Pilot-scale in-situ biomethanation of sewage sludge: Effects of gas recirculation method

The methanation efficiency and operational stability of a 2 m3 pilot-scale in-situ biomethanation reactor were investigated using on-site sewage sludge as the substrate, at a wastewater treatment plant. In parallel, a laboratory-scale study was conducted. Hydrogen conversion efficiencies of 96.7 and 97.5 %, and average methane contents of 84.2 and 83.2 % were obtained, for the laboratory and pilot experiments, respectively. The pilot-scale digester was operated at various conditions for 137 d, of which the last 30 d were stable with a high biomethanation efficiency and an average pH of 8.2. Gas recirculation increased the hydrogen conversion efficiency. When hydrogen injection and gas recirculation were applied separately, a 96 % lower gas recirculation rate was needed to achieve the same hydrogen conversion efficiency, compared to a mixture of hydrogen injection and gas recirculation in the same line. These findings may facilitate the selection of suitable gas recirculation concepts for practical biomethanation applications.

Atrimonas thermophila gen. nov., sp. nov., a novel anaerobic thermophilic bacterium of the phylum Atribacterota isolated from deep subsurface gas field and proposal of Atrimonadaceae fam. nov. within the class Atribacteria in the phylum Atribacterota

A novel anaerobic, thermophilic bacterium of the class Atribacteria, strain M15T, was isolated from a high-temperature gas reservoir, Japan. Cells of strain M15T were gram-negative, short oval-shaped, and lacked flagella. Growth occurred at 45-75 °C (optimum 70-75 °C) and pH 6.5-8.5 (optimum pH 7.5-8.0) and was fast under optimal conditions (doubling time 11.4 h). Yeast extract was required for growth. Fermentative growth with glucose, arabinose, xylose, and cellobiose was observed. The major fermentative end products of glucose were acetate and hydrogen. The major cellular fatty acids were C16:0, iso-C15:0, and C18:0. The genomic G + C content was 46.0 mol%. Fluorescence and electron microscopy observations revealed the intracellular localization of genomic DNA surrounded by a membrane in the cells of strain M15T as reported in a sole validly described species of the class Atribacteria in the phylum Atribacterota, Atribacter laminatus strain RT761T, suggesting that the unique morphological traits are widely shared in this class. Phylogenetic analyses indicated that strain M15T belongs to a distinct family-level lineage in the class Atribacteria and shows low similarities to Atribacter laminatus strain RT761T (16S rRNA gene sequence identity of 90.1 %, average nucleotide identity [ANI] of 66.1 %, average amino acid identity [AAI] of 55.8 %). Phenotypic traits of strain M15T (thermophilic, fast-growing, relatively high G + C content, etc.) were clearly distinct from A. laminatus. Based on these phenotypic and genomic properties, we propose a novel genus and species, Atrimonas thermophila gen. nov., sp. nov. for strain M15T (=JCM39389T, =KCTC25731T) representing a novel family Atrimonadaceae fam., nov. in the class Atribacteria.

Diverse electron carriers drive syntrophic interactions in an enriched anaerobic acetate-oxidizing consortium

In many anoxic environments, syntrophic acetate oxidation (SAO) is a key pathway mediating the conversion of acetate into methane through obligate cross-feeding interactions between SAO bacteria (SAOB) and methanogenic archaea. The SAO pathway is particularly important in engineered environments such as anaerobic digestion (AD) systems operating at thermophilic temperatures and/or with high ammonia. Despite the widespread importance of SAOB to the stability of the AD process, little is known about their in situ physiologies due to typically low biomass yields and resistance to isolation. Here, we performed a long-term (300-day) continuous enrichment of a thermophilic (55 °C) SAO community from a municipal AD system using acetate as the sole carbon source. Over 80% of the enriched bioreactor metagenome belonged to a three-member consortium, including an acetate-oxidizing bacterium affiliated with DTU068 encoding for carbon dioxide, hydrogen, and formate production, along with two methanogenic archaea affiliated with Methanothermobacter_A. Stable isotope probing was coupled with metaproteogenomics to quantify carbon flux into each community member during acetate conversion and inform metabolic reconstruction and genome-scale modeling. This effort revealed that the two Methanothermobacter_A species differed in their preferred electron donors, with one possessing the ability to grow on formate and the other only consuming hydrogen. A thermodynamic analysis suggested that the presence of the formate-consuming methanogen broadened the environmental conditions where ATP production from SAO was favorable. Collectively, these results highlight how flexibility in electron partitioning during SAO likely governs community structure and fitness through thermodynamic-driven mutualism, shedding valuable insights into the metabolic underpinnings of this key functional group within methanogenic ecosystems.

