Product inhibition of butyrate metabolism by acetate and hydrogen in a thermophilic coculture

Regulation of the methanogenesis pathways by hydrogen at transcriptomic level in time

The biomethane formation from 4 H2 + CO2 by pure cultures of two methanogens, Methanocaldococcus fervens and Methanobacterium thermophilum, has been studied. The goal of the study was to understand the regulation of the enzymatic steps associated with biomethane biosynthesis by H2, using metagenomic, pan-genomic, and transcriptomic approaches. Methanogenesis in the autotrophic methanogen M. fervens could be easily "switched off" and "switched on" by H2/CO2 within about an hour. In contrast, the heterotrophic methanogen M. thermophilum was practically insensitive to the addition of the H2/CO2 trigger although this methanogen also converted H2/CO2 to CH4. From practical points of view, the regulatory function of H2/CO2 suggests that in the power-to-gas (P2G) renewable excess electricity conversion and storage systems, the composition of the biomethane-generating methanogenic community is essential for sustainable operation. In addition to managing the specific hydrogenotrophic methanogenesis biochemistry, H2/CO2 affected several, apparently unrelated, metabolic pathways. The redox-regulated overall biochemistry and symbiotic relationships in the methanogenic communities should be explored in order to make the P2G technology more efficient. KEY POINTS : • Hydrogenotrophic methanogens may respond distinctly to H2/CO2 in bio-CH4 formation. • H2/CO2 can also activate metabolic routes, which are apparently unrelated to methanogenesis. • Sustainable conversion of the fluctuating renewable electricity to bio-CH4 is an option.

Complexity of temperature dependence in methanogenic microbial environments

There is virtually no environmental process that is not dependent on temperature. This includes the microbial processes that result in the production of CH₄, an important greenhouse gas. Microbial CH₄ production is the result of a combination of many different microorganisms and microbial processes, which together achieve the mineralization of organic matter to CO₂ and CH₄. Temperature dependence applies to each individual step and each individual microbe. This review will discuss the different aspects of temperature dependence including temperature affecting the kinetics and thermodynamics of the various microbial processes, affecting the pathways of organic matter degradation and CH₄ production, and affecting the composition of the microbial communities involved. For example, it was found that increasing temperature results in a change of the methanogenic pathway with increasing contribution from mainly acetate to mainly H₂/CO₂ as immediate CH₄ precursor, and with replacement of aceticlastic methanogenic archaea by thermophilic syntrophic acetate-oxidizing bacteria plus thermophilic hydrogenotrophic methanogenic archaea. This shift is consistent with reaction energetics, but it is not obligatory, since high temperature environments exist in which acetate is consumed by thermophilic aceticlastic archaea. Many studies have shown that CH₄ production rates increase with temperature displaying a temperature optimum and a characteristic apparent activation energy (E_a). Interestingly, CH₄ release from defined microbial cultures, from environmental samples and from wetland field sites all show similar E_a values around 100 kJ mol^-1 indicating that CH₄ production rates are limited by the methanogenic archaea rather than by hydrolysis of organic matter. Hence, the final rather than the initial step controls the methanogenic degradation of organic matter, which apparently is rarely in steady state.

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.

The Anaerobic Fungi: Challenges and Opportunities for Industrial Lignocellulosic Biofuel Production

Lignocellulose is a promising feedstock for biofuel production as a renewable, carbohydrate-rich and globally abundant source of biomass. However, challenges faced include environmental and/or financial costs associated with typical lignocellulose pretreatments needed to overcome the natural recalcitrance of the material before conversion to biofuel. Anaerobic fungi are a group of underexplored microorganisms belonging to the early diverging phylum Neocallimastigomycota and are native to the intricately evolved digestive system of mammalian herbivores. Anaerobic fungi have promising potential for application in biofuel production processes due to the combination of their highly effective ability to hydrolyse lignocellulose and capability to convert this substrate to H2 and ethanol. Furthermore, they can produce volatile fatty acid precursors for subsequent biological conversion to H2 or CH4 by other microorganisms. The complex biological characteristics of their natural habitat are described, and these features are contextualised towards the development of suitable industrial systems for in vitro growth. Moreover, progress towards achieving that goal is reviewed in terms of process and genetic engineering. In addition, emerging opportunities are presented for the use of anaerobic fungi for lignocellulose pretreatment; dark fermentation; bioethanol production; and the potential for integration with methanogenesis, microbial electrolysis cells and photofermentation.

Deep insights into the network of acetate metabolism in anaerobic digestion: focusing on syntrophic acetate oxidation and homoacetogenesis

Acetate is a pivotal intermediate product during anaerobic decomposition of organic matter. Its generation and consumption network is quite complex, which almost covers the most steps in anaerobic digestion (AD) process. Besides acidogenesis, acetogenesis and methanogenesis, syntrophic acetate oxidation (SAO) replaced acetoclastic methanogenesis to release the inhibition of AD at some special conditions, and the importance of considering homoacetogenesis had also been proved when analysing anaerobic fermentations. Syntrophic acetate-oxidizing bacteria (SAOB), with function of SAO, can survive under high temperature and ammonia/ volatile fatty acids (VFAs) concentrations, while, homoacetogens, performed homoacetogenesis, are more active under acidic, alkaline and low temperature (10°C-20°C) conditions, This review summarized the roles of SAO and homoacetogenesis in AD process, which contains the biochemical reactions, metabolism pathways, physiological characteristics and energy conservation of functional bacteria. The specific roles of these two processes in the subprocess of AD (i.e., acidogenesis, acetogenesis and methanogenesis) were also analyzed in detail. A two phases anaerobic digester is proposed for protein-rich waste(water) treatment by enhancing the functions of homoacetogens and SAOB compared to the traditional two-phases anaerobic digesters, in which the first phase is fermentation phase including acidogens and homoacetogens for acetate production, and second phase is a mixed culture coupling syntrophic fatty acids bacteria, SAOB and hydrogenotrophic methanogens for methane production. This review provides a new insight into the network on production and consumption of acetate in AD process.

