Conversion of acetic acid to methane by thermophiles

Evidence for nontraditional mcr-containing archaea contributing to biological methanogenesis in geothermal springs

Recent discoveries of methyl-coenzyme M reductase-encoding genes (mcr) in uncultured archaea beyond traditional euryarchaeotal methanogens have reshaped our view of methanogenesis. However, whether any of these nontraditional archaea perform methanogenesis remains elusive. Here, we report field and microcosm experiments based on ¹³C-tracer labeling and genome-resolved metagenomics and metatranscriptomics, revealing that nontraditional archaea are predominant active methane producers in two geothermal springs. Archaeoglobales performed methanogenesis from methanol and may exhibit adaptability in using methylotrophic and hydrogenotrophic pathways based on temperature/substrate availability. A five-year field survey found Candidatus Nezhaarchaeota to be the predominant mcr-containing archaea inhabiting the springs; genomic inference and mcr expression under methanogenic conditions strongly suggested that this lineage mediated hydrogenotrophic methanogenesis in situ. Methanogenesis was temperature-sensitive , with a preference for methylotrophic over hydrogenotrophic pathways when incubation temperatures increased from 65° to 75°C. This study demonstrates an anoxic ecosystem wherein methanogenesis is primarily driven by archaea beyond known methanogens, highlighting diverse nontraditional mcr-containing archaea as previously unrecognized methane sources.

Effects of florfenicol on methane accumulation and changes in the structure of the prokaryotic community in a water-sediment system

Florfenicol has been widely used in the veterinary and aquaculture to control bacterial diseases because of its high efficacy, quick effect, and low cost. The water-sediment system has become an important sink for florfenicol, and the anaerobic environment of lake sediments is favorable for methane (CH₄) production. Although antibiotics may impact methanogenesis under anaerobic conditions, the influence of florfenicol on CH₄ accumulation in anaerobic water-sediment system remains uncertain. This study evaluated how florfenicol affects CH₄ accumulation and the structure of the prokaryotic community in a water-sediment system. Anaerobic systems with different florfenicol concentrations (0, 0.2, 1, 5 and 10 mg/L) were incubated and CH₄ accumulation, pH, total organic carbon content, degradation ratio of florfenicol, and structure of the prokaryotic community were monitored. It was found that CH₄ accumulation raised in low florfenicol (0.2 and 1 mg/L) systems during the growth period, while CH₄ accumulation declined in high florfenicol (5 and 10 mg/L) systems. In the first 13 d, 83.67-99.30 % of florfenicol degraded in different treatments. The addition of florfenicol also influenced the structure of the prokaryotic community of the sediments. Proteobacteria and Chloroflexi were dominant at the phylum level. The dominant taxa at the order level gradually changed from Methanomicrobiales to Methanobacteriales, and finally to Methanosarcinales, indicating the dynamic transformation of methanogens in the reactor. This study reveals the effects of florfenicol on CH₄ production under anaerobic conditions and provides a theoretical basis for further research on the underlying mechanisms. The findings also provide some basic data on the impact of new pollutants on the global carbon cycle and greenhouse gas emission.

A comprehensive review of the recent development and challenges of a solar-assisted biodigester system

The extensive use of fossil fuels and the environmental effect of their combustion products have attracted researchers to look into renewable energy sources. In addition, global mass production of waste has motivated communities to recycle and reuse the waste in a sustainable way to lower landfill waste and associated problems. The development of waste to energy (WtE) technology including the production of bioenergy, e.g. biogas produced from various waste through Anaerobic Digestion (AD), is considered one of the potential measures to achieve the sustainable development goals of the United Nations (UN). Therefore, this study reviews the most recent studies from relevant academic literature on WtE technology (particularly AD technology) for biogas production and the application of a solar-assisted biodigester (SAB) system aimed at improving performance. In addition, socio-economic factors, challenges, and perspectives have been reported. From the analysis of different technologies, further work on effective low-cost technologies is recommended, especially using SAB system upgrading and leveraging the opportunities of this system. The study found that the performance of the AD system is affected by a variety of factors and that different approaches can be applied to improve performance. It has also been found that solar energy systems efficiently raise the biogas digester temperature and through this, they maximize the biogas yield under optimum conditions. The study revealed that the solar-assisted AD system produces less pollution and improves performance compared to the conventional AD system.