Novel quantitative method for individual isotopomer of organic acids from 13C tracer experiments determines carbon flow in acetogenesis

Anaerobic microbial acetogenesis is ubiquitous on Earth, and thus plays an important role in the global carbon cycle. The mechanism of carbon fixation in acetogens has attracted great interest from various studies for combatting climate change, and even for studying ancient metabolic pathways. Here, we developed a new, simple method for investigating carbon flows in the metabolic reaction of acetogen by conveniently and accurately determining the relative abundance of individual acetate- and/or formate-isotopomers formed in ¹³C labeling experiments. We measured the underivatized analyte by gas chromatography-mass spectrometry (GC-MS) coupled with a direct aqueous sample injection technique. The individual abundance of analyte isotopomers was calculated by the mass spectrum analysis using the least-squares approach. The validity of the method was demonstrated by determining known mixtures of unlabeled and ¹³C-labeled analytes. The developed method was applied to study the carbon fixation mechanism of the well-known acetogen Acetobacterium woodii grown on methanol and bicarbonate. We provided a quantitative reaction model for methanol metabolism of A. woodii, which indicated that methanol was not the sole carbon precursor of the acetate methyl group and that 20-22% of the methyl group was formed from CO₂. In contrast, the carboxyl group of acetate appeared to form exclusively by CO₂ fixation. Thus, our simple method, without laborious analytical procedures, has broad utility for the study of biochemical and chemical processes related to acetogenesis on Earth.

Isolation of a Novel Thermophilic Methanogen and the Evolutionary History of the Class Methanobacteria

Methanogens can produce methane in anaerobic environments via the methanogenesis pathway, and are regarded as one of the most ancient life forms on Earth. They are ubiquitously distributed across distinct ecosystems and are considered to have a thermophilic origin. In this study, we isolated, pure cultured, and completely sequenced a single methanogen strain DL9LZB001, from a hot spring at Tengchong in Southwest China. DL9LZB001 is a thermophilic and hydrogenotrophic methanogen with an optimum growth temperature of 65 °C. It is a putative novel species, which has been named Methanothermobacter tengchongensis-a Class I methanogen belonging to the class Methanobacteria. Comparative genomic and ancestral analyses indicate that the class Methanobacteria originated in a hyperthermal environment and then evolved to adapt to ambient temperatures. This study extends the understanding of methanogens living in geothermal niches, as well as the origin and evolutionary history of these organisms in ecosystems with different temperatures.

Draft Genome Sequence of the Toluene-Degrading, Dissimilatory Sulfate-Reducing Bacterium Desulforhabdus sp. Strain TSK

The draft genome sequence of Desulforhabdus sp. strain TSK, which oxidizes toluene under dissimilatory sulfate-reducing conditions, had an estimated size of 4,933,642 bp.

Complete Genomic Sequence of the Thermophilic Hydrogen-Oxidizing Methanogen Methanothermobacter tenebrarum Strain RMAS T

Methanothermobacter tenebrarum strain RMAS^T has a complete genomic length of 1,472,762 bp, a GC content of 42.1%, 1,599 coding DNA sequences (CDSs), 1 CRISPR array, 3 rRNAs, and 38 tRNAs.

Characterization of biogas production and microbial community in thermophilic anaerobic co-digestion of sewage sludge and paper waste

To recover the biogas from sewage sludge and paper waste (PW), the methanogenic performance of thermophilic anaerobic co-digestion of sewage sludge with PW was assessed by a continuous experiment. The effects on the biogas production and microbial community were investigated by changing the PW ratio from 0 to 66.7%. The optimal performance was obtained at the ratio of sewage sludge: PW = 4:6 (total solids), where the COD removal efficiency and biogas production increased from 58.34±5.90% to 72.92±0.08% and 438±53 to 594±72 mL/g-VS_added, respectively. By investigating the trend of carbohydrate and protein degradation rates, the competition between carbohydrate and protein degradation was quantified. The critical PW addition ratio was about (63.64%), where the protein degradation rate decreased to zero with increasing PW addition. Meanwhile, the microbial analysis showed that cellulolytic bacteria outcompeted proteolytic bacteria and to be the predominant group after PW addition.

The process of methanogenesis in paddy fields under different elevated CO2 concentrations

Understanding the process of methanogenesis in paddy fields under the scenarios of future climate change is of great significance for reducing greenhouse gas emissions and regulating the soil carbon cycle. Methyl Coenzyme M Reductase subunit A (mcrA) of methanogens is a rate-limiting enzyme that catalyzes the final step of CH₄ production. However, the mechanism of methanogenesis change in the paddy fields under different elevated CO₂ concentrations (e[CO₂]) is rarely explored in earlier studies. In this research, we explored how the methanogens affect CH₄ flux in paddy fields under various (e[CO₂]). CH₄ flux and CH₄ production potential (MPP), and mcrA gene abundance were quantitatively analyzed under C (ambient CO₂ concentration), C₁ (C + 160 ppm CO₂), and C₂ (C + 200 ppm CO₂) treatments. Additionally, the community composition and structure of methanogens were also compared with Illumina MiSeq sequencing. The results showed that C₂ treatment significantly increased CH₄ flux and MPP at the tillering stage. E[CO₂] had a positive effect on the abundance of methanogens, but the effect was insignificant. We detected four known dominant orders of methanogenesis in this study, such as Methanosarcinales, Methanobacteriales, Methanocellales, and Methanomicrobiales. Although e[CO₂] did not significantly change the overall community structure and diversity of methanogens, C₂ treatment significantly reduced the relative abundance of two uncultured genera compared to C treatment. A linear regression model of DOC, methanogenic abundance, and MPP can explain 67.2% of the variation of CH₄ flux under e[CO₂]. Overall, our results demonstrated that CH₄ flux in paddy fields under e[CO₂] was mainly controlled by soil unstable C substrate and the abundance and activity of methanogens in rhizosphere soil.

