales bacterium UTCHB2, and Syntrophobacter fumaroxidans, along with functional proteins such as cytochrome c and e-pili, established mutualistic relationships with Methanothrix soehngenii. This synergy facilitated a multi-pathway metabolic process that converted acetate and CO2 into methane. The study sheds light on the intricate microbial dynamics within the biocathode, suggesting promising prospects for the scalable integration of AD-MEC and its potential in sustainable energy production.

Biochar-mediated synergistic promotion of DIET and IHT: Kinetic and thermodynamic insights into propionate two-step syntrophic methanogenesis

Biochar can effectively enhance the thermophilic anaerobic degradation of propionate. Nevertheless, previous studies frequently ascribed this enhancement to direct interspecies electron transfer (DIET) without considering the underlying kinetics and thermodynamics involved. Additionally, quantitative evaluation of biochar’s impact on interspecies electron transfer (IET) has been limited. In this study, we developed and optimized models for analyzing thermodynamics and electron transfer flux to elucidate the mechanism by which biochar enhances the two-step syntrophic methanogenesis of propionate. Findings revealed that biochar primarily facilitated DIET (1.78 × 10−4 A), leveraging its redox capacity. Additionally, it synergistically enhanced interspecies hydrogen transfer (4.08 × 10−10 A), significantly improving IET efficiency. This synergistic effect led to accelerated oxidative degradation rates of propionate (39.2 %) and intermediate acetate (13.5 %), expanding the thermodynamic windows for their syntrophic methanogenesis (221 % and 103 %, respectively). Accordingly, biochar increased the maximum rate of propionate methanation (71.9 %) and reduced the lag phase time (51.1 %). Moreover, biochar promoted the specific growth rates of acetogenic bacteria and methanogens while significantly increasing the abundance of Pelotomaculum, Methanoculleus, and Methanosarcina. These findings provide new insights and theoretical support for the use of biochar in promoting anaerobic digestion and facilitating IET during the process.

The Stereochemical Course of DmdC, an Enzyme Involved in the Degradation of Dimethylsulfoniopropionate.

The acyl-CoA dehydrogenase DmdC is involved in the degradation of the marine sulfur metabolite dimethylsulfonio propionate (DMSP) through the demethylation pathway. The stereochemical course of this reaction was investigated through the synthesis of four stereoselectively deuterated substrate surrogates carrying stereoselective deuterations at the α- or the β-carbon. Analysis of the products revealed a specific abstraction of the 2-pro-R proton and of the 3-pro-S hydride, establishing an anti elimination for the DmdC reaction.

Underlying the inhibition mechanisms of sulfate and lincomycin on long-term anaerobic digestion: Microbial response and antibiotic resistance genes distribution

This study evaluated the resilience of a long-term anaerobic treatment system exposed to sulfate, lincomycin (LCM) and their combined stress. LCM was found to impede anaerobic propionate degradation, while sulfate for restraining methanogenic acetate utilization. The combined stress, with influent LCM of 200 mg/L and sulfate of 1404 mg/L, revealed severer inhibition on anaerobic digestion than individual inhibition, leading to 73.9 % and 38.5 % decrease in methane production and sulfate removal, respectively. Suppression on propionate-oxidizing bacteria like unclassified_f__Anaerolineae and unclassified_f__Syntrophaceae further demonstrated LCM's inhibitory effect on propionate degradation. Besides, the down-regulation of genes encoding dissimilatory sulfate reduction enzymes caused by LCM triggered great inhibition on sulfate reduction. A notable increase in ARGs was detected under sulfate-stressed condition, owing to its obvious enrichment of tetracycline-resistant genes. Genera including unclassified_f__Syntrophaceae, unclassified_f__Geobacteraceae and unclassified_f__Anaerolineaceae were identified as dominant host of ARGs and enriched by sulfate addition. Overall, these results could provide the theoretical basis for further enhancement on anaerobic digestion of pharmaceutical wastewater containing sulfate and lincomycin.

Mathematical model of anaerobic propionate degradation by incorporating the r/K selection theory

Although anaerobic digestion mathematical model (ADMM) has been widely applied for system optimization and mechanism understanding, the selection of key kinetic and stoichiometric parameter values for ADMM is still a challenge. An ADMM incorporating the r/K selection theory was proposed to examine the enrichment of r- and K-strategists in completely mixed reactors (CMRs) and sequencing batch reactors (SBRs). With the influent propionate concentration (PC) of 0.75 g COD/L, K-strategists predominated at the sludge retention time (SRT) higher than 13 d for propionate oxidizing bacteria (POB) and 9 d for acetoclastic methanogens (AM) in CMRs, while r-strategists dominated at the SRT less than 20 d for POB and 5 d for AM in SBRs. Under the SRT of 20 d, in CMRs, both K-strategists of POB and AM increased with increasing PCs, while r-strategists of POB only appeared when the PC exceeded 6 g COD/L and r-strategists of AM could not be enriched under all PC conditions; in SBRs, K-strategists of POB and AM showed the superiority when the PC was lower than 0.5 and 1.5 g COD/L, respectively. Only K-strategists of hydrogenotrophic methanogens (HM) were enriched in all simulated scenarios. Based on sensitivity analysis, SRT and the first order decay rate had a high influence on the enrichment of HM, AM, and POB. Overall, the selection of r-/K-strategists of POB and AM could be achieved by regulating SRT, operational mode, and PC.