Growth Characteristics and Thermodynamics of Syntrophic Acetate Oxidizers

Syntrophic acetate oxidation (SAO) plays a pivotal role in biogas production processes when aceticlastic methanogens are inhibited. Despite the importance of SAO, the metabolic interactions and syntrophic growth of the organisms involved are still poorly understood. Therefore, we studied growth parameters and interactions within constructed defined cocultures comprising the methanogen Methanoculleus bourgensis and one, or several, of the syntrophic acetate oxidizers Syntrophaceticus schinkii, [ Clostridium] ultunense, and Tepidanaerobacter acetatoxydans and a novel, uncharacterized bacterium. Cultivation experiments in a design-of-experiment approach revealed positive effects on methane production rate of increased ammonium levels (up to 0.2 M), temperature (up to 45 °C), and acetate concentrations (0.15-0.30 M). Molecular analyses and thermodynamic calculations demonstrated close interlinkages between the microorganisms, with available energies of -10 kJ/mol for acetate oxidation and -20 kJ/mol for hydrogenotrophic methanogenesis. The estimated generation time varied between 3 and 20 days for all syntrophic microorganisms involved, and the acetate minimum threshold level was 0.40-0.45 mM. The rate of methanogenesis depended on the SAO bacteria present in the culture. These data are beneficial for interpretation of SAO prevalence and competiveness against aceticlastic methanogens in anaerobic environments.

Metagenome, metatranscriptome, and metaproteome approaches unraveled compositions and functional relationships of microbial communities residing in biogas plants

The production of biogas by anaerobic digestion (AD) of agricultural residues, organic wastes, animal excrements, municipal sludge, and energy crops has a firm place in sustainable energy production and bio-economy strategies. Focusing on the microbial community involved in biomass conversion offers the opportunity to control and engineer the biogas process with the objective to optimize its efficiency. Taxonomic profiling of biogas producing communities by means of high-throughput 16S rRNA gene amplicon sequencing provided high-resolution insights into bacterial and archaeal structures of AD assemblages and their linkages to fed substrates and process parameters. Commonly, the bacterial phyla Firmicutes and Bacteroidetes appeared to dominate biogas communities in varying abundances depending on the apparent process conditions. Regarding the community of methanogenic Archaea, their diversity was mainly affected by the nature and composition of the substrates, availability of nutrients and ammonium/ammonia contents, but not by the temperature. It also appeared that a high proportion of 16S rRNA sequences can only be classified on higher taxonomic ranks indicating that many community members and their participation in AD within functional networks are still unknown. Although cultivation-based approaches to isolate microorganisms from biogas fermentation samples yielded hundreds of novel species and strains, this approach intrinsically is limited to the cultivable fraction of the community. To obtain genome sequence information of non-cultivable biogas community members, metagenome sequencing including assembly and binning strategies was highly valuable. Corresponding research has led to the compilation of hundreds of metagenome-assembled genomes (MAGs) frequently representing novel taxa whose metabolism and lifestyle could be reconstructed based on nucleotide sequence information. In contrast to metagenome analyses revealing the genetic potential of microbial communities, metatranscriptome sequencing provided insights into the metabolically active community. Taking advantage of genome sequence information, transcriptional activities were evaluated considering the microorganism's genetic background. Metaproteome studies uncovered enzyme profiles expressed by biogas community members. Enzymes involved in cellulose and hemicellulose decomposition and utilization of other complex biopolymers were identified. Future studies on biogas functional microbial networks will increasingly involve integrated multi-omics analyses evaluating metagenome, transcriptome, proteome, and metabolome datasets.

Unraveling the microbiome of a thermophilic biogas plant by metagenome and metatranscriptome analysis complemented by characterization of bacterial and archaeal isolates

BACKGROUND

One of the most promising technologies to sustainably produce energy and to mitigate greenhouse gas emissions from combustion of fossil energy carriers is the anaerobic digestion and biomethanation of organic raw material and waste towards biogas by highly diverse microbial consortia. In this context, the microbial systems ecology of thermophilic industrial-scale biogas plants is poorly understood.

RESULTS

The microbial community structure of an exemplary thermophilic biogas plant was analyzed by a comprehensive approach comprising the analysis of the microbial metagenome and metatranscriptome complemented by the cultivation of hydrolytic and acido-/acetogenic Bacteria as well as methanogenic Archaea. Analysis of metagenome-derived 16S rRNA gene sequences revealed that the bacterial genera Defluviitoga (5.5 %), Halocella (3.5 %), Clostridium sensu stricto (1.9 %), Clostridium cluster III (1.5 %), and Tepidimicrobium (0.7 %) were most abundant. Among the Archaea, Methanoculleus (2.8 %) and Methanothermobacter (0.8 %) were predominant. As revealed by a metatranscriptomic 16S rRNA analysis, Defluviitoga (9.2 %), Clostridium cluster III (4.8 %), and Tepidanaerobacter (1.1 %) as well as Methanoculleus (5.7 %) mainly contributed to these sequence tags indicating their metabolic activity, whereas Hallocella (1.8 %), Tepidimicrobium (0.5 %), and Methanothermobacter (<0.1 %) were transcriptionally less active. By applying 11 different cultivation strategies, 52 taxonomically different microbial isolates representing the classes Clostridia, Bacilli, Thermotogae, Methanomicrobia and Methanobacteria were obtained. Genome analyses of isolates support the finding that, besides Clostridium thermocellum and Clostridium stercorarium, Defluviitoga tunisiensis participated in the hydrolysis of hemicellulose producing ethanol, acetate, and H2/CO2. The latter three metabolites are substrates for hydrogentrophic and acetoclastic archaeal methanogenesis.

CONCLUSIONS

Obtained results showed that high abundance of microorganisms as deduced from metagenome analysis does not necessarily indicate high transcriptional or metabolic activity, and vice versa. Additionally, it appeared that the microbiome of the investigated thermophilic biogas plant comprised a huge number of up to now unknown and insufficiently characterized species.

The effect of feeding high fat diet to beef cattle on manure composition and gaseous emission from a feedlot pen surface

BACKGROUND

Dietary manipulation is a common practice to mitigate gaseous emission from livestock production facilities, and the variation of fat level in the diet has shown great influence on ruminal volatile fatty acids (VFA) and enteric methane generation. The changes in dietary fat levels influence rumen chemistry that could modify manure nutrient composition along with odor and gaseous emissions from manure management facilities.

METHODS

A field experiment was carried out on beef cattle feedlots to investigate the effect of four levels of dietary fat concentrations (3 to 5.5 %) on the manure composition and gaseous emissions (methane-CH4, nitrous oxide-N2O, carbon dioxide-CO2 and hydrogen sulfide-H2S) from the feedlot pen surface. The experiment was carried out over a 5-month period from June to October during North Dakota's summer-fall climatic condition. Air and manure sampling was conducted five times at a 20-30 day intervals.

RESULTS

Overall, this research indicated that fat levels in diet have no or little effect on the nutrient composition of manure and gaseous emission from the pens with cattle fed with different diet. Though significant variation of gaseous emission and manure composition were observed between different sampling periods, no effect of high fat diet was observed on manure composition and gaseous emission.

CONCLUSIONS

It can be concluded that addition of fat to animal diet may not have any impact on gaseous emission and manure compositions.

Searching for links in the biotic characteristics and abiotic parameters of nine different biogas plants

To find links between the biotic characteristics and abiotic process parameters in anaerobic digestion systems, the microbial communities of nine full‐scale biogas plants in South Tyrol (Italy) and Vorarlberg (Austria) were investigated using molecular techniques and the physical and chemical properties were monitored. DNA from sludge samples was subjected to microarray hybridization with the ANAEROCHIP microarray and results indicated that sludge samples grouped into two main clusters, dominated either by Methanosarcina or by Methanosaeta, both aceticlastic methanogens. Hydrogenotrophic methanogens were hardly detected or if detected, gave low hybridization signals. Results obtained using denaturing gradient gel electrophoresis (DGGE) supported the findings of microarray hybridization. Real‐time PCR targeting Methanosarcina and Methanosaeta was conducted to provide quantitative data on the dominating methanogens. Correlation analysis to determine any links between the microbial communities found by microarray analysis, and the physicochemical parameters investigated was conducted. It was shown that the sludge samples dominated by the genus Methanosarcina were positively correlated with higher concentrations of acetate, whereas sludge samples dominated by representatives of the genus Methanosaeta had lower acetate concentrations. No other correlations between biotic characteristics and abiotic parameters were found. Methanogenic communities in each reactor were highly stable and resilient over the whole year.

Quantitative analysis of methanogenic community dynamics in three anaerobic batch digesters treating different wastewaters

Quantitative changes in methanogenic community structures, associated with performance data, were investigated in three anaerobic batch digesters treating synthetic glucose medium, whey permeate, and liquefied sewage sludge. All digesters were initially seeded with anaerobic sludge obtained from a local municipal wastewater treatment plant. Dynamics of methanogenic populations were monitored, at order and family levels, using real-time PCR based on the 16S rRNA gene. The molecular monitoring revealed that, in each digester, the quantitative structure of methanogenic community varied continuously over treatment time and the variation corresponded well to the changes in chemical profiles. Biphasic production of methane, associated with successive increases in aceticlastic (mainly Methanosarcinaceae) and hydrogenotrophic (mainly Methanomicrobiales) methanogenic groups, was observed in each digester. This corresponded to the diauxic utilization of acetate and longer-chain volatile fatty acids (C(3)-C(6)), mainly propionate. Additionally, the non-metric multidimensional scaling (NMDS) analysis of the quantification results demonstrated that the community shift patterns in three digesters were totally different from each other. Considering that the operating conditions in all trials were identical except substrates, the differences in quantitative shift profiles were suggested to be due to the different substrate compositions. This implied that the composition of wastewater could affect the evolution of quantitative methanogenic community structure in an anaerobic process. Overall, our results suggested that more attention to quantitative as well as qualitative approaches on microbial communities is needed for fundamental understanding of anaerobic processes, particularly under dynamic or transitional conditions.

Methanogenic community change in a full-scale UASB reactor operated at a low F/M ratio

A full-scale upflow anaerobic sludge blanket (UASB) reactor was investigated in terms of archaeal composition, acetoclastic methanogenic capacity and performance over a 2-year period. Performance of the reactor in terms of COD removal efficiency varied between 60% and 80% at organic loading rates (OLRs) in the range of 2.5-12 kg COD m-3 d-1. The reactor had been operated under a F/M (food to microorganisms) ratio of 0.02-0.03 gCOD gTVS-1 d-1, which is much lower than the typical values reported for similar reactors. According to specific methanogenic activity (SMA) tests the anaerobic sludge was operating at only 12-34% of its potential acetoclastic methanogenic capacity. These results demonstrated that the UASB reactor was under loaded compared to its maximum loading capacity. All other operational parameters had been maintained within their desired ranges. The SMA test and Fluorescence in situ hybridization (FISH) results revealed that a decrease in the acetoclastic methanogenic activity of the UASB sludge from 344 mL CH4 gTVS-1 d-1 to 109 mL CH4 gTVS-1 d-1 coincided with a decrease in the relative abundance of acetoclastic Methanosaeta from 90%+/-1.2 to 79%+/-1.4 of the archaeal population, and an increase in the relative abundance of hydrogenotrophic Methanobacteriales from non-detectable levels to 24%+/-0.7% of the archaeal population during the 2-year operation of the reactor. The relative abundance of archaeal cells within the UASB sludge was in the range of 15-17%.

Acidification of methanol-fed anaerobic granular sludge bioreactors by cobalt deprivation: Induction and microbial community dynamics

The acidification of mesophilic (30 degrees C) methanol-fed upflow anaerobic sludge bed (UASB) reactors induced by cobalt deprivation from the influent was investigated by coupling the reactor performance (pH 7.0; organic loading rate 4.5 g COD . L(-1) . d(-1)) to the microbial ecology of the bioreactor sludge. The latter was investigated by specific methanogenic activity (SMA) measurements and fluorescence in situ hybridization (FISH) to quantify the abundance of key organisms over time. This study hypothesized that under cobalt limiting conditions, the SMA on methanol of the sludge gradually decreases, which ultimately results in methanol accumulation in the reactor effluent. Once the methanol accumulation surpasses a threshold value (about 8.5 mM for the sludge investigated), reactor acidification occurs because acetogens outcompete methylothrophic methanogens at these elevated methanol concentrations. Methanogens present in granular sludge at the time of the acidification do not use methanol as the direct substrate and are unable to degrade acetate. Methylotrophic/acetoclastic methanogenic activity was found to be lost within 10 days of reactor operation, coinciding with the disappearance of the Methanosarcina population. The loss of SMA on methanol can thus be used as an accurate parameter to predict reactor acidification of methanol-fed UASB reactors operating under cobalt limiting conditions.

Microbial community in methanogenic packed-bed reactor successfully operating at short hydraulic retention time

The microbial community in a thermophilic anaerobic packed-bed reactor, which had been successfully operated to convert acetic and butyric acids to methane at a short hydraulic retention time (from 24 h to 1.9 h), was investigated. Archaea closely related to known methanogens were detected by 16S rRNA gene analyses of the effluents, together with diverse types of unidentified bacteria.

Influence of extrinsic factors on granulation in UASB reactor

The aim of this mini-review is to synthesize and analyze information on how the process of granulation is affected by environmental and operational conditions in the reactor. The factors reviewed are temperature, pH, alkalinity, organic loading rate, upflow velocity, nature and strength of substrate, nutrients, multivalent cations and heavy metals, microbial ecology of seed sludge, exo-cellular polymer, and addition of natural and synthetic polymers. Careful temperature control and adequate alkalinity is required for generation and maintenance of granules. Nature and strength of substrate in conjunction with intra-granular diffusion to a large extent determines the microstructure of the granules. The divalent cations such as calcium and iron may enhance granulation by ionic bridging and linking exo-cellular polymers. However, their presence in excess may lead to cementation due to precipitation leading to increased ash content and mass transfer limitation. The addition of external additives such as ionic polymers may enhance granulation in the upflow anaerobic sludge blanket reactors.

Dynamic transition of a methanogenic population in response to the concentration of volatile fatty acids in a thermophilic anaerobic digester

In this study, the microbial community succession in a thermophilic methanogenic bioreactor under deteriorative and stable conditions that were induced by acidification and neutralization, respectively, was investigated using PCR-mediated single-strand conformation polymorphism (SSCP) based on the 16S rRNA gene, quantitative PCR, and fluorescence in situ hybridization (FISH). The SSCP analysis indicated that the archaeal community structure was closely correlated with the volatile fatty acid (VFA) concentration, while the bacterial population was impacted by pH. The archaeal community consisted mainly of two species of hydrogenotrophic methanogen (i.e., a Methanoculleus sp. and a Methanothermobacter sp.) and one species of aceticlastic methanogen (i.e., a Methanosarcina sp.). The quantitative PCR of the 16S rRNA gene from each methanogen revealed that the Methanoculleus sp. predominated among the methanogens during operation under stable conditions in the absence of VFAs. Accumulation of VFAs induced a dynamic transition of hydrogenotrophic methanogens, and in particular, a drastic change (i.e., an approximately 10,000-fold increase) in the amount of the 16S rRNA gene from the Methanothermobacter sp. The predominance of the one species of hydrogenotrophic methanogen was replaced by that of the other in response to the VFA concentration, suggesting that the dissolved hydrogen concentration played a decisive role in the predominance. The hydrogenotrophic methanogens existed close to bacteria in aggregates, and a transition of the associated bacteria was also observed by FISH analyses. The degradation of acetate accumulated during operation under deteriorative conditions was concomitant with the selective proliferation of the Methanosarcina sp., indicating effective acetate degradation by the aceticlastic methanogen. The simple methanogenic population in the thermophilic anaerobic digester significantly responded to the environmental conditions, especially to the concentration of VFAs.

Biofilm microbial community of a thermophilic trickling biofilter used for continuous biohydrogen production

Molecular methods were employed to investigate the microbial community of a biofilm obtained from a thermophilic trickling biofilter reactor (TBR) that was operated long-term to produce H(2). Biomass concentration in the TBR gradually decreased as reactor bed height increased. Despite this difference in biomass concentration, samples from the bottom and middle of the TBR bed revealed similar microbial populations as determined by PCR-DGGE analysis of 16S rRNA genes. Nucleotide sequences of most DGGE bands were affiliated with the classes Clostridia and Bacilli in the phylum Firmicutes, and the most dominant bands showed a high sequence similarity to Thermoanaerobacterium thermosaccharolyticum.

Comparison of two-stage thermophilic (68°C/55°C) anaerobic digestion with one-stage thermophilic (55°C) digestion of cattle manure

A two-stage 68 degrees C/55 degrees C anaerobic degradation process for treatment of cattle manure was studied. In batch experiments, an increase of the specific methane yield, ranging from 24% to 56%, was obtained when cattle manure and its fractions (fibers and liquid) were pretreated at 68 degrees C for periods of 36, 108, and 168 h, and subsequently digested at 55 degrees C. In a lab-scale experiment, the performance of a two-stage reactor system, consisting of a digester operating at 68 degrees C with a hydraulic retention time (HRT) of 3 days, connected to a 55 degrees C reactor with 12-day HRT, was compared with a conventional single-stage reactor running at 55 degrees C with 15-days HRT. When an organic loading of 3 g volatile solids (VS) per liter per day was applied, the two-stage setup had a 6% to 8% higher specific methane yield and a 9% more effective VS-removal than the conventional single-stage reactor. The 68 degrees C reactor generated 7% to 9% of the total amount of methane of the two-stage system and maintained a volatile fatty acids (VFA) concentration of 4.0 to 4.4 g acetate per liter. Population size and activity of aceticlastic methanogens, syntrophic bacteria, and hydrolytic/fermentative bacteria were significantly lower in the 68 degrees C reactor than in the 55 degrees C reactors. The density levels of methanogens utilizing H2/CO2 or formate were, however, in the same range for all reactors, although the degradation of these substrates was significantly lower in the 68 degrees C reactor than in the 55 degrees C reactors. Temporal temperature gradient electrophoresis profiles (TTGE) of the 68 degrees C reactor demonstrated a stable bacterial community along with a less divergent community of archaeal species.

Long-term effects of operating temperature and sulphate addition on the methanogenic community structure of anaerobic hybrid reactors

The diversity, population dynamics, and activity profiles of methanogens in anaerobic granular sludges from two anaerobic hybrid reactors treating a molasses wastewater both mesophilically (37 degrees C) and thermophilically (55 degrees C) during a 1081 day trial were determined. The influent to one of the reactors was supplemented with sulphate, after an acclimation period of 112 days, to determine the effect of competition with sulphate-reducing bacteria on the methanogenic community structure. Sludge samples were removed from the reactors at intervals throughout the operational period and examined by amplified ribosomal DNA (rDNA) restriction analysis (ARDRA) and partial sequencing of 16S rRNA genes. In total, 18 operational taxonomic units (OTUs) were identified, 12 of which were sequenced. The methanogenic communities in both reactors changed during the operational period. The seed sludge and the reactor biomass sampled during mesophilic operation, both in the presence and absence of sulphate, was characterised by a predominance of Methanosaeta spp. Following temperature elevation, the dominant methanogenic sequences detected in the non-sulphate supplemented reactor were closely related to Methanocorpusculum parvum. By contrast, the dominant OTUs detected in the sulphate-supplemented reactor upon temperature increase were related to the hydrogen-utilising methanogen, Methanobacterium thermoautotrophicum. The observed methanogenic community structure in the reactors correlated with the operational performance of the reactors during the trial and with physiological measurements of the reactor biomass. Both reactors achieved chemical oxygen demand (COD) removal efficiencies of over 90% during mesophilic operation, with or without sulphate supplementation. During thermophilic operation, the presence of sulphate resulted in decreased reactor performance (effluent acetate concentrations of >3000 mg/l and biogas methane content of <25%). It was demonstrated that methanogenic conversion of acetate at 55 degrees C was extremely sensitive to inhibition by sulphide (50% inhibition at 8-17 mg/l unionised sulphide at pH 7.6-8.0), while the conversion of H(2)/CO(2) methanogenically was favoured. The combination of experiments carried out demonstrated the presence of specific methanogenic populations during periods of successful operational performance.

Perspectives for anaerobic digestion

The modern society generates large amounts of waste that represent a tremendous threat to the environment and human and animal health. To prevent and control this, a range of different waste treatment and disposal methods are used. The choice of method must always be based on maximum safety, minimum environmental impact and, as far as possible, on valorization of the waste and final recycling of the end products. One of the main trends of today's waste management policies is to reduce the stream of waste going to landfills and to recycle the organic material and the plant nutrients back to the soil. Anaerobic digestion (AD) is one way of achieving this goal and it will furthermore, reduce energy consumption or may even be net energy producing. This chapter aims at provide a basic understanding of the world in which anaerobic digestion is operating today. The newest process developments as well as future perspectives will be discussed.

Growth kinetics of thermophilic Methanosarcina spp. isolated from full-scale biogas plants treating animal manures

This study determines the growth kinetics of thermophilic strains of Methanosarcina spp. from full-scale thermophilic biogas plants. The complete set of kinetic parameters, including maximum specific growth rate µ(max), half saturation constant K(S), acetate threshold concentration and cell growth yield Y(X/S), were determined for six Methanosarcina strains newly isolated from full-scale reactors and the type strain Methanosarcina thermophila TM-1(T). The kinetic experiments were performed in media supplemented with acetate and activated carbon at the optimum growth temperatures of the individual strains, 50-55 degrees C. The µ(max) values of the isolates were in the range of 0.044-0.064 h(-1), the K(S) ranged from 6.5 to 24.7 mM acetate and the threshold for acetate utilization from 0.11 to 0.40 mM. The cell growth yields of the strains were between 0.78 and 2.97 g dry weight cells mol(-1) acetate. The six isolates exhibited significantly higher µ(max) and had higher affinity to acetate than the type strain M. thermophila TM-1(T). Generally, the affinities of thermophilic Methanosarcina strains tested in this study cover a similar range to those reported in the literature for mesophilic Methanosarcina spp. with acetate as substrate. The strains isolated from plants treating mixtures of animal manures and industrial organic wastes had higher affinity for acetate and lower thresholds than strains isolated from reactors operating solely on manures.

Immobilization patterns and dynamics of acetate-utilizing methanogens immobilized in sterile granular sludge in upflow anaerobic sludge blanket reactors

Sterile granular sludge was inoculated with either Methanosarcina mazeii S-6, Methanosaeta concilii GP-6, or both species in acetate-fed upflow anaerobic sludge blanket (UASB) reactors to investigate the immobilization patterns and dynamics of aceticlastic methanogens in granular sludge. After several months of reactor operation, the methanogens were immobilized, either separately or together. The fastest immobilization was observed in the reactor containing M. mazeii S-6. The highest effluent concentration of acetate was observed in the reactor with only M. mazeii S-6 immobilized, while the lowest effluent concentration of acetate was observed in the reactor where both types of methanogens were immobilized together. No changes were observed in the kinetic parameters (Ks and mumax) of immobilized M. concilii GP-6 or M. mazeii S-6 compared with suspended cultures, indicating that immobilization does not affect the growth kinetics of these methanogens. An enzyme-linked immunosorbent assay using polyclonal antibodies against either M. concilii GP-6 or M. mazeii S-6 showed significant variations in the two methanogenic populations in the different reactors. Polyclonal antibodies were further used to study the spatial distribution of the two methanogens. M. concilii GP-6 was immobilized only on existing support material without any specific pattern. M. mazeii S-6, however, showed a different immobilization pattern: large clumps were formed when the concentration of acetate was high, but where the acetate concentration was low this strain was immobilized on support material as single cells or small clumps. The data clearly show that the two aceticlastic methanogens immobilize differently in UASB systems, depending on the conditions found throughout the UASB reactor.

Whole-cell hybridization of Methanosarcina cells with two new oligonucleotide probes

Two new oligonucleotide probes targeting the 16S rRNA of the methanogenic genus Methanosarcina were developed. The probes have the following sequences (Escherichia coli numbering): probe SARCI551, 5'-GAC CCAATAATCACGATCAC-3', and probe SARCI645, 5'-TCCCGGTTCCAAGTCTGGC-3'. In situ hybridization with the fluorescently labelled probes required several modifications of standard procedures. Cells of Methanosarcina mazeii S-6 were found to lyse during the hybridization step if fixed in 3% formaldehyde and stored in 50% ethanol. Lysis was, however, not observed with cells fixed and stored in 1.6% formaldehyde-0.85% NaCl. Extensive autofluorescence of the cells was found upon hybridization in the presence of 5 mM EDTA, but successful hybridization could be obtained without addition of this compound. The mounting agent Citifluor AF1, often used in conjugation with the fluorochrome fluorescein, was found to wash the labelled probes out of the cells. Stable labelling could be obtained with rhodamine-labelled probes when the specimen was mounted in immersion oil, and high hybridization intensities of the Methanosarcina cells were found even in the presence of biomass from an anaerobic reactor. The inherent high autofluorescence of the biomass could be lowered by use of a highly specific narrow-band filter. The probes were found to be specific for Methanosarcina and useful for detection of this genus in samples from anaerobic reactors.

Limitations of thermophilic anaerobic wastewater treatment and the consequences for process design

Thermophilic anaerobic digestion offers an attractive alternative for the treatment of medium- and high-strength wastewaters. However, literature reports reveal that thermophilic wastewater treatment systems are often more sensitive to environmental changes than the well-defined high-rate reactors at the mesophilic temperature range. Also, in many cases a poorer effluent quality is experienced while the carry over of suspended solids in the effluent is relatively high. In this paper recent achievements are discussed regarding the process stability of thermophilic anaerobic wastewater treatment systems. Laboratory experiments reveal a relatively low sensitivity to temperature changes if high-rate reactors with immobilized biomass are used. Other results show that if a staged process is applied, thermophilic reactors can be operated for prolonged periods of time under extreme loading conditions (80-100 kg chemical oxygen demand.m-3.day-1), while the concentrations of volatile fatty acids in the effluent remain at a low level.

Acetate treatment in 70 degrees C upflow anaerobic sludge-blanket (UASB) reactors: start-up with thermophilic inocula and the kinetics of the UASB sludges

This study focused on the use the thermophilic anaerobic granulae in the start-up of 70 degrees C acetate-fed upflow anaerobic sludge-blanket (UASB) reactors and the kinetics of granulae grown at 70 degrees C. In the UASB reactors, chemical oxygen demand removal commenced within 48 h of the start-up. The maximum reduction in chemical oxygen demand was 84% with the feed containing yeast and 71% without a yeast supplement. In the bioassays, the yeast-grown sludge converted 98% of the acetate consumed to methane as compared to 92% for the sludge grown without yeast. The highest initial specific methane production rate (mu-CH4) of the UASB sludges grown at 70 degrees C was 0.088 h(-1) at an acetate concentration of 4.6mM. The higher initial acetate concentration was found to prolong the lag-phase in methane production significantly and to decrease mu-CH4. The half-saturation constant (Ks), the inhibition constant (Ki), the inhibition response coefficient (n) and the mu-CH4-max, calculated according to a modified Haldane equation, were 1.5 mM, 2.8 mM, 0.8 and 0.28 h(-1), respectively. The prolonged starvation of the 70 degrees C sludge (15 days) decreased the mu-CH4 from about 0.022 h(-1) to 0.011 h(-1) and increased the lag phase in methane production from 6 h to 24 h as compared with non-starved sludge.

Methanogenesis in thermophilic biogas reactors

Methanogenesis in thermophilic biogas reactors fed with different wastes is examined. The specific methanogenic activity with acetate or hydrogen as substrate reflected the organic loading of the specific reactor examined. Increasing the loading of thermophilic reactors stabilized the process as indicated by a lower concentration of volatile fatty acids in the effluent from the reactors. The specific methanogenic activity in a thermophilic pilot-plant biogas reactor fed with a mixture of cow and pig manure reflected the stability of the reactor. The numbers of methanogens counted by the most probable number (MPN) technique with acetate or hydrogen as substrate were further found to vary depending on the loading rate and the stability of the reactor. The numbers of methanogens counted with antibody probes in one of the reactor samples was 10 times lower for the hydrogen-utilizing methanogens compared to the counts using the MPN technique, indicating that other non-reacting methanogens were present. Methanogens that reacted with the probe against Methanobacterium thermoautotrophicum were the most numerous in this reactor. For the acetate-utilizing methanogens, the numbers counted with the antibody probes were more than a factor of 10 higher than the numbers found by MPN. The majority of acetate utilizing methanogens in the reactor were Methanosarcina spp. single cells, which is a difficult form of the organism to cultivate in vitro. No reactions were observed with antibody probes raised against Methanothrix soehngenii or Methanothrix CALS-1 in any of the thermophilic biogas reactors examined. Studies using 2-14C-labeled acetate showed that at high concentrations (more than approx. 1 mM) acetate was metabolized via the aceticlastic pathway, transforming the methyl-group of acetate into methane. When the concentration of acetate was less than approx. 1 mM, most of the acetate was oxidized via a two-step mechanism (syntrophic acetate oxidation) involving one organism oxidizing acetate into hydrogen and carbon dioxide and a hydrogen-utilizing methanogen forming the products of the first microorganism into methane. In thermophilic biogas reactors, acetate oxidizing cultures occupied the niche of Methanothrix species, aceticlastic methanogens which dominate at low acetate concentrations in mesophilic systems. Normally, thermophilic biogas reactors are operated at temperatures from 52 to 56 degrees C. Experiments using biogas reactors fed with cow manure showed that the same biogas yield found at 55 degrees C could be obtained at 61 degrees C after a long adaptation period. However, propionate degradation was inhibited by increasing the temperature.

Acetate Degradation at 70°C in Upflow Anaerobic Sludge Blanket Reactors and Temperature Response of Granules Grown at 70°C

Anaerobic acetate degradation at 70°C and at 55°C (as a reference) was studied by running laboratory upflow anaerobic sludge blanket (UASB) reactors inoculated with mesophilic granular sludge. In UASB reactors fed with acetate-containing media (3 g of chemical oxygen demand [COD] per liter, corresponding to 47 mM acetate) approximately 50 days was needed at 70°C and less than 15 days was needed at 55°C to achieve an effluent COD of 500 to 700 mg/liter. In the UASB reactors at both 70 and 55°C up to 90% of the COD was removed. Batch assays showed that sludges from two 70°C UASB reactors, one run at a low effluent acetate concentration and the other run at a high effluent acetate concentration, exhibited slightly different responses to temperatures in the range from 37 to 70°C. Both 70°C sludges, as well as the 55°C sludge, produced methane at temperatures of 37 to 73°C. The 55°C sludge exhibited shorter lag phases than the 70°C sludges and higher specific methane production rates between 37 and 65°C.

Characterization of an extremely thermostable glutamate dehydrogenase: a key enzyme in the primary metabolism of the hyperthermophilic archaebacterium, Pyrococcus furiosus

Glutamate dehydrogenase (L-glutamate:NAD(P)+ oxidoreductase, deaminating, EC 1.4.1.3) from the hyperthermophilic Archeon Pyrococcus furiosus was purified to homogeneity by chromatography on anion-exchange, molecular-exclusion and hydrophobic-interaction media. The purified native enzyme had an M(r) of 270,000 +/- 15,000 and was shown to be a hexamer with identical subunits of M(r) 46,000. The enzyme was exceptionally thermostable, having a half-life of 3.5 to more than 10 h at 100 degrees C, depending on the concentration of enzyme. The Km of the enzyme for ammonia was high (9.5 mM), indicating that the enzyme is probably active in the deaminating, catabolic direction. The coenzyme utilization of the enzyme resembled the equivalent enzymes from eukaryotes rather than eubacteria, since both NADH and NADPH were recognized with high affinity. The enzyme displayed a preference for NADP+ over NAD+ that was more pronounced at low assay temperatures (50-70 degrees C) compared with the optimal temperature for enzyme activity, 95 degrees C.