Dynamic shifts within volatile fatty acid-degrading microbial communities indicate process imbalance in anaerobic digesters

Buildup of volatile fatty acids (VFAs) in anaerobic digesters (ADs) often results in acidification and process failure. Understanding the dynamics of microbial communities involved in VFA degradation under stable and overload conditions may help optimize anaerobic digestion processes. In this study, five triplicate mesophilic completely mixed AD sets were operated at different organic loading rates (OLRs; 1-6 g chemical oxygen demand [COD] L_R^-1day^-1), and changes in the composition and abundance of VFA-degrading microbial communities were monitored using amplicon sequencing and taxon-specific quantitative PCRs, respectively. AD sets operated at OLRs of 1-4 g COD L_R^-1day^-1 were functionally stable throughout the operational period (120 days) whereas process instability (characterized by VFA buildup, pH decline, and decreased methane production rate) occurred in digesters operated at ≥ 5 g COD L_R^-1day^-1. Though microbial taxa involved in propionate (Syntrophobacter and Pelotomaculum) and butyrate (Syntrophomonas) degradation were detected across all ADs, their abundance decreased with increasing OLR. The overload conditions also inhibited the proliferation of the acetoclastic methanogen, Methanosaeta, and caused a microbial community shift to acetate oxidizers (Tepidanaerobacter acetatoxydans) and hydrogenotrophic methanogens (Methanoculleus). This study's results highlight the importance of operating ADs with conditions that promote the maintenance of microbial communities involved in VFA degradation.

Anaerobic digestion of mixed urban biowaste: The microbial community shift towards stability

Anaerobic digestion is applied worldwide to treat food waste (FW) with the aim of obtaining renewable bioenergy by exploiting the methane gas produced. However, there are several problems in practical applications, primarily due to system instability. Although exhaustive knowledge regarding anaerobic microbial community composition has been established, few studies have investigated long-term correlations between microbial consortia, operative conditions and feedstock characteristics. Here, microbial community shifts as a response to feedstock variations were investigated in long-term semi-continuous systems, which were evaluated by an in situ cell detection method and 16S rRNA gene amplicon sequencing. FW digestion showed progressive system instability caused by the inhibition of methanogens, which resulted in volatile fatty acid accumulation and process failure at the low organic loading rate (OLR). Conversely, by co-digesting FW with waste-activated sludge (WAS), a stable process with methane yields of up to 0.27 Nm³  kg^-1VS_fed for OLR = 1.7 gVS L^-1d^-1 was achieved. This stabilizing effect was not related to the buffering capacity of WAS, but to its capacity to avoid volatile fatty acid accumulation and falls in pH by overcoming methanogenic activity inhibition. WAS addition promoted the establishment of a stable and active archaeal population in anaerobic co-digestion (AcoD) reactors. The continuous supply of trace elements together with the seeding of microbial functional groups were the main drivers that positively affected process stability.

Specific quorum sensing signal molecules inducing the social behaviors of microbial populations in anaerobic digestion

A series of dominant functional floras involved in anaerobic digestion was obtained by subculturing anaerobic sludge with specific substrates, respectively, and their specific quorum sensing signals (AHLs) were identified. It was found that most of the AHLs were secreted in starvation of the functional floras. One or two AHLs were found to have a significant positive correlation with the behavior of hydrolytic-fermentative bacteria, homoacetogens, syntrophic propionate-oxidizing bacteria, syntrophic butyrate-oxidizing bacteria and aceticlastic methanogens, respectively. Hydrogenotrophic methanogens had a various self-organization means and broader relations with the other populations, which was induced by six AHLs at least. Performance of an upflow anaerobic sludge blanket in the startup process revealed that the dominant populations involved in anaerobic digestion would secret specific AHLs to induce K-strategy in resource-limited situation for reproduction and regulating their cooperation. This work presents a novel perspective on the social behaviors of microbial populations in anaerobic digestion.

Concerted Metabolic Shifts Give New Insights Into the Syntrophic Mechanism Between Propionate-Fermenting Pelotomaculum thermopropionicum and Hydrogenotrophic Methanocella conradii

Microbial syntrophy is a thermodynamically-based cooperation between microbial partners that share the small amounts of free energy for anaerobic growth. To gain insights into the mechanism by which syntrophic microorganisms coordinate their metabolism, we constructed cocultures of propionate-oxidizing Pelotomaculum thermopropionicum and hydrogenotrophic Methanocella conradii and compared them to monocultures. Transcriptome analysis was performed on these cultures using strand-specific mRNA sequencing (RNA-Seq). The results showed that in coculture both P. thermopropionicum and M. conradii significantly upregulated the expression of genes involved in catabolism but downregulated those for anabolic biosynthesis. Specifically, genes coding for the methylmalonyl-CoA pathway in P. thermopropionicum and key genes for methanogenesis in M. conradii were substantially upregulated in coculture compared to monoculture. The putative flavin-based electron bifurcation/confurcation systems in both organisms were also upregulated in coculture. Formate dehydrogenase encoding genes in both organisms were markedly upregulated, indicating that formate was produced and utilized by P. thermopropionicum and M. conradii, respectively. The inhibition of syntrophic activity by formate and 2-bromoethanesulphonate (2-BES) but not H₂/CO₂ also suggested that formate production was used by P. thermopropionicum for the recycling of intracellular redox mediators. Finally, flagellum-induced signal transduction and amino acids exchange was upregulated for syntrophic interactions. Together, our study suggests that syntrophic organisms employ multiple strategies including global metabolic shift, utilization of electron bifurcation/confurcation and employing formate as an alternate electron carrier to optimize their metabolisms for syntrophic growth.