Leveraging anaerobic biodegradation of tetracycline in anaerobic digestion systems with different operational modes

Antibiotics such as tetracycline in wastewater may cause the operation failure of anaerobic digestion systems. With two types of operational modes, continuous-flow reactors (CFRs) and sequencing batch reactors (SBRs) were operated with or without the addition of tetracycline to examine the influence of tetracycline on system performance, tetracycline removal, metabolic activity, and microbial communities. Tetracycline inhibited the removal of chemical oxygen demand (COD) by 23.9% in CFRs and 20.5% in SBRs, and led to the accumulation of volatile fatty acids (VFAs). Also, the maximum methane production rate was inhibited by 24.5% in CFRs and 48.8% in SBRs due to the existence of tetracycline, while the maximum methane production was reduced by 32.8% in CFRs and 13.8% in SBRs. CFRs exhibited a better recovery capacity during COD shock loading and demonstrated superior propionate degradation and methane production capabilities. Moreover, CFRs achieved better tetracycline removal capacity compared to SBRs. Biodegradation was found to be the dominant tetracycline removal pathway, accounting for 88.9% (CFRs) and 82.0% (SBRs). Long-term exposure to tetracycline decreased archaea abundances and inhibited the enrichment of hydrolytic-acidogenic bacteria (Mesotoga and Syner-01). Meanwhile, CFRs provided a more conducive environment for the enrichment of these microorganisms. The presence of Geobacter in conjunction with the detected ethanol in batch experients of CFRs suggested the potential presence of direct interspecies electron transfer, elucidating the enhanced pollutant removal of CFRs. Additionally, Trichococcus, a genus associated with tetracycline removal, was found to be highly enriched with the long-term dosage of tetracycline.

Syntrophic entanglements for propionate and acetate oxidation under thermophilic and high-ammonia conditions

Propionate is a key intermediate in anaerobic digestion processes and often accumulates in association with perturbations, such as elevated levels of ammonia. Under such conditions, syntrophic ammonia-tolerant microorganisms play a key role in propionate degradation. Despite their importance, little is known about these syntrophic microorganisms and their cross-species interactions. Here, we present metagenomes and metatranscriptomic data for novel thermophilic and ammonia-tolerant syntrophic bacteria and the partner methanogens enriched in propionate-fed reactors. A metagenome for a novel bacterium for which we propose the provisional name ‘Candidatus Thermosyntrophopropionicum ammoniitolerans’ was recovered, together with mapping of its highly expressed methylmalonyl-CoA pathway for syntrophic propionate degradation. Acetate was degraded by a novel thermophilic syntrophic acetate-oxidising candidate bacterium. Electron removal associated with syntrophic propionate and acetate oxidation was mediated by the hydrogen/formate-utilising methanogens Methanoculleus sp. and Methanothermobacter sp., with the latter observed to be critical for efficient propionate degradation. Similar dependence on Methanothermobacter was not seen for acetate degradation. Expression-based analyses indicated use of both H2 and formate for electron transfer, including cross-species reciprocation with sulphuric compounds and microbial nanotube-mediated interspecies interactions. Batch cultivation demonstrated degradation rates of up to 0.16 g propionate L−1 day−1 at hydrogen partial pressure 4–30 Pa and available energy was around −20 mol−1 propionate. These observations outline the multiple syntrophic interactions required for propionate oxidation and represent a first step in increasing knowledge of acid accumulation in high-ammonia biogas production systems.

Fermentation Characteristics, Digestibility, and Estimation of Ruminant Methane from Saponin: A Quantitative Study

The effect of using saponins on ruminants' performance differed from several published research data based on the level of saponins added to the feed. This research was conducted to analyze the effect of saponins on fermentation characteristics, digestibility, and estimation of methane in ruminants with a mixed model approach from published journal articles—a total of 127 studies from 32 journals national and international. The variable measured included the level of saponins (%), dry matter intake, Average Daily Gain (ADG), Dry Matter Digestibility (DMD), Organic Matter Digestibility (OMD), Crude protein (CP), Neutral Detergent Fiber (NDF), Acid Detergent Fiber (ADF), protozoa population, bacterial population, Volatile Fatty Acid (VFA), acetate/C2, propionate/C3, butyrate/C4, valerate/C5, acetate/propionate (C2/C3), NH3, pH, and methane gas production. The results showed that using saponins in ruminants increased ADG, CP, ADF, NDF degradation, Total VFA, and proportion of propionate. The addition of saponins level reduced the protozoa population, acetate proportion, and the ratio of acetate: to propionate (C2/C3). In contrast, feed intake and digestibility decreased with the administration of saponin. The bacterial population were similar among treatments, and methane production increased by increasing saponins. In conclusion, the administration of saponin level recommended is 0.3-3.1% of the total ration to improve performance and increase feed efficiency.