Reduction of alkalinity supplementation for acid-based wastewater treatment using a thermophilic multi-feed upflow anaerobic sludge blanket reactor

Generally, Shochu distillery wastewater treatment is required the addition of alkalinity agents for an increase of pH in the UASB reactor. However, to reduce the cost of alkalinity supplementation, cost-effective reactor operation method has been desired. This study aimed to reduce the alkalinity supplementation for a thermophilic (55°C) multi-feed up-flow anaerobic sludge blanket (MF UASB) reactor for the low-cost treatment of the wastewater from the production of the Japanese distilled alcohol called shochu. Shochu distillery wastewater contains high concentrations of organics (46,500-57,600 mgCOD L^-1; COD: chemical oxygen demand) and volatile fatty acids (16,200-25,000 mgCOD L^-1), and low pH (4.1-4.5). With alkalinity supplementation of 0.045 mgCaCO₃ mgCOD^-1 using 24% NaOH, the MF UASB reactor achieved an 87 ± 2% COD removal rate with an organic loading rate of 24 kgCOD m^-3 day^-1 for 554 days reactor operation (hydraulic retention time of 10 h and influent concentration of 10,000 mgCOD L^-1). The organic removal rate decreased to 19 ± 3% in the MF UASB reactor when alkalinity supplementation was reduced to 0.015 mgCaCO₃·mgCOD^-1. In this study , the minimum alkalinity supplementation was 0.045 mgCaCO₃ mgCOD^-1 at an organic loading rate of 24 kgCOD m^-3 day^-1.

Response of high-, mid- and low-abundant taxa and potential pathogens to eight disinfection methods and their interactions in domestic hot water system

Eight disinfection methods were applied to control biofilm contamination in domestic hot water system. The inactivation efficiency, responses of high- (≥1%), mid- (0.1% ~ 1%) and low-abundant taxa (≤0.1%) to disinfection, and interactions within and across three sub-communities were investigated. Ultraviolet was the most effective disinfection method for total bacteria and Escherichia coli, and chlorine dioxide had the highest inactivation efficiency on heterotrophic bacteria, while silver ions exhibited poor performance on all of them. At the phylum level, the responses of microorganisms to eight disinfection methods were different, but Proteobacteria and Firmicutes dominated in most samples. Eight disinfection methods had a greater impact on the proportion of high- and mid-abundant taxa than that of low-abundant taxa, and led to dissimilar transformations of genera among high-, mid- and low-abundant taxa in each sample. High-, mid- and low-abundant taxa of different samples showed similar structures and were roughly clustered into three Groups. Moreover, high-abundant taxa had more complex internal interactions than mid- and low-abundant taxa, and mainly presented co-occurrence patterns. The associations between high- and low-abundant taxa were close, and some low-abundant genera were identified as hub bacteria, such as Paracoccus, Thioalkalispira and Flavitalea. Furthermore, a total of 23 potential pathogens were detected in this study, and they mainly showed positive interactions, with Mycobacteria and Streptococcus as keystone genera. These results highlight the dissimilar responses of high-, mid- and low-abundant taxa to disinfection, and the critical role of some low-abundant genera in the microbial network, as well as the co-occurrence patterns among potential pathogens.

Koleobacter methoxysyntrophicus gen. nov., sp. nov., a novel anaerobic bacterium isolated from deep subsurface oil field and proposal of Koleobacteraceae fam. nov. and Koleobacterales ord. nov. within the class Clostridia of the phylum Firmicutes

An anaerobic thermophilic, rod-shaped bacterium possessing a unique non-lipid sheathed-like structure enveloping a single-membraned cell, designated strain NRmbB1^T was isolated from at the deep subsurface oil field located in Yamagata Prefecture, Japan. Growth occurred with 40-60°C (optimum, 55°C), 0-2% (2%), NaCl and pH 6.0-8.5 (8.0). Fermentative growth with various sugars was observed. Glucose-grown cells generated acetate, hydrogen, pyruvate and lactate as the main end products. Syntrophic growth occurred with glucose, pyruvate and 3,4,5-trimethoxybenzoate in the presence of an H₂-scavenging partner, and growth on 3,4,5-trimethoxybenzoate was only observed under syntrophic condition. The predominant cellular fatty acids were C_16:0, iso-C_16:0, anteiso-C_15:0, and iso-C_14:0. Respiratory quinone was not detected. The genomic G+C content was 40.8mol%. Based on 16S rRNA gene phylogeny, strain NRmbB1^T belongs to a distinct order-level clade in the class Clostridia of the phylum Firmicutes, sharing low similarity with other isolated organisms (i.e., 87.5% for top hit Moorella thermoacetica DSM 2955^T). In total, chemotaxonomic, phylogenetic and genomic characterization revealed that strain NRmbB1^T (=KCTC 25035^T, =JCM 39120^T) represents a novel species of a new genus. In addition, we also propose the associated family and order as Koleobacteraceae fam. nov and Koleobacterales ord. nov., respectively.