Magnetite accelerates syntrophic acetate oxidation in methanogenic systems with high ammonia concentrations

Ammonia accumulation is a major inhibitory substance causing anaerobic digestion upset and failure in CH4 production. At high ammonia levels, CH4 production through syntrophic acetate oxidization (SAO) pathways is more tolerant to ammonia toxicity than the acetoclastic methanogenesis pathway, but the low CH4 production rate through SAO constitutes the main reason for the low efficiency of energy recovery in anaerobic digesters treating ammonia-rich substrates. In this study, we showed that acetate fermentation to CH4 and CO2 occurred through SAO pathway in the anaerobic reactors containing a high ammonia concentration (5.0 g l-1 NH4+ -N), and the magnetite nanoparticles supplementation increased the CH4 production rates from acetate by 36-58%, compared with the anaerobic reactors without magnetite under the same ammonia level. The mechanism of facilitated methanogenesis was proposed to be the establishment of direct interspecies electron transfer (DIET) for SAO, in which magnetite facilitated DIET between syntrophic acetate oxidizing bacteria and methanogens. High-throughput 16S rRNA gene sequencing analysis revealed that the bacterial Geobacteraceae and the archaeal Methanosarcinaceae and Methanobacteriaceae might be involved in magnetite-mediated DIET for SAO and CH4 production. This study demonstrated that magnetite supplementation might provide an effective approach to accelerate CH4 production rates in the anaerobic reactors treating wastewater containing high ammonia.

Acidogenic fermentation of the organic fraction of municipal solid waste and cheese whey for bio-plastic precursors recovery - Effects of process conditions during batch tests

The problem of fossil fuels dependency is being addressed through sustainable bio-fuels and bio-products production worldwide. At the base of this bio-based economy there is the efficient use of biomass as non-virgin feedstock. Through acidogenic fermentation, organic waste can be valorised in order to obtain several precursors to be used for bio-plastic production. Some investigations have been done but there is still a lack of knowledge that must be filled before moving to effective full scale plants. Acidogenic fermentation batch tests were performed using food waste (FW) and cheese whey (CW) as substrates. Effects of nine different combinations of substrate to inoculum (S/I) ratio (2, 4, and 6) and initial pH (5, 7, and 9) were investigated for metabolites (acetate, butyrate, propionate, valerate, lactate, and ethanol) productions. Results showed that the most abundant metabolites deriving from FW fermentation were butyrate and acetate, mainly influenced by the S/I ratio (acetate and butyrate maximum productions of 21.4 and 34.5g/L, respectively, at S/I=6). Instead, when dealing with CW, lactate was the dominant metabolite significantly correlated with pH (lactate maximum production of 15.7g/L at pH = 9).

BEAP profiles as rapid test system for status analysis and early detection of process incidents in biogas plants

A method was developed to quantify the performance of microorganisms involved in different digestion levels in biogas plants. The test system was based on the addition of butyrate (BCON), ethanol (ECON), acetate (ACON) or propionate (PCON) to biogas sludge samples and the subsequent analysis of CH₄ formation in comparison to control samples. The combination of the four values was referred to as BEAP profile. Determination of BEAP profiles enabled rapid testing of a biogas plant's metabolic state within 24 h and an accurate mapping of all degradation levels in a lab-scale experimental setup. Furthermore, it was possible to distinguish between specific BEAP profiles for standard biogas plants and for biogas reactors with process incidents (beginning of NH₄⁺-N inhibition, start of acidification, insufficient hydrolysis and potential mycotoxin effects). Finally, BEAP profiles also functioned as a warning system for the early prediction of critical NH₄⁺-N concentrations leading to a drop of CH₄ formation.

Improved biomass production by humic analog anthraquinone-2-sulfonate from kitchen waste in a two-phase system

The volatile fatty acids from kitchen waste were used as substrates of anoxygenic photosynthetic bacteria (APB) in a dark-photo fermentation reactor, and anthraquinone-2-sulfonate (AQS) was firstly applied to boost the biomass yield.

Biomass hydrolysis inhibition at high hydrogen partial pressure in solid-state anaerobic digestion

In solid-state anaerobic digestion, so-called ss-AD, biogas production is inhibited at high total solids contents. Such inhibition is likely caused by a slow diffusion of dissolved reaction intermediates that locally accumulate. In this study, we investigated the effect of H2 and CO2 partial pressure on ss-AD. Partial pressure of H2 and/or CO2 was artificially fixed, from 0 to 1 557mbars for H2 and from 0 to 427mbars for CO2. High partial pressure of H2 showed a significant effect on methanogenesis, while CO2 had no impact. At high [Formula: see text] , the overall substrate degradation decreased with no accumulation of metabolites from acidogenic bacteria, indicating that the hydrolytic activity was specifically impacted. Interestingly, such inhibition did not occur when CO2 was added with H2. This result suggests that CO2 gas transfer is probably a key factor in ss-AD from biomass.

Increase of methane formation by ethanol addition during continuous fermentation of biogas sludge

Very recently, it was shown that the addition of acetate or ethanol led to enhanced biogas formation rates during an observation period of 24 h. To determine if increased methane production rates due to ethanol addition can be maintained over longer time periods, continuous reactors filled with biogas sludge were developed which were fed with the same substrates as the full-scale reactor from which the sludge was derived. These reactors are well reflected conditions of a full-scale biogas plant during a period of 14 days. When the fermenters were pulsed with 50–100 mM ethanol, biomethanation increased by 50–150 %, depending on the composition of the biogas sludge. It was also possible to increase methane formation significantly when 10–20 mM pure ethanol or ethanolic solutions (e.g. beer) were added daily. In summary, the experiments revealed that “normal” methane production continued to take place, but ethanol led to production of additional methane.

Modeling the pH effect on sulfidogenesis in anaerobic sewer biofilm

Wastewater pH affects the transfer of H2S from liquid to gas and hence impacts on the extent of odour and corrosion. Sewage pH may change significantly in sewer networks due to, for example, the fermentation of organic compounds, particularly when the hydraulic retention time is long, the discharge of trade wastes that are organic-rich or with extreme pH, and the intentional addition of alkali to raise the sewage pH. Such a variation of pH is expected to affect sulfate reduction by sewer biofilms. In this work, experimental studies were carried out to reveal the detailed effects of pH on sulfidogenesis by anaerobic sewer biofilms developed in a laboratory sewer reactor fed with real sewage, and the potential causes for the inhibitory effects observed under both high and low pH conditions. The data clearly showed that the sewer biofilm had the highest sulfate reduction rate at around neutral pH (6.5-7.5), and the activity decreased when pH was higher or lower. The data also suggested that the inhibiting effect at a higher pH is likely due to the inhibitory effects of free ammonia, while at a lower pH it is more likely due to the pH itself. An inhibition model formulated based on the above hypotheses was able to adequately describe the measured sulfide production rates under different pH levels and different free ammonia concentrations, as well as data previously reported in literature. The calibrated model was used to simulate sulfide production in a sewer network receiving alkali dosage. The model adequately predicted sulfide production in the network, which could not be achieved with a model not considering the pH effect.

The kinetics of process dependent ammonia inhibition of methanogenesis from acetic acid

Advanced anaerobic digestion processes aimed at improving the methanization of sewage sludge may be potentially impaired by the production of inhibitory compounds (e.g. free ammonia). The result of methanogenic inhibition is relatively high effluent concentrations of acetic acid and other soluble organics, as well as reduced methane yields. An extreme example of such an advanced process is the thermal hydrolytic pretreatment of sludge prior to high solids digestion (THD). Compared to a conventional mesophilic anaerobic digestion process (MAD), THD operates in a state of constant inhibition driven by high free ammonia concentrations, and elevated pH values. As such, previous investigations of the kinetics of methanogenesis from acetic acid under uninhibited conditions do not necessarily apply well to the modeling of extreme processes such as THD. By conducting batch ammonia toxicity assays using biomass from THD and MAD reactors, we compared the response of these communities over a broad range of ammonia inhibition. For both processes, increased inhibitor concentrations resulted in a reduction of biomass growth rate (r(max) = μ(max)∙X) and a resulting decrease in the substrate half saturation coefficient (K(S)). These two parameters exhibited a high degree of correlation, suggesting that for a constant transport limited system, the K(S) was mostly a linear function of the growth rate. After correcting for reactor pH and temperature, we found that the THD and MAD biomass were both able to perform methanogenesis from acetate at high free ammonia concentrations (equivalent to 3-5 g/L total ammonia nitrogen), albeit at less than 30% of their respective maximum rates. The reduction in methane production was slightly less pronounced for the THD biomass than for MAD, suggesting that the long term exposure to ammonia had selected for a methanogenic pathway less dependent on those organisms most sensitive to ammonia inhibition (i.e. aceticlastic methanogens).

Biostimulation of anaerobic BTEX biodegradation under fermentative methanogenic conditions at source-zone groundwater contaminated with a biodiesel blend (B20)

Field experiments were conducted to assess the potential for anaerobic biostimulation to enhance BTEX biodegradation under fermentative methanogenic conditions in groundwater impacted by a biodiesel blend (B20, consisting of 20 % v/v biodiesel and 80 % v/v diesel). B20 (100 L) was released at each of two plots through an area of 1 m(2) that was excavated down to the water table, 1.6 m below ground surface. One release was biostimulated with ammonium acetate, which was added weekly through injection wells near the source zone over 15 months. The other release was not biostimulated and served as a baseline control simulating natural attenuation. Ammonium acetate addition stimulated the development of strongly anaerobic conditions, as indicated by near-saturation methane concentrations. BTEX removal began within 8 months in the biostimulated source zone, but not in the natural attenuation control, where BTEX concentrations were still increasing (due to source dissolution) 2 years after the release. Phylogenetic analysis using quantitative PCR indicated an increase in concentration and relative abundance of Archaea (Crenarchaeota and Euryarchaeota), Geobacteraceae (Geobacter and Pelobacter spp.) and sulfate-reducing bacteria (Desulfovibrio, Desulfomicrobium, Desulfuromusa, and Desulfuromonas) in the biostimulated plot relative to the control. Apparently, biostimulation fortuitously enhanced the growth of putative anaerobic BTEX degraders and associated commensal microorganisms that consume acetate and H2, and enhance the thermodynamic feasibility of BTEX fermentation. This is the first field study to suggest that anaerobic-methanogenic biostimulation could enhance source zone bioremediation of groundwater aquifers impacted by biodiesel blends.

Interspecies electron transfer via hydrogen and formate rather than direct electrical connections in cocultures of Pelobacter carbinolicus and Geobacter sulfurreducens

Direct interspecies electron transfer (DIET) is an alternative to interspecies H(2)/formate transfer as a mechanism for microbial species to cooperatively exchange electrons during syntrophic metabolism. To understand what specific properties contribute to DIET, studies were conducted with Pelobacter carbinolicus, a close relative of Geobacter metallireducens, which is capable of DIET. P. carbinolicus grew in coculture with Geobacter sulfurreducens with ethanol as the electron donor and fumarate as the electron acceptor, conditions under which G. sulfurreducens formed direct electrical connections with G. metallireducens. In contrast to the cell aggregation associated with DIET, P. carbinolicus and G. sulfurreducens did not aggregate. Attempts to initiate cocultures with a genetically modified strain of G. sulfurreducens incapable of both H(2) and formate utilization were unsuccessful, whereas cocultures readily grew with mutant strains capable of formate but not H(2) uptake or vice versa. The hydrogenase mutant of G. sulfurreducens compensated, in cocultures, with significantly increased formate dehydrogenase gene expression. In contrast, the transcript abundance of a hydrogenase gene was comparable in cocultures with that for the formate dehydrogenase mutant of G. sulfurreducens or the wild type, suggesting that H(2) was the primary electron carrier in the wild-type cocultures. Cocultures were also initiated with strains of G. sulfurreducens that could not produce pili or OmcS, two essential components for DIET. The finding that P. carbinolicus exchanged electrons with G. sulfurreducens via interspecies transfer of H(2)/formate rather than DIET demonstrates that not all microorganisms that can grow syntrophically are capable of DIET and that closely related microorganisms may use significantly different strategies for interspecies electron exchange.

Thermodynamics of formate-oxidizing metabolism and implications for H2 production

Formate-dependent proton reduction to H(2) (HCOO(-) + H(2)O → HCO(3)(-) + H(2)) has been reported for hyperthermophilic Thermococcus strains. In this study, a hyperthermophilic archaeon, Thermococcus onnurineus strain NA1, yielded H(2) accumulation to a partial pressure of 1 × 10(5) to 7 × 10(5) Pa until the values of Gibbs free energy change (ΔG) reached near thermodynamic equilibrium (-1 to -3 kJ mol(-1)). The bioenergetic requirement for the metabolism to conserve energy was demonstrated by ΔG values as small as -5 kJ mol(-1), which are less than the biological minimum energy quantum, -20 kJ mol(-1), as calculated by Schink (B. Schink, Microbiol. Mol. Biol. Rev. 61:262-280, 1997). Considering formate as a possible H(2) storage material, the H(2) production potential of the strain was assessed. The volumetric H(2) production rate increased linearly with increasing cell density, leading to 2,820 mmol liter(-1) h(-1) at an optical density at 600 nm (OD(600)) of 18.6, and resulted in the high specific H(2) production rates of 404 ± 6 mmol g(-1) h(-1). The H(2) productivity indicates the great potential of T. onnurineus strain NA1 for practical application in comparison with H(2)-producing microbes. Our result demonstrates that T. onnurineus strain NA1 has a highly efficient metabolic system to thrive on formate in hydrothermal systems.

Total solids content drives high solid anaerobic digestion via mass transfer limitation

The role of the total solids (TS) content on anaerobic digestion was investigated in batch reactors. A range of TS contents from 10% to 35% was evaluated, four replicates were performed. The total methane production slightly decreased with TS concentrations increasing from 10% to 25% TS. Two behaviors were observed at 30% TS: two replicates had similar performances to that at 25% TS; for the two other replicates, the methane production was inhibited as observed at 35% TS. This difference suggested that 30% TS content corresponded to a threshold of the solids content, above which methanogenesis was strongly inhibited. The Anaerobic Digestion Model No. 1 (ADM1) was used to describe the experimental data. The effects of hydrolysis step and liquid/gas mass transfer were particularly investigated. The simulations showed that mass transfer limitation could explain the low methane production at high TS, and that hydrolysis rate constants slightly decreased with increasing TS.

Effects of hydrogen and acetate on benzene mineralisation under sulphate-reducing conditions

Syntrophic mineralisation of benzene, as recently proposed for a sulphate-reducing enrichment culture, was tested in product inhibition experiments with acetate and hydrogen, both putative intermediates of anaerobic benzene fermentation. Using [(13)C(6)]-benzene enabled tracking the inhibition of benzene mineralisation sensitively by analysis of (13)CO(2). In noninhibited cultures, hydrogen was detected at partial pressures of 2.4 × 10(-6) ± 1.5 × 10(-6) atm. Acetate was detected at concentrations of 17 ± 2 μM. Spiking with 0.1 atm hydrogen produced a transient inhibitory effect on (13)CO(2) formation. In cultures spiked with higher amounts of hydrogen, benzene mineralisation did not restart after hydrogen consumption, possibly due to the toxic effects of the sulphide produced. An inhibitory effect was also observed when acetate was added to the cultures (0.3, 3.5 and 30 mM). Benzene mineralisation resumed after acetate was degraded to concentrations found in noninhibited cultures, indicating that acetate is another key intermediate in anaerobic benzene mineralisation. Although benzene mineralisation by a single sulphate reducer cannot be ruled out, our results strongly point to an involvement of syntrophic interactions in the process. Thermodynamic calculations revealed that, under in situ conditions, benzene fermentation to hydrogen and acetate yielded a free energy change of ΔG'=-83.1 ± 5.6 kJ mol(-1). Benzene mineralisation ceased when ΔG' values declined below -61.3 ± 5.3 kJ mol(-1) in the presence of acetate, indicating that ATP-consuming reactions are involved in the pathway.

BTEX plume dynamics following an ethanol blend release: geochemical footprint and thermodynamic constraints on natural attenuation

In this 10 year study, Brazilian gasoline (100 L, containing 24% ethanol by volume) was released to a sandy aquifer to evaluate the natural attenuation of benzene, toluene, ethylbenzene, and total xylenes (BTEX) in the presence of ethanol. Groundwater concentrations of BTEX, ethanol, and degradation products (e.g., acetate and methane) were measured over the entire plume using an array of monitoring well clusters, to quantify changes in plume mass and region of influence. Ethanol biodegradation coincided with the development of methanogenic conditions while acetate (a common anaerobic metabolite) accumulated. The benzene plume expanded beyond the 30 m long monitored area and began to recede after 2.7 years, when ethanol had disappeared. Theoretical calculations suggest that the transient accumulation of acetate (up to 166 mg L(-1)) may have hindered the thermodynamic feasibility of benzene degradation under methanogenic conditions. Yet, benzene removal proceeded relatively fast compared to literature values (and faster than the alkylbenzenes present at this site) after acetate concentrations had decreased below inhibitory levels. Thus, site investigations of ethanol blend releases should consider monitoring acetate concentrations. Overall, this study shows that inhibitory effects of ethanol and acetate are relatively short-lived, and demonstrates that monitored natural attenuation can be a viable option to deal with ethanol blend releases.

Thermophilic two-stage dry anaerobic digestion of model garbage with ammonia stripping

To avoid the inhibition of methane production by ammonia that occurs during the degradation of garbage, anaerobic digestion with prior ammonia production and subsequent stripping was investigated. In the ammonia production phase, the maximum ammonia concentration was approximately 2800 mg N/kg of total wet sludge in the range of 4 days of sludge retention time, indicating that only 43% of total nitrogen in the model garbage was converted to ammonia. The model garbage from which ammonia was produced and stripped was subjected to semi-continuous thermophilic dry anaerobic digestion over 180 days. The gas yield was in the range of 0.68 to 0.75 Nm(3)/kg volatile solid, and it decreased with the decrease of the sludge retention time. The ammonia-nitrogen concentration in the sludge was kept below 3000 mg N/kg total wet sludge. Microbial community structure analysis revealed that the phylum Firmicutes dominated in the ammonia production, but the community structure changed at different sludge retention times. In dry anaerobic digestion, the dominant bacteria shifted from the phylum Thermotogae to Firmicutes. The dominant archaeon was the genus Methanothermobacter, but the ratio of Methanosarcina increased during the process of dry anaerobic digestion.

Anaerobic degradation of propionate by a mesophilic acetogenic bacterium in coculture and triculture with different methanogens

A mesophilic acetogenic bacterium (MPOB) oxidized propionate to acetate and CO(2) in cocultures with the formate- and hydrogen-utilizing methanogens Methanospirillum hungatei and Methanobacterium formicicum. Propionate oxidation did not occur in cocultures with two Methanobrevibacter strains, which grew only with hydrogen. Tricultures consisting of MPOB, one of the Methanobrevibacter strains, and organisms which are able to convert formate into H(2) plus CO(2) (Desulfovibrio strain G11 or the homoacetogenic bacterium EE121) also degraded propionate. The MPOB, in the absence of methanogens, was able to couple propionate conversion to fumarate reduction. This propionate conversion was inhibited by hydrogen and by formate. Formate and hydrogen blocked the energetically unfavorable succinate oxidation to fumarate involved in propionate catabolism. Low formate and hydrogen concentrations are required for the syntrophic degradation of propionate by MPOB. In triculture with Methanospirillum hungatei and the aceticlastic Methanothrix soehngenii, propionate was degraded faster than in biculture with Methanospirillum hungatei, indicating that low acetate concentrations are favorable for propionate oxidation as well.

Enrichment of Thermophilic Propionate-Oxidizing Bacteria in Syntrophy with Methanobacterium thermoautotrophicum or Methanobacterium thermoformicicum

Thermophilic propionate-oxidizing, proton-reducing bacteria were enriched from the granular methanogenic sludge of a bench-scale upflow anaerobic sludge bed reactor operated at 55 degrees C with a mixture of volatile fatty acids as feed. Thermophilic hydrogenotrophic methanogens had a high decay rate. Therefore, stable, thermophilic propionate-oxidizing cultures could not be obtained by using the usual enrichment procedures. Stable and reproducible cultivation was possible by enrichment in hydrogen-pregrown cultures of Methanobacterium thermoautotrophicum DeltaH which were embedded in precipitates of FeS, achieved by addition of FeCl(2) to the media. The propionate-oxidizing bacteria formed spores which resisted pasteurization for 30 min at 90 degrees C or 10 min at 100 degrees C. Highly purified cultures were obtained with either M. thermoautotrophicum DeltaH or Methanobacterium thermoformicicum Z245 as the syntrophic partner organism. The optimum temperature for the two cultures was 55 degrees C. Maximum specific growth rates of cultures with M. thermoautotrophicum DeltaH were somewhat lower than those of cultures with M. thermoformicicum Z245 (0.15 and 0.19 day, respectively). Growth rates were even higher (0.32 day) when aceticlastic methanogens were present as well. M. thermoautotrophicum DeltaH is an obligately hydrogen-utilizing methanogen, showing that interspecies hydrogen transfer is the mechanism by which reducing equivalents are channelled from the acetogens to this methanogen. Boundaries of hydrogen partial pressures at which propionate oxidation occurred were between 6 and 34 Pa. Formate had a strong inhibitory effect on propionate oxidation in cultures with M. thermoautotrophicum. Inhibition by formate was neutralized by addition of the formate-utilizing methanogen or by addition of fumarate. Results indicate that formate inhibited succinate oxidation to fumarate, an intermediate step in the biochemical pathway of propionate oxidation.

Interspecies acetate transfer influences the extent of anaerobic benzoate degradation by syntrophic consortia

Benzoate degradation by an anaerobic, syntrophic bacterium, strain SB, in coculture with Desulfovibrio sp. strain G-11 reached a threshold value which depended on the amount of acetate added and ranged from about 2.5 to 29.9 (mu)M. Increasing acetate concentrations also uncompetitively inhibited benzoate degradation. The apparent V(infmax) and apparent K(infm) for benzoate degradation decreased with increasing acetate concentration, but the benzoate degradation capacities (V(infmax)/K(infm)) of cell suspensions remained comparable. The addition of an acetate-using bacterium to cocultures after the threshold was reached resulted in the degradation of benzoate to below the detection limit. Mathematical simulations showed that the benzoate threshold was not predicted by the inhibitory effect of acetate on benzoate degradation kinetics. With nitrate instead of sulfate as the terminal electron acceptor, no benzoate threshold was observed in the presence of 20 mM acetate even though the kinetics of benzoate degradation were slower with nitrate rather than sulfate as the electron acceptor. When strain SB was grown with Desulfovibrio sp. strain DG2 that had a fourfold-lower V(infmax) for hydrogen use than strain G-11, the V(infmax) for benzoate degradation was 37-fold lower than that of strain SB-G-11 cocultures. The Gibb's free energy for benzoate degradation was less negative in cell suspensions with a threshold than in suspensions without a threshold. These studies showed that the threshold was not a function of the inhibition of benzoate degradation by acetate or the toxicity of the undissociated form of acetate. Rather, a critical or minimal Gibb's free energy may exist where thermodynamic constraints preclude further benzoate degradation.

Influence of mass transfer limitations on determination of the half saturation constant for hydrogen uptake in a mixed-culture CH(4)-producing enrichment

There is strong evidence in the literature supporting the existence of significant mass transfer limitations on the kinetics of exogenous H(2) consumption by methanogens. The half saturation constant for H (2) uptake by a mixed-culture, CH(4) producing enrichment was measured using an experimental protocol that avoided internal mass transfer limitations. The value obtained was two orders of magnitude smaller than any other previously reported. A mathematical model for acetogenic syntrophic associations was developed to check the capacity of H(2) as electron transporter between syntrophic partners. It was found that H(2) diffusion could account for the rate of transport of electrons between the syntrophic microorganisms and that formate is not a necessary intermediate. The possibility that formate may be an intermediate in this system was not ruled out. A Monod-type kinetic equation was modified to include the observed H(2) threshold effect. This modified equation was used to predict the CH(4)-production rate in a batch-fed digester. The results show that the external and internal H(2) pools are kinetically coupled.