Elucidation of the coumarin degradation by Pseudomonas sp. strain NyZ480

As a phytotoxin and synthetic chemical, coumarin (COU) is known for its hepatotoxicity and carcinogenicity. However, no thorough characterization of its microbial degradation has been reported. Here, Pseudomonas sp. strain NyZ480 was isolated for its capability of utilizing COU as the sole carbon source. Studies on its growth and degradation efficiency of COU under various conditions suggested that strain NyZ480 performed the optimum degradation at 30 ℃, pH 7, and 0.5 mM COU was completely removed within 4 h with 1% inoculum. HPLC and LC-MS analyses indicated that dihydrocoumarin (DHC), melilotic acid (MA) and 3-(2,3-dihydroxyphenyl)propionate (DHPP) were the upstream biotransformation intermediates of COU. Enzyme assay established that the initial reaction transforming COU to DHC required an NAD(P)H-dependent reductase, followed by the hydrolysis of DHC to generate MA, and the third reaction catalyzing the monooxygenation of MA to DHPP utilized a strict NADH-dependent hydroxylase. Combining genomics and transcriptomics, we proposed that the COU downstream degradation (from DHPP) was catalyzed by enzymes encoded by a gene cluster homologous to the mhp cluster for 3(3-hydroxyphenyl)propionate degradation via DHPP in E. coli. This study thoroughly identified the intermediates from the COU catabolism, providing essential insights into the molecular evidences of its biodegradation pathway.

Homoacetogenesis is altering the metabolic pathway of acidogenic microbiome and combating volatile fatty acid accumulation in anaerobic reactors

Volatile fatty acids (VFAs) (especially propionate and butyrate) accumulation is known to be bottleneck limiting optimal use of anaerobic digestion (AD). Previous studies targeting optimization of the AD process, mainly focused on strategies for enhancing the degradation of propionate and butyrate. In this study, an alternative method to reduce risk of accumulation of higher VFA, was to promote homoacetogens, which are generating small molecules (such as formate, acetate) that can be easily utilized by methanogens by methanogens. Enrichment of homoacetogens would be advantageous for AD process. Two AD reactors operated at two different temperatures (35 ± 1 °C and 24 ± 1 °C) inoculated with heat-pretreated inoculum were used for this study. After 182 days operation, the quantity of homoacetogens, measured by the number of fhs gene, representing homoacetogens, reached 1.24 × 1014 copies/g wet sludge in ambient temperature (24 °C) reactor, which was much higher than it in mesophilic (35 °C) reactor (1.89 ×1012 copies/g wet sludge). The higher copies at ambient temperature compared to mesophilic, was because homoacetogens were surviving at this temperature. At ambient temperature, the acidification rate increased from 30.57 % to 34.25 %, with the composition of acetate and formate of total VFAs raised from 34.86 % to 55.70 %. It meant that enrichment of homoacetogens was resulting in re-routing carbon to methanogenic precursors, which had a significant positive effect on reducing the risk of VFAs accumulation. The genus Acetobacterium was the main homoacetogen enriched in 24 °C reactors. The abundance of Acetobacterium had a positive correlation with acetate concentration while it had a negative correlation with temperature. This study provides a new approach for mitigation the risk of VFAs accumulation.

Inhibitory mechanisms on dry anaerobic digestion: Ammonia, hydrogen and propionic acid relationship

Inhibitory pathways in dry anaerobic digestion are still understudied and current knowledge on wet processes cannot be easily transferred. This study forced instability in pilot-scale digesters by operating at short retention times (40 and 33 days) in order to understand inhibition pathways over long term operation (145 days). The first sign of inhibition at elevated total ammonia concentrations (8 g/l) was a headspace hydrogen level over the thermodynamic limit for propionic degradation, causing propionic accumulation. The combined inhibitory effect of propionic and ammonia accumulation resulted in further increased hydrogen partial pressures and n-butyric accumulation. The relative abundance of Methanosarcina increased while that of Methanoculleus decreased as digestion deteriorated. It was hypothesized that high ammonia, total solids and organic loading rate inhibited syntrophic acetate oxidisers, increasing their doubling time and resulting in its wash out, which in turn inhibited hydrogenotrophic methanogenesis and shifted the predominant methanogenic pathway towards acetoclastic methanogenesis at free ammonia over 1.5 g/l. C/N increases to 25 and 29 reduced inhibitors accumulation but did not avoid inhibition or the washout of syntrophic acetate oxidising bacteria.