Similar Methanogenic Shift but Divergent Syntrophic Partners in Anaerobic Digesters Exposed to Direct versus Successive Ammonium Additions

During anaerobic digestion (AD) of protein-rich wastewater, ammonium (NH4+) is released by amino acid degradation. High NH4+ concentrations disturb the AD microbiome balance, leading to process impairments. The sensitivity of the AD microbiome to NH4+ and the inhibition threshold depend on multiple parameters, especially the previous microbial acclimation to ammonium stress. However, little is known about the effect of different NH4+ acclimation strategies on the differential expression of key active microbial taxa. Here, we applied NH4+ inputs of increasing intensity (from 1.7 to 15.2 g N-NH4+ liters-1) in batch assays fed with synthetic wastewater, according to two different strategies: (i) direct independent inputs at a unique target concentration and (ii) successive inputs in a stepwise manner. In both strategies, along the NH4+ gradient, the active methanogens shifted from acetoclastic Methanosaeta to Methanosarcina and eventually hydrogenotrophic Methanoculleus. Despite shorter latency times, the successive input modality led to lower methane production rate, lower soluble chemical oxygen demand (sCOD) removal efficiency, and lower half maximal inhibitory concentration, together with higher volatile fatty acid (VFA) accumulation, compared to the independent input modality. These differential performances were associated with a drastically distinct succession pattern of the active bacterial partners in both experiments. In particular, the direct exposure modality was characterized by a progressive enrichment of VFA producers (mainly Tepidimicrobium) and syntrophic VFA oxidizers (mainly Syntrophaceticus) with increasing NH4+ concentration, while the successive exposure modality was characterized by a more dynamic succession of VFA producers (mainly Clostridium, Sporanaerobacter, Terrisporobacter) and syntrophic VFA oxidizers (mainly Tepidanaerobacter, Syntrophomonas). These results bring relevant insights for improved process management through inoculum adaptation, bioaugmentation, or community-driven optimization. IMPORTANCE Anaerobic digestion (AD) is an attractive biotechnological process for wastewater bioremediation and bioenergy production in the form of methane-rich biogas. However, AD can be inhibited by ammonium generated by protein-rich effluent, commonly found in agro-industrial activities. Insights in the microbial community composition and identification of AD key players are crucial for anticipating process impairments in response to ammonium stress. They can also help in defining an optimal microbiome adapted to high ammonium levels. Here, we compared two strategies for acclimation of AD microbiome to increasing ammonium concentration to better understand the effect of this stress on the methanogens and their bacterial partners. Our results suggest that long-term cumulative exposure to ammonia disrupted the AD microbiome more strongly than direct (independent) ammonium additions. We identified bioindicators with different NH4+ tolerance capacity among VFA producers and syntrophic VFA oxidizers.

Early response of methanogenic archaea to H2 as evaluated by metagenomics and metatranscriptomics

BACKGROUND

The molecular machinery of the complex microbiological cell factory of biomethane production is not fully understood. One of the process control elements is the regulatory role of hydrogen (H2). Reduction of carbon dioxide (CO2) by H2 is rate limiting factor in methanogenesis, but the community intends to keep H2 concentration low in order to maintain the redox balance of the overall system. H2 metabolism in methanogens becomes increasingly important in the Power-to-Gas renewable energy conversion and storage technologies.

RESULTS

The early response of the mixed mesophilic microbial community to H2 gas injection was investigated with the goal of uncovering the first responses of the microbial community in the CH4 formation and CO2 mitigation Power-to-Gas process. The overall microbial composition changes, following a 10 min excessive bubbling of H2 through the reactor, was investigated via metagenome and metatranscriptome sequencing. The overall composition and taxonomic abundance of the biogas producing anaerobic community did not change appreciably 2 hours after the H2 treatment, indicating that this time period was too short to display differences in the proliferation of the members of the microbial community. There was, however, a substantial increase in the expression of genes related to hydrogenotrophic methanogenesis of certain groups of Archaea. As an early response to H2 exposure the activity of the hydrogenotrophic methanogenesis in the genus Methanoculleus was upregulated but the hydrogenotrophic pathway in genus Methanosarcina was downregulated. The RT-qPCR data corroborated the metatranscriptomic RESULTS: H2 injection also altered the metabolism of a number of microbes belonging in the kingdom Bacteria. Many Bacteria possess the enzyme sets for the Wood-Ljungdahl pathway. These and the homoacetogens are partners for syntrophic community interactions between the distinct kingdoms of Archaea and Bacteria.