Carbon balance of anaerobic granulation process: carbon credit

The concept of carbon credit arose out of increasing awareness of the need to reduce emissions of greenhouse gases to combat global warming which was formalized in the Kyoto protocol. In addition to contribution to sustainable development with energy recovery in the form of methane, carbon credits can be claimed by application of advanced anaerobic processes in wastewater treatment for reducing emissions of greenhouse gases. As anaerobic granular systems are capable of handling high organic loadings concomitant with high strength wastewater and short hydraulic retention time, they could render much more carbon credits than other conventional anaerobic systems. This study investigated the potential carbon credit derived from laboratory-scale upflow anaerobic sludge blanket (UASB) reactors based on a carbon balance analysis. Methane emission reduction could be calculated by calculating the difference of UASB reactors and open lagoon treatment systems. Based on the 2.5l bench-scale reactor, the total CH(4) emissions reduction was calculated as 29 kg CO(2)/year. On scaling up to a typical full-scale anaerobic digester, the total CH(4) emissions reduction could achieve 46,420 tons CO(2) reduction/year. The estimated carbon credits would amount to 278,500 US$ per year by assuming a carbon price of 6 US$ per metric ton CO(2) reduction. The analysis postulated that it is financially viable to invest in advanced anaerobic granular treatment system from the revenue generated from carbon credits.

Physiology, ecology, phylogeny, and genomics of microorganisms capable of syntrophic metabolism

Syntrophic metabolism is diverse in two respects: phylogenetically with microorganisms capable of syntrophic metabolism found in the Deltaproteobacteria and in the low G+C gram-positive bacteria, and metabolically given the wide variety of compounds that can be syntrophically metabolized. The latter includes saturated fatty acids, unsaturated fatty acids, alcohols, and hydrocarbons. Besides residing in freshwater and marine anoxic sediments and soils, microbes capable of syntrophic metabolism also have been observed in more extreme habitats, including acidic soils, alkaline soils, thermal springs, and permanently cold soils, demonstrating that syntrophy is a widely distributed metabolic process in nature. Recent ecological and physiological studies show that syntrophy plays a far larger role in carbon cycling than was previously thought. The availability of the first complete genome sequences for four model microorganisms capable of syntrophic metabolism provides the genetic framework to begin dissecting the biochemistry of the marginal energy economies and interspecies interactions that are characteristic of the syntrophic lifestyle.

Feasibility study of the anaerobic digestion of dewatered pig slurry by means of polyacrylamide

Liquid livestock waste can be managed by separating liquid and solid fractions then treating each separately by applying best available technology, such as anaerobic digestion for the solid fraction. There is an increasing use of polyacrylamide (PAM) as a flocculant agent to improve solid-liquid separation. In the present work, the anaerobic toxicity of PAM residues and the optimal range of total solids concentration for maximum methane production were studied as a function of PAM dosage. Results showed that dry matter and its volatile solids content increased significantly with increasing PAM dosage. Batch anaerobic tests showed that methane yield decreased linearly with increasing total solids, while the methane production per unit of raw substrate reached a maximum at 16.4% total solids. No PAM toxicity was measured for PAM concentrations below 415 g/kg total solids, but some indirect inhibitory phenomena were observed, such as a limited hydrolysis rate due to particle aggregation, and inhibition of methanogenesis by high ammonia concentration.

Homoacetogenesis as the alternative pathway for H2 sink during thermophilic anaerobic degradation of butyrate under suppressed methanogenesis

Butyrate degradation for hydrogen production under conditions suppressing methanogenesis was evaluated in continuously fed-tank reactors operated at 55 degrees C and started up with digested manure as inoculum. This study shows that the reaction of butyrate degradation to acetate and hydrogen could happen when gas sparging was applied. Gas sparging was very important for reducing hydrogen partial pressure and made the reaction thermodynamically possible. Almost no hydrogen or methane (methane production was prevented by the addition of 2-bromoethane-sulfonic acid) was detected, indicating that the H2 produced from butyrate oxidation was consumed in a subsequent step. It was found by isotope experiments that hydrogen produced from butyrate degradation reacted immediately with CO2 to form acetate via homoacetogenesis. When CO2/HCO(3-) was not provided in the system, butyrate degradation was no longer possible and butyrate-degrading cultures were washed out. It was furthermore found that the microorganisms responsible for homoacetogenesis were likely present in normal anaerobic environments, such as biogas reactors.

Biotechnological intensification of biogas production

The importance of syntrophic relationships among microorganisms participating in biogas formation has been emphasized, and the regulatory role of in situ hydrogen production has been recognized. It was assumed that the availability of hydrogen may be a limiting factor for hydrogenotrophic methanogens. This hypothesis was tested under laboratory and field conditions by adding a mesophilic (Enterobacter cloacae) or thermophilic hydrogen-producing (Caldicellulosyruptor saccharolyticus) strain to natural biogas-producing consortia. The substrates were waste water sludge, dried plant biomass from Jerusalem artichoke, and pig manure. In all cases, a significant intensification of biogas production was observed. The composition of the generated biogas did not noticeably change. In addition to being a good hydrogen producer, C. saccharolyticus has cellulolytic activity; hence, it is particularly suitable when cellulose-containing biomass is fermented. The process was tested in a 5-m(3) thermophilic biogas digester using pig manure slurry as a substrate. Biogas formation increased at least 160-170% upon addition of the hydrogen-producing bacteria as compared to the biogas production of the spontaneously formed microbial consortium. Using the hydrogenase-minus control strain provided evidence that the observed enhancement was due to interspecies hydrogen transfer. The on-going presence of C. saccharolyticus was demonstrated after several months of semicontinuous operation.

Ecological control analysis: being(s) in control of mass flux and metabolite concentrations in anaerobic degradation processes

Identification of the functional groups of microorganisms that are predominantly in control of fluxes through, and concentrations in, microbial networks would benefit microbial ecology and environmental biotechnology: the properties of those controlling microorganisms could be studied or monitored specifically or their activity could be modulated in attempts to manipulate the behaviour of such networks. Herein we present ecological control analysis (ECA) as a versatile mathematical framework that allows for the quantification of the control of each functional group in a microbial network on its process rates and concentrations of intermediates. In contrast to current views, we show that rates of flow of matter are not always limited by a single functional group; rather flux control can be distributed over several groups. Also, control over intermediate concentrations is always shared. Because of indirect interactions, through other functional groups, the concentration of an intermediate can also be controlled by functional groups not producing or consuming it. Ecological control analysis is illustrated by a case study on the anaerobic degradation of organic matter, using experimental data obtained from the literature. During anaerobic degradation, fermenting microorganisms interact with terminal electron-accepting microorganisms (e.g. halorespirers, methanogens). The analysis indicates that flux control mainly resides with fermenting microorganisms, but can shift to the terminal electron-accepting microorganisms under less favourable redox conditions. Paradoxically, halorespiring microorganisms do not control the rate of perchloroethylene and trichloroethylene degradation even though they catalyse those processes themselves.

Responses of the biogas process to pulses of oleate in reactors treating mixtures of cattle and pig manure

The effect of oleate on the anaerobic digestion process was investigated. Two thermophilic continuously stirred tank reactors (CSTR) were fed with mixtures of cattle and pig manure with different total solid (TS) and volatile solid (VS) content. The reactors were subjected to increasing pulses of oleate. Following pulses of 0.5 and 1.0 g oleate/L, the most distinct increase in volatile fatty acid (VFA) concentrations were observed in the reactor with the lowest TS/VS content. This suggests a higher adsorption of oleate on the surfaces of biofibers in the reactor with the highest TS/VS and a less pronounced inhibition of the anaerobic digestion process. On the other hand, addition of 2.0 g oleate/L severely inhibited the process in both reactors, and a significant increase in all VFA concentrations combined with an immediate drop in methane production was noticed. However, 20 days after the reactors had been exposed to oleate both reactors showed a lower VFA concentration along with a higher methane production than before the pulses. This indicates that oleate had a stimulating effect on the overall process. The improved acetogenic and methanogenic activity in the reactors was confirmed in batch activity tests. In addition to this, toxicity tests revealed that the oleate pulses induced an increase in the tolerance level of acetotrophic methanogens towards oleate. When evaluating the usability of different process parameters (i.e., VFA and methane production) as indicators of process recovery, following the inhibition by oleate, propionate was found to be most suitable.

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.

Improvement of acetate production from lactose by growing Clostridium thermolacticum in mixed batch culture

AIMS

The objective of this study was to increase the acetate production by Clostridium thermolacticum growing on lactose, available as a renewable resource in the milk and whey permeate from the cheese industry.

METHODS AND RESULTS

Experiments for increased acetate productivity by thermophilic anaerobes grown on lactose were carried out in batch cultures. Lactose at concentration of 30 mmol l(-1) (10 g l(-1)) was completely degraded by Cl. thermolacticum and growth rate was maximal. High concentrations of by-products, ethanol, lactate, hydrogen and carbon dioxide were generated. By using an efficient hydrogenotroph, Methanothermobacter thermoautotrophicus, in a defined thermophilic anaerobic consortium (58 degrees C) with Cl. thermolacticum and the acetogenic Moorella thermoautotrophica, the hydrogen partial pressure was dramatically lowered. As a consequence, by-products concentrations were significantly reduced and acetate production was increased.

CONCLUSION

Through efficient in situ hydrogen scavenging in the consortium, the metabolic pattern was modified in favour of acetate production, at the expense of reduced by-products like ethanol.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of this thermophilic anaerobic consortium opens new opportunities for the efficient valorization of lactose, the main waste from the cheese industry, and production of calcium-magnesium acetate, an environmentally friendly road de-icer.

Monitoring and control of anaerobic reactors

The current status in monitoring and control of anaerobic reactors is reviewed. The influence of reactor design and waste composition on the possible monitoring and control schemes is examined. After defining the overall control structure, and possible control objectives, the possible process measurements are reviewed in detail. In the sequel, possible manipulated variables, such as the hydraulic retention time, the organic loading rate, the sludge retention time, temperature, pH and alkalinity are evaluated with respect to the two main reactor types: high-rate and low-rate. Finally, the different control approaches that have been used are comprehensively described. These include simple and adaptive controllers, as well as more recent developments such as fuzzy controllers, knowledge-based controllers and controllers based on neural networks.

Energetics of syntrophic propionate oxidation in defined batch and chemostat cocultures

Propionate consumption was studied in syntrophic batch and chemostat cocultures of Syntrophobacter fumaroxidans and Methanospirillum hungatei. The Gibbs free energy available for the H(2)-consuming methanogens was <-20 kJ mol of CH(4)(-1) and thus allowed the synthesis of 1/3 mol of ATP per reaction. The Gibbs free energy available for the propionate oxidizer, on the other hand, was usually >-10 kJ mol of propionate(-1). Nevertheless, the syntrophic coculture grew in the chemostat at steady-state rates of 0.04 to 0. 07 day(-1) and produced maximum biomass yields of 2.6 g mol of propionate(-1) and 7.6 g mol of CH(4)(-1) for S. fumaroxidans and M. hungatei, respectively. The energy efficiency for syntrophic growth of S. fumaroxidans, i.e., the biomass produced per unit of available Gibbs free energy was comparable to a theoretical growth yield of 5 to 12 g mol of ATP(-1). However, a lower growth efficiency was observed when sulfate served as an additional electron acceptor, suggesting inefficient energy conservation in the presence of sulfate. The maintenance Gibbs free energy determined from the maintenance coefficient of syntrophically grown S. fumaroxidans was surprisingly low (0.14 kJ h(-1) mol of biomass C(-1)) compared to the theoretical value. On the other hand, the Gibbs free-energy dissipation per mole of biomass C produced was much higher than expected. We conclude that the small Gibbs free energy available in many methanogenic environments is sufficient for syntrophic propionate oxidizers to survive on a Gibbs free energy that is much lower than that theoretically predicted.