Cross-Stress Adaptation in a Piezophilic and Hyperthermophilic Archaeon From Deep Sea Hydrothermal Vent

Hyperthermophiles, living in environments above 80°C and usually coupling with multi-extreme environmental stresses, have drawn great attention due to their application potential in biotechnology and being the primitive extant forms of life. Studies on their survival and adaptation mechanisms have extended our understanding on how lives thrive under extreme conditions. During these studies, the "cross-stress" behavior in various organisms has been observed between the extreme high temperature and other environmental stresses. Despite the broad observation, the global view of the cross-stress behavior remains unclear in hyperthermophiles, leaving a knowledge gap in our understanding of extreme adaptation. In this study, we performed a global quantitative proteomic analysis under extreme temperatures, pH, hydrostatic pressure (HP), and salinity on an archaeal strain, Thermococcus eurythermalis A501, which has outstanding growth capability on a wide range of temperatures (50-100°C), pH (4-9), and HPs (0.1-70 MPa), but a narrow range of NaCl (1.0-5.0 %, w/v). The proteomic analysis (79.8% genome coverage) demonstrated that approximately 61.5% of the significant differentially expressed proteins (DEPs) responded to multiple stresses. The responses to most of the tested stresses were closely correlated, except the responses to high salinity and low temperature. The top three enriched universal responding processes include the biosynthesis and protection of macromolecules, biosynthesis and metabolism of amino acids, ion transport, and binding activities. In addition, this study also revealed that the specific dual-stress responding processes, such as the membrane lipids for both cold and HP stresses and the signal transduction for both hyperosmotic and heat stresses, as well as the sodium-dependent energetic processes might be the limiting factor of the growth range in salinity. The present study is the first to examine the global cross-stress responses in a piezophilic hyperthermophile at the proteomic level. Our findings provide direct evidences of the cross-stress adaptation strategy (33.5% of coding-genes) to multiple stresses and highlight the specific and unique responding processes (0.22-0.63% of coding genes for each) to extreme temperature, pH, salinity, and pressure, which are highly relevant to the fields of evolutionary biology as well as next generation industrial biotechnology (NGIB).

pH and Phosphate Induced Shifts in Carbon Flow and Microbial Community during Thermophilic Anaerobic Digestion

pH is a central environmental factor influencing CH4 production from organic substrates, as every member of the complex microbial community has specific pH requirements. Here, we show how varying pH conditions (5.0-8.5, phosphate buffered) and the application of a phosphate buffer per se induce shifts in the microbial community composition and the carbon flow during nine weeks of thermophilic batch digestion. Beside monitoring the methane production as well as volatile fatty acid concentrations, amplicon sequencing of the 16S rRNA gene was conducted. The presence of 100 mM phosphate resulted in reduced CH4 production during the initial phase of the incubation, which was characterized by a shift in the dominant methanogenic genera from a mixed Methanosarcina and Methanoculleus to a pure Methanoculleus system. In buffered samples, acetate strongly accumulated in the beginning of the batch digestion and subsequently served as a substrate for methanogens. Methanogenesis was permanently inhibited at pH values ≤5.5, with the maximum CH4 production occurring at pH 7.5. Adaptations of the microbial community to the pH variations included shifts in the archaeal and bacterial composition, as less competitive organisms with a broad pH range were able to occupy metabolic niches at unfavorable pH conditions.

Isolation of an Obligate Mixotrophic Methanogen That Represents the Major Population in Thermophilic Fixed-Bed Anaerobic Digesters

Methanothermobacter Met2 is a metagenome-assembled genome (MAG) that encodes a putative mixotrophic methanogen constituting the major populations in thermophilic fixed-bed anaerobic digesters. In order to characterize its physiology, the present work isolated an archaeon (strain Met2-1) that represents Met2-type methanogens by using a combination of enrichments under a nitrogen atmosphere, colony formation on solid media and limiting dilution under high partial pressures of hydrogen. Strain Met2-1 utilizes hydrogen and carbon dioxide for methanogenesis, while the growth is observed only when culture media are additionally supplemented with acetate. It does not grow on acetate in the absence of hydrogen. The results demonstrate that Methanothermobacter sp. strain Met2-1 is a novel methanogen that exhibits obligate mixotrophy.