CONCLUSIONS

External H2 regulates the functional activity of certain Bacteria and Archaea. The syntrophic cross-kingdom interactions in H2 metabolism are important for the efficient operation of the Power-to-Gas process. Therefore, mixed communities are recommended for the large scale Power-to-Gas process rather than single hydrogenotrophic methanogen strains. Fast and reproducible response from the microbial community can be exploited in turn-off and turn-on of the Power-to-Gas microbial cell factories.

Performance of different methanogenic species for the microbial electrosynthesis of methane from carbon dioxide

Microbial electrosynthesis (MES) is a promising technology to convert CO₂ and electricity into the biofuel methane using methanogens. Until now, most investigations on electro-methanogenesis are "proof-of-principle" studies. In this paper, different strains were quantitatively compared in regard to final methane concentration, yields based on CO₂-conversion, productivities as well as Coulombic efficiencies in order to identify suitable organisms for MES. Methanococcus vannielii, Methanococcus maripaludis, Methanolacinia petrolearia, Methanobacterium congolense, and Methanoculleus submarinus were able to produce methane via MES at -700 mV vs. standard hydrogen electrode (SHE). Beside methane also biological H₂ production was detected during MES, which might be due to the involvement of hydrogenases. A direct electron transfer pathway is most likely. Obviously, M. maripaludis is the most resource efficient methane producer in microbial electrosynthesis regarding the methane productivity (8.81 ± 0.51 mmol m^-2 d^-1) and the Coulombic efficiency (58.9 ± 0.8%).

The time response of anaerobic digestion microbiome during an organic loading rate shock

Knowledge of connections between operational conditions, process stability, and microbial community dynamics is essential to enhance anaerobic digestion (AD) process efficiency and management. In this study, the detailed temporal effects of a sudden glycerol-based organic overloading on the AD microbial community and process imbalance were investigated in two replicate anaerobic digesters by a time-intensive sampling scheme. The microbial community time response to the overloading event was shorter than the shifts of reactor performance parameters. An increase in bacterial community dynamics and in the abundances of several microbial taxa, mainly within the Firmicutes, Tenericutes, and Chloroflexi phyla and Methanoculleus genera, could be detected prior to any shift on the reactor operational parameters. Reactor acidification already started within the first 24 h of the shock and headed the AD process to total inhibition in 72 h alongside with the largest shifts on microbiome, mostly the increase of Anaerosinus sp. and hydrogenotrophic methanogenic Archaea. In sum, this work proved that AD microbial community reacts very quickly to an organic overloading and some shifts occur prior to alterations on the performance parameters. The latter is very interesting as it can be used to improve AD process management protocols.

Stimulation of long-term ammonium nitrogen deposition on methanogenesis by Methanocellaceae in a coastal wetland

Atmospheric nitrogen deposition caused by human activities has been receiving much attention. Here, after long-term simulated ammonium and nitrate nitrogen deposition (NH₄Cl, KNO₃, and NH₄NO₃) in the Yellow River Delta (YRD), a sensitive coastal wetland ecosystem typified by a distinct wet and dry season, methane fluxes were measured, by adopting a closed static chamber technique. The results showed that deposition of ammonium nitrogen accelerated methane emissions all year round. Ammonium nitrogen deposition transformed the YRD from a methane sink into a source during the dry season. Methanocellaceae is the only methanogen with increased abundance after the application of NH₄Cl and NH₄NO₃, which promoted methane emissions, during the wet season. The findings suggested that Methanocellaceae may facilitate methane emissions in response to increased ammonium nitrogen deposition. Other methanogens might have profited from ammonium supplementation, such as Methanosarcinaceae. Deposition of nitrate nitrogen did not affect methane flux significantly. To the best of our knowledge, this study is the first to show that Methanocellaceae may be responsible for methane production in coastal wetland system. This study highlights the significant effect of ammonium nitrogen and slight effect of nitrate nitrogen on methane emission in the YRD and it will be helpful to understand the microbial mechanism responding to increased nitrogen deposition in the sensitive coastal wetland ecosystem.