Genome Analyses and Genome-Centered Metatranscriptomics of Methanothermobacter wolfeii Strain SIV6, Isolated from a Thermophilic Production-Scale Biogas Fermenter

In the thermophilic biogas-producing microbial community, the genus Methanothermobacter was previously described to be frequently abundant. The aim of this study was to establish and analyze the genome sequence of the archaeal strain Methanothermobacter wolfeii SIV6 originating from a thermophilic industrial-scale biogas fermenter and compare it to related reference genomes. The circular chromosome has a size of 1,686,891 bases, featuring a GC content of 48.89%. Comparative analyses considering three completely sequenced Methanothermobacter strains revealed a core genome of 1494 coding sequences and 16 strain specific genes for M. wolfeii SIV6, which include glycosyltransferases and CRISPR/cas associated genes. Moreover, M. wolfeii SIV6 harbors all genes for the hydrogenotrophic methanogenesis pathway and genome-centered metatranscriptomics indicates the high metabolic activity of this strain, with 25.18% of all transcripts per million (TPM) belong to the hydrogenotrophic methanogenesis pathway and 18.02% of these TPM exclusively belonging to the mcr operon. This operon encodes the different subunits of the enzyme methyl-coenzyme M reductase (EC: 2.8.4.1), which catalyzes the final and rate-limiting step during methanogenesis. Finally, fragment recruitment of metagenomic reads from the thermophilic biogas fermenter on the SIV6 genome showed that the strain is abundant (1.2%) within the indigenous microbial community. Detailed analysis of the archaeal isolate M. wolfeii SIV6 indicates its role and function within the microbial community of the thermophilic biogas fermenter, towards a better understanding of the biogas production process and a microbial-based management of this complex process.

Biological hydrogen methanation systems - an overview of design and efficiency

The rise in intermittent renewable electricity production presents a global requirement for energy storage. Biological hydrogen methanation (BHM) facilitates wind and solar energy through the storage of otherwise curtailed or constrained electricity in the form of the gaseous energy vector biomethane. Biological methanation in the circular economy involves the reaction of hydrogen - produced during electrolysis - with carbon dioxide in biogas to produce methane (4H2 + CO2 = CH4 + 2H2), typically increasing the methane output of the biogas system by 70%. In this paper, several BHM systems were researched and a compilation of such systems was synthesized, facilitating comparison of key parameters such as methane evolution rate (MER) and retention time. Increased retention times were suggested to be related to less efficient systems with long travel paths for gases through reactors. A significant lack of information on gas-liquid transfer co-efficient was identified.

The microbial zonation of SRB and soNRB enhanced the performance of SR-DSR process under the micro-aerobic condition

The micro-aerobic condition has proven to effectively enhance the COD removal and elemental sulfur (S⁰) transformation rate in the sulfate reduction-denitrifying sulfide removal (SR-DSR) process. However, the mechanisms of how micro-aerobic condition enhances S⁰ transformation remain largely unknown. Therefore in this work an integrated investigation was performed to document the mechanisms and the effect of different startup modes (micro-aerobic startup (termed as mSR-DSR) and anaerobic startup (termed as aSR-DSR)) on bioreactor performance and microbial community dynamics. The results showed that micro-aerobic startup achieved a shorter period to reach a stable performance for SR-DSR, which could be one of the factors affecting the choice of the bioreactor startup mode considering engineering application. For all the tested conditions, removal of nitrate, sulfate and lactate were 100%, >80% and 100%, respectively. The maximum transformation rate of elemental sulfur in mSR-DSR was 57%, which was higher than that in aSR-DSR. The mechanism explorations revealed that micro-aerobic condition not only particularly enriched the sulfide-oxidizing, nitrate-reducing bacteria (soNRB) but also promoted the microbial zonation of sulfate-reducing bacteria (SRB) and soNRB, thereby achieving more S⁰ transformation in the effluent. Under micro-aerobic condition, SRB were mainly distributed in the bottom and middle part of the reactor, while soNRB were assembled in the top. The relative abundance of soNRB in both aSR-DSR and mSR-DSR notably increased to 41.5% and 23.7% at the top when 5 mL air min^-1 L_reactor^-1 was applied. Furthermore, the degradation of organic carbon was also accelerated under micro-aerobic condition, possibly due to the enrichment of organic compounds degrading bacteria Bacteroidetes_vadin HA17.

Characterization of microbial community and main functional groups of prokaryotes in thermophilic anaerobic co-digestion of food waste and paper waste

The thermophilic anaerobic co-digestion of food waste and paper waste was successfully operated with a 0% to 70% fraction of paper waste. The variation of functional microbial community was investigated by 16S rRNA gene analysis. The results indicated that the hydrolyzing bacterial community changed from carbohydrate/protein-degrading bacteria to cellulose-degrading bacteria when the paper waste ratio was higher than 50%. Significant changes in the taxon responsible for cellulose degradation were found depending on the paper waste fraction. Cellulose-degrading bacteria outcompeted lactic acid bacteria in the degradation of monosaccharide, resulting in a decline in the proportion of lactic acid bacteria and the absence of an accumulation of lactic acid. At high paper waste ratios, because the cellulose-degrading bacteria, such as Defluviitoga tunisiensis, were more likely to degrade monosaccharides directly to acetate and hydrogen rather than to propionate and butyrate, the abundance of syntrophs was reduced. The variation of those bacteria with high H₂-producing ability significantly influenced the proportion of hydrogenotrophic archaea. The change in the microbial community as the paper waste fraction increased was illustrated with regard to anaerobic degradation steps.

Effects of CO on hydrogenotrophic methanogenesis under thermophilic and extreme-thermophilic conditions: Microbial community and biomethanation pathways

Coke oven gas is considered as a potential hydrogen source for biogas bio-upgrading. In this study, the effects of CO on biomethanation performance and microbial community structure of hydrogenotrophic mixed cultures were investigated under thermophilic (55 °C) and extreme-thermophilic (70 °C) conditions. 5% (v/v) CO did not inhibit hydrogenotrophic methanogenesis during semi-continuous operation, and 83-97% CO conversion to CH₄ was achieved. Methanothermobacter thermoautotrophicus was the dominant methanogen at both temperatures and was the main functional archaea associated with CO biomethanation. Specific methanogenic activity test results showed that long-term 5% CO acclimation shortened the lag phase from 5 h to 1 h at 55 °C and 15 h to 3 h at 70 °C. CO₂ was the preferred carbon source over CO for hydrogenotrophic methanogens and CO consumption only started when CO₂ was completely depleted. M. thermoautotrophicus dominated mixed cultures showed a great potential in simultaneous hydrogenotrophic methanogenesis and CO biomethanation.

Addition of granular activated carbon and trace elements to favor volatile fatty acid consumption during anaerobic digestion of food waste

The effect of supplementing granular activated carbon and trace elements on the anaerobic digestion performance of consecutive batch reactors treating food waste was investigated. The results from the first batch suggest that addition of activated carbon favored biomass acclimation, improving acetic acid consumption and enhancing methane production. Adding trace elements allowed a faster consumption of propionic acid. A second batch proved that a synergy existed when activated carbon and trace elements were supplemented simultaneously. The degradation kinetics of propionate oxidation were particularly improved, reducing significantly the batch duration and improving the average methane productivities. Addition of activated carbon favored the growth of archaea and syntrophic bacteria, suggesting that interactions between these microorganisms were enhanced. Interestingly, microbial analyses showed that hydrogenotrophic methanogens were predominant. This study shows for the first time that addition of granular activated carbon and trace elements may be a feasible solution to stabilize food waste anaerobic digestion.

Responses of Microbial Community Composition to Temperature Gradient and Carbon Steel Corrosion in Production Water of Petroleum Reservoir

Oil reservoir production systems are usually associated with a temperature gradient and oil production facilities frequently suffer from pipeline corrosion failures. Both bacteria and archaea potentially contribute to biocorrosion of the oil production equipment. Here the response of microbial populations from the petroleum reservoir to temperature gradient and corrosion of carbon steel coupons were investigated under laboratory condition. Carbon steel coupons were exposed to production water from a depth of 1809 m of Jiangsu petroleum reservoir (China) and incubated for periods of 160 and 300 days. The incubation temperatures were set at 37, 55, and 65°C to monitoring mesophilic, thermophilic and hyperthermophilic microorganisms associated with anaerobic carbon steel corrosion. The results showed that corrosion rate at 55°C (0.162 ± 0.013 mm year^-1) and 37°C (0.138 ± 0.008 mm year^-1) were higher than that at 65°C (0.105 ± 0.007 mm year^-1), and a dense biofilm was observed on the surface of coupons under all biotic incubations. The microbial community analysis suggests a high frequency of bacterial taxa associated with families Porphyromonadaceae, Enterobacteriaceae, and Spirochaetaceae at all three temperatures. While the majority of known sulfate-reducing bacteria, in particular Desulfotignum, Desulfobulbus and Desulfovibrio spp., were predominantly observed at 37°C; Desulfotomaculum spp., Thermotoga spp. and Thermanaeromonas spp. as well as archaeal members closely related to Thermococcus and Archaeoglobus spp. were substantially enriched at 65°C. Hydrogenotrophic methanogens of the family Methanobacteriaceae were dominant at both 37 and 55°C; acetoclastic Methanosaeta spp. and methyltrophic Methanolobus spp. were enriched at 37°C. These observations show that temperature changes significantly alter the microbial community structure in production fluids and also affected the biocorrosion of carbon steel under anaerobic conditions.

Impact of temperatures on microbial community structures of sewage sludge biological hydrolysis

This study investigated the biological hydrolysis performance at 35°C (BH35), 42°C (BH42), and 55°C (BH55) and the effect of temperatures on microbial communities of the hydrolyzed sludge. The results showed that the suspended solid reduction, volatile fatty acids (VFA) production, and biogas production increased with the BH temperatures. VFAs produced in the sludge BH included acetic acid, propionic acid, isobutyric acid, butyric acid, and isovaleric acid with the fractions of acetic acid increased with BH temperatures. The Illumina MiSeq sequencing analysis showed that the microbial taxonomic structures of the BH systems varied with BH temperatures. It was found that Acidaminobacter at 35°C, Proteiniphilum and Lutispor at 42°C, and Gelria at 55°C were the main protein fermenting bacteria genera, while the carbohydrate fermenting bacteria might belong to the genera of Macellibacteroides and Paludibacter at 35°C, Fronticella at 42°C, and Tepidimicrobium at 55°C.

Petrothermobacter organivorans gen. nov., sp. nov., a thermophilic, strictly anaerobic bacterium of the phylum Deferribacteres isolated from a deep subsurface oil reservoir

A novel thermophilic, anaerobic, chemoheterotrophic, acetate-oxidizing and iron(III)-, manganese(IV)-, nitrate- and sulfate-reducing bacterium, designated strain ANA^T, was isolated from a deep subsurface oil field in Japan (Yabase oil field, Akita Pref.). Cells of strain ANA^T were Gram-stain-negative, non-motile, non-spore forming and slightly curved or twisted rods (1.5-5.0 µm long and 0.6-0.7 µm wide). The isolate grew at 25-60 °C (optimum 55 °C) and pH 6.0-8.0 (optimum pH 7.0). The isolate was capable of reducing iron(III), manganese(IV), nitrate and sulfate as an electron acceptor. The isolate utilized a limited range of electron donors such as acetate, lactate, pyruvate and yeast extract for iron reduction. Strain ANA^T also used pyruvate, fumarate, succinate, malate, yeast extract and peptone for fermentative growth. The major respiratory quinones were menaquinone-7(H8) and menaquinone-8. The strain contained C18 : 0, iso-C18 : 0 and C16 : 0 as the major cellular fatty acids. The G+C content of the genomic DNA was 34.3 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain ANA^T was closely related to Calditerrivibrio nitroreducens in the phylum Deferribacteres with low sequence similarities (89.5 %), and formed a distinct clade within the family Deferribacteraceae. In addition, the isolate is the first sulfate-reducing member of the phylum Deferribacteres. Based on phenotypic, chemotaxonomic and phylogenetic properties, a novel genus and species, Petrothermobacter organivorans gen. nov., sp. nov., is proposed for the isolate (type strain=ANA^T= NBRC 112621^T=DSM 105015^T).

Non-autotrophic methanogens dominate in anaerobic digesters

Anaerobic digesters are man-made habitats for fermentative and methanogenic microbes, and are characterized by extremely high concentrations of organics. However, little is known about how microbes adapt to such habitats. In the present study, we report phylogenetic, metagenomic, and metatranscriptomic analyses of microbiomes in thermophilic packed-bed digesters fed acetate as the major substrate, and we have shown that acetoclastic and hydrogenotrophic methanogens that utilize acetate as a carbon source dominate there. Deep sequencing and precise binning of the metagenomes reconstructed complete genomes for two dominant methanogens affiliated with the genera Methanosarcina and Methanothermobacter, along with 37 draft genomes. The reconstructed Methanosarcina genome was almost identical to that of a thermophilic acetoclastic methanogen Methanosarcina thermophila TM-1, indicating its cosmopolitan distribution in thermophilic digesters. The reconstructed Methanothermobacter (designated as Met2) was closely related to Methanothermobacter tenebrarum, a non-autotrophic hydrogenotrophic methanogen that grows in the presence of acetate. Met2 lacks the Cdh complex required for CO2 fixation, suggesting that it requires organic molecules, such as acetate, as carbon sources. Although the metagenomic analysis also detected autotrophic methanogens, they were less than 1% in abundance of Met2. These results suggested that non-autotrophic methanogens preferentially grow in anaerobic digesters containing high concentrations of organics.

Biologically Produced Methane as a Renewable Energy Source

Methanogens are a unique group of strictly anaerobic archaea that are more metabolically diverse than previously thought. Traditionally, it was thought that methanogens could only generate methane by coupling the oxidation of products formed by fermentative bacteria with the reduction of CO₂. However, it has recently been observed that many methanogens can also use electrons extruded from metal-respiring bacteria, biocathodes, or insoluble electron shuttles as energy sources. Methanogens are found in both human-made and natural environments and are responsible for the production of ∼71% of the global atmospheric methane. Their habitats range from the human digestive tract to hydrothermal vents. Although biologically produced methane can negatively impact the environment if released into the atmosphere, when captured, it can serve as a potent fuel source. The anaerobic digestion of wastes such as animal manure, human sewage, or food waste produces biogas which is composed of ∼60% methane. Methane from biogas can be cleaned to yield purified methane (biomethane) that can be readily incorporated into natural gas pipelines making it a promising renewable energy source. Conventional anaerobic digestion is limited by long retention times, low organics removal efficiencies, and low biogas production rates. Therefore, many studies are being conducted to improve the anaerobic digestion process. Researchers have found that addition of conductive materials and/or electrically active cathodes to anaerobic digesters can stimulate the digestion process and increase methane content of biogas. It is hoped that optimization of anaerobic digesters will make biogas more readily accessible to the average person.

Assessing the Ecophysiology of Methanogens in the Context of Recent Astrobiological and Planetological Studies

Among all known microbes capable of thriving under extreme and, therefore, potentially extraterrestrial environmental conditions, methanogens from the domain Archaea are intriguing organisms. This is due to their broad metabolic versatility, enormous diversity, and ability to grow under extreme environmental conditions. Several studies revealed that growth conditions of methanogens are compatible with environmental conditions on extraterrestrial bodies throughout the Solar System. Hence, life in the Solar System might not be limited to the classical habitable zone. In this contribution we assess the main ecophysiological characteristics of methanogens and compare these to the environmental conditions of putative habitats in the Solar System, in particular Mars and icy moons. Eventually, we give an outlook on the feasibility and the necessity of future astrobiological studies concerning methanogens.

Prokaryotic phylogenetic diversity of Hungarian deep subsurface geothermal well waters

Geothermal wells characterized by thermal waters warmer than 30°C can be found in more than 65% of the area of Hungary. The examined thermal wells located nearby Szarvas are used for heating industrial and agricultural facilities because of their relatively high hydrocarbon content. The aim of this study was to reveal the prokaryotic community structure of the water of SZR18, K87 and SZR21 geothermal wells using molecular cloning methods and Denaturing Gradient Gel Electrophoresis (DGGE). Water samples from the outflow pipes were collected in 2012 and 2013. The phylogenetic distribution of archaeal molecular clones was very similar in each sample, the most abundant groups belonged to the genera Methanosaeta, Methanothermobacter and Thermofilum. In contrast, the distribution of bacterial molecular clones was very diverse. Many of them showed the closest sequence similarities to uncultured clone sequences from similar thermal environments. From the water of the SZR18 well, phylotypes closely related to genera Fictibacillus and Alicyclobacillus (Firmicutes) were only revealed, while the bacterial diversity of the K87 well water was much higher. Here, the members of the phyla Thermodesulfobacteria, Proteobacteria, Nitrospira, Chlorobi, OP1 and OPB7 were also detected besides Firmicutes.

Roles of thermophilic thiosulfate-reducing bacteria and methanogenic archaea in the biocorrosion of oil pipelines

Thermophilic sulfide-producing microorganisms from an oil pipeline network were enumerated with different sulfur oxyanions as electron acceptors at 55°C. Most-probable number (MPN) analysis showed that thiosulfate-reducing bacteria were the most numerous sulfidogenic microorganisms in pipeline inspection gauge (PIG) scrapings. Thiosulfate-reducing and methanogenic enrichments were obtained from the MPN cultures that were able to use yeast extract as the electron donor. Molecular analysis revealed that both enrichments harbored the same dominant bacterium, which belonged to the genus Anaerobaculum. The dominant archaeon in the methanogenic enrichment was affiliated with the genus Methanothermobacter. With yeast extract as the electron donor, the general corrosion rate by the thiosulfate-reducing enrichment (8.43 ± 1.40 milli-inch per year, abbreviated as mpy) was about 5.5 times greater than the abiotic control (1.49 ± 0.15 mpy), while the comparable measures for the methanogenic culture were 2.03 ± 0.49 mpy and 0.62 ± 0.07 mpy, respectively. Total iron analysis in the cultures largely accounted for the mass loss of iron measured in the weight loss determinations. Profilometry analysis of polished steel coupons incubated in the presence of the thiosulfate-reducing enrichment revealed 59 pits over an area of 71.16 mm(2), while only 6 pits were evident in the corresponding methanogenic incubations. The results show the importance of thiosulfate-utilizing, sulfide-producing fermentative bacteria such as Anaerobaculum sp. in the corrosion of carbon steel, but also suggest that Anaerobaculum sp. are of far less concern when growing syntrophically with methanogens.

Surface appendages of archaea: structure, function, genetics and assembly

Organisms representing diverse subgroupings of the Domain Archaea are known to possess unusual surface structures. These can include ones unique to Archaea such as cannulae and hami as well as archaella (archaeal flagella) and various types of pili that superficially resemble their namesakes in Bacteria, although with significant differences. Major advances have occurred particularly in the study of archaella and pili using model organisms with recently developed advanced genetic tools. There is common use of a type IV pili-model of assembly for several archaeal surface structures including archaella, certain pili and sugar binding structures termed bindosomes. In addition, there are widespread posttranslational modifications of archaellins and pilins with N-linked glycans, with some containing novel sugars. Archaeal surface structures are involved in such diverse functions as swimming, attachment to surfaces, cell to cell contact resulting in genetic transfer, biofilm formation, and possible intercellular communication. Sometimes functions are co-dependent on other surface structures. These structures and the regulation of their assembly are important features that allow various Archaea, including thermoacidophilic, hyperthermophilic, halophilic, and anaerobic ones, to survive and thrive in the extreme environments that are commonly inhabited by members of this domain.