Anaerobic codigestion of pretreated wheat straw with cattle manure and analysis of the microbial community

Wheat straw (WS) was pretreated with four concentrations of H2O2 (1%, 2%, 3%, and 4%) and was anaerobically codigested with dairy cattle manure (CM) at various ratios from 100:0 to 0:100. Wet-state H2O2 pretreatment effectively enhanced the biodegradability and methane yield of the WS. The optimal concentration of H2O2 for treating WS was 3%. The methane yield was higher with the codigestion of CM and H2O2-treated WS than with untreated WS and higher than with H2O2-treated WS alone or CM alone. A 40:60 ratio of H2O2-treated WS mixed with CM produced the highest yield of methane (320.8 mL g volatile solid (VS)(-1)). Results of high-throughput sequencing indicated that the methanogenic community shifted during the codigestion from the acetoclastic methanogens, Methanosarcina, to the hydrogenotrophic methanogens, Methanosphaera and Methanoculleus.

Methane yields and methanogenic community changes during co-fermentation of cattle slurry with empty fruit bunches of oil palm

The biomethane potential and structural changes of the methanogenic community in a solid-state anaerobic digestion process co-digesting cattle slurry and empty fruit bunches were investigated under mesophilic (37°C) and thermophilic (55°C) conditions. Phylogenetic microarrays revealed the presence of two hydrogenotrophic genera (Methanoculleus and Methanobrevibacter) and one acetoclastic genus (Methanosarcina). Methanosarcina numbers were found to increase in both mesophilic and thermophilic treatments of empty fruit bunches. Methanobrevibacter, which dominated in the cattle slurry, remained constant during anaerobic digestion (AD) at 37°C and decreased in numbers during digestion at 55°C. Numbers of Methanoculleus remained constant at 37°C and increased during the thermophilic digestion. Physicochemical data revealed non-critical concentrations for important monitoring parameters such as total ammonia nitrogen, free ammonia nitrogen and volatile fatty acids in all treatments after AD. The biomethane potential of empty fruit bunches was higher under thermophilic conditions than under mesophilic conditions.

Performance and microbial community analysis of the anaerobic reactor with coke oven gas biomethanation and in situ biogas upgrading

A new method for simultaneous coke oven gas (COG) biomethanation and in situ biogas upgrading in anaerobic reactor was developed in this study. The simulated coke oven gas (SCOG) (92% H2 and 8% CO) was injected directly into the anaerobic reactor treating sewage sludge through hollow fiber membrane (HFM). With pH control at 8.0, the added H2 and CO were fully consumed and no negative effects on the anaerobic degradation of sewage sludge were observed. The maximum CH4 content in the biogas was 99%. The addition of SCOG resulted in enrichment and dominance of homoacetogenetic genus Treponema and hydrogenotrophic genus Methanoculleus in the liquid, which indicated that H2 were converted to methane by both direct (hydrogenotrophic methanogenesis) and indirect (homoacetogenesis+aceticlastic methanogenesis) pathways in the liquid. However, the aceticlasitic genus Methanosaeta was dominant for archaea in the biofilm on the HFM, which indicated indirect (homoacetogenesis+aceticlastic methanogenesis) H2 conversion pathway on the biofilm.

Bioaugmentation with an acetate-oxidising consortium as a tool to tackle ammonia inhibition of anaerobic digestion

Ammonia is the major inhibitor of anaerobic digestion (AD) process in biogas plants. In the current study, the bioaugmentation of the ammonia tolerant SAO co-culture (i.e. Clostridium ultunense spp. nov. in association with Methanoculleus spp. strain MAB1) in a mesophilic up-flow anaerobic sludge blanket (UASB) reactor subjected to high ammonia loads was tested. The co-cultivation in fed-batch reactors of a fast-growing hydrogenotrophic methanogen (i.e. Methanoculleus bourgensis MS2(T)) with the SAO co-culture was also investigated. Results demonstrated that bioaugmentation of SAO co-culture in a UASB reactor was not possible most likely due to the slow maximum growth rate (μmax=0.007 h(-1)) of the culture caused by the methanogenic partner. The addition of M. bourgensis to SAO led to 42% higher growth rate (μmax=0.01 h(-1)) in fed-batch reactors. This indicates that methanogens were the slowest partners of the SAO co-culture and therefore were the limiting factor during bioaugmentation in the UASB reactor.

Comparison of methanogenic community structure and anaerobic process performance treating swine wastewater between pilot and optimized lab scale bioreactors

To investigate methanogenic community structure and process performance of anaerobic digestion treating swine wastewater at different scale, a pilot plant with 20 m(3) of effective working volume and lab scale methanogenic digester with 6L working volume were operated for 71 days and 6 turnover periods, respectively. During the steady state of anaerobic digestion, COD and VS removal efficiency in pilot plant were 65.3±3.2, 51.6±4.3%, respectively, which was similar to those in lab scale. However, calculated VFAs removal efficiency and methane yield were lower in pilot plant than in lab scale digester. Also, organics removal efficiencies, which consist of total carbohydrates, proteins, and lipids, were different between pilot and lab scale. These results were thought to be due to the ratio of carbohydrates to proteins in the raw swine wastewater. As a result of qualitative microbial analysis, Methanoculleus receptaculii, and Methanoculleus bourgensis, were commonly concerned with methane production.

Impact of protein-, lipid- and cellulose-containing complex substrates on biogas production and microbial communities in batch experiments

In the present study, nine complex organic substrates from three classes (protein-, lipid-, and cellulose-rich) were investigated in batch experiments and compared with a control in order to evaluate their potential for utilisation as substrates for biogas production. High methane production was observed from protein-rich substrates; problems arose from lipid-containing, lactose and cellulose fermentation. Using DGGE analysis it could be shown that different classes of substrate resulted in different microbial communities, whereupon similar substrates tended to show a similar microbial structure. By means of qPCR Methanoculleus sp., a hydrogenotrophic methanogen was found to be the most abundant organism in the batch experiments. Additionally, it could be demonstrated that methanogenic organisms withstood adverse environmental conditions for at least an incubation period of 55 days, pointing to a high stability of the archaeal community even in times of decreasing or even failing fermenter performance.

Pulp mill wastewater sediment reveals novel methanogenic and cellulolytic populations

Pulp mill wastewater generated from wheat straw is characterized as high alkalinity and very high COD pollution load. A naturally developed microbial community in a pulp mill wastewater storage pool that had been disused were investigated in this study. Owing to natural evaporation and a huge amount of lignocellulose's deposition, the wastewater sediment contains high concentrations of organic matters and sodium ions, but low concentrations of chloride and carbonate. The microbiota inhabiting especially anaerobic community, including methanogenic arhcaea and cellulolytic species, was studied. All archaeal sequences fall into 2 clusters of family Halobacteriaceae and methanogenic archaeon in the phylum Euryarchaeota. In the methanogenic community, phylogenetic analysis of methyl coenzyme M reductase A (mcrA) genes targeted to novel species in genus Methanoculleus or novel genus of order Methanomicrobiales. The predominance of Methanomicrobiales suggests that methanogenesis in this system might be driven by the hydrogenotrophic pathway. As the important primary fermenter for methane production, the cellulolytic community of enzyme GHF48 was found to be dominated by narrower breadth of novel clostridial cellulase genes. Novel anoxic functional members in such extreme sediment provide the possibility of enhancing the efficiency of anoxic treatment of saline and alkaline wastewaters, as well as benefiting to the biomass transformation and biofuel production processes.

[Analysis of methanogenic community of anaerobic granular sludge based on mcrA gene]

The methanogenic community in anaerobic granular sludge from a full-scale UASB treating avernectin wastewater was analyzed based on mcrA gene, compared to 16S rRNA gene. The results indicated that the diversity indices of methanogenic community, including Shannon diversity index, Margalef richness index and Berger-Parker dominance index, were no difference between mcrA gene-based and 16S rRNA gene-based PCR products analysis by DGGE, although their DGGE band patterns were different, implying that the diversity analysis of methanogenic community based on mcrA genes was consistent with 16S rRNA gene. The phylogenetic analysis of dominant methanogenic populations based on these two target genes also showed resemble and Methanobacteriales and Methanosarcinales were determined to be the main orders of methanogenic populations in anaerobic granular sludge. On the other hand, the difference in phylogenetic analysis suggested simultaneously some group-specific of the two target genes. The hybridization of methanogenic community in FISH analysis based on two target genes was almost identical except a little different hybridization areas. The average relative abundance of methanogenic community was 24.25% +/- 6. 47% detected by FISH based on mcrA gene, lower than that based on 16S rRNA gene (33.42% +/- 2.34%). Then it could be concluded that the analysis of methanogenic community based on mcrA gene and 16S rRNA gene exhibited high resemblance and mcrA gene could used to be target gene for methanogenic community, as an alternative of 16S rRNA gene.

Effects of various fatty acid amendments on a microbial digester community in batch culture

Since biogas production is becoming increasingly important the understanding of anaerobic digestion processes is fundamental. However, large-scale digesters often lack online sensor equipment to monitor key parameters. Furthermore the possibility to selectively change fermenting parameter settings in order to investigate methane output or microbial changes is limited. In the present study we examined the possibility to investigate the microbial community of a large scale (750,000 L) digester within a laboratory small-scale approach. We studied the short-term response of the downscaled communities on various fatty acids and its effects on gas production and compared it with data from the original digester sludge. Even high loads of formic acid led to distinct methane formation, whereas high concentrations of other acids (acetic, butyric, propionic acid) caused a marked inhibition of methanogenesis coupled with an increase in hydrogen concentration. Molecular microbial techniques (DGGE/quantitative real-time-PCR) were used to monitor the microbial community changes which were related to data from GC and HPLC analysis. DGGE band patterns showed that the same microorganisms which were already dominant in the original digester re-established again in the lab-scale experiment. Very few microorganisms dominated the whole fermenting process and species diversity was not easily influenced by moderate varying fatty acid amendments--Methanoculleus thermophilus being the most abundant species throughout the variants. MCR-copy number determined via quantitative real-time-PCR--turned out to be a reliable parameter for quantification of methanogens, even in a very complex matrix like fermenter sludge. Generally the downscaled batch approach was shown to be appropriate to investigate microbial communities from large-scale digesters.

[Analysis of methanogenic community of anaerobic granular sludge in a full-scale UASB treating avermectin wastewater]

Methanogens is considered to be important functional microbial population in anaerobic granular sludge. Fluorescence in situ hybridization was used to analyze methanogens of anaerobic granular sludge in a full-scale UASB treating avermectin wastewater. The results indicated that the distribution forms of methanogens, Methanobacteriales and Methanosarcinales were identical on the surface and inner face of granular sludge samples in different formation stages (with different diameters) , although the relative abundances of these methanogens were different. The relative abundances of methanogens on the inner face were larger than these on the surface of granular sludge samples. The relative abundances of Methanobacteriales were larger than these of Methanosarcinales. The relative abundances of methanogens in all granular sludge samples ranged from (25.50 +/- 8.63)% to (48.67 +/- 8.87)%. The maximum relative abundances of methanogens were obtained in mature granular sludge with diameter of 1.0-2.0 mm, (47.08 +/- 8.26)% on the surface and (48.67 +/- 8.87)% on the inner face, respectively. The avermectin residue in wastewater showed possible inhibition effect on methanogens. The maximum specific methanogenic activities of granular sludge samples ranged from 1.311 to 1.562 g/(g x d), varying as same as COD removal ratios and relative abundances of methanogens, implying the strong correlation of methanogens with bioactivity of granular sludge.

Focused-Pulsed sludge pre-treatment increases the bacterial diversity and relative abundance of acetoclastic methanogens in a full-scale anaerobic digester

The low yield of methane in anaerobic digestion systems represents a loss of energy that can be captured as renewable energy when the input sludge is pre-treated to make it more bioavailable. We investigated Focused-Pulsed (FP) pre-treatment, which make complex biological solids more bioavailable by exposing them to rapid pulses of a very strong electric field. We investigated how the microbial ecology in full-scale anaerobic digesters was altered when the digesters' methane production rate was significantly increased by FP pre-treatment. Using clone libraries and quantitative PCR, we demonstrated a shift in methanogenic genera to the acetate-cleaving Methanosaeta and away from the H(2)-oxidizing Methanoculleus. In addition, the acetate concentration in the effluent was very low, probably due to the dominance of Methanosaeta, which are capable of scavenging low acetate concentrations. By analyzing 36,797 pyrosequencing tags from the V6 region of the bacterial 16S rRNA gene, along with archaeal and bacterial clone libraries and quantitative PCR, we compared the microbial community composition before and after FP treatment. The bacterial community became more diverse after FP pre-treatment and was populated more by phylotypes associated with cellulose fermentation (Ruminococcus), scavenging of biomass-derived organic carbon (Chloroflexi), and homo-acetogenesis (Treponema). We interpret that, as the overall activity of the community was stimulated by addition of more bioavailable organic matter, the bacterial community became more phylogenetically diverse to take advantage of the added input of biodegradable material and in response to the more efficient utilization of acetate by Methanosaeta.

Magnetic resonance microscopy of iron transport in methanogenic granules

Interactions between anaerobic biofilms and heavy metals such as iron, cobalt or nickel are largely unknown. Magnetic resonance imaging (MRI) is a non-invasive method that allows in situ studies of metal transport within biofilm matrixes. The present study investigates quantitatively the penetration of iron (1.7 5mM) bound to ethylenediaminetetraacetate (EDTA) into the methanogenic granules (spherical biofilm). A spatial resolution of 109x109x218 microm(3) and a temporal resolution of 11 min are achieved with 3D Turbo Spin Echo (TSE) measurements. The longitudinal relaxivity, i.e. the slope the dependence of the relaxation rate (1/T(1)) on the concentration of paramagnetic metal ions, was used to measure temporal changes in iron concentration in the methanogenic granules. It took up to 300 min for the iron-EDTA complex ([FeEDTA](2-)) to penetrate into the methanogenic granules (3-4mm in diameter). The diffusion was equally fast in all directions with irregularities such as diffusion-facilitating channels and diffusion-resistant zones. Despite these irregularities, the overall process could be modeled using Fick's equations for diffusion in a sphere, because immobilization of [FeEDTA](2-) in the granular matrix (or the presence of a reactive barrier) was not observed. The effective diffusion coefficient (D(ejf)) of [FeEDTA](2-) was found to be 2.8x10(-11)m(2)s(-1), i.e. approximately 4% of D(ejf) of [FeEDTA](2-) in water. The Fickian model did not correspond to the processes taking place in the core of the granule (3-5% of the total volume of the granule), where up to 25% over-saturation by iron (compare to the concentration in the bulk solution) occurred.

Estimates of biogenic methane production rates in deep marine sediments at Hydrate Ridge, Cascadia margin

Methane hydrate found in marine sediments is thought to contain gigaton quantities of methane and is considered an important potential fuel source and climate-forcing agent. Much of the methane in hydrates is biogenic, so models that predict the presence and distribution of hydrates require accurate rates of in situ methanogenesis. We estimated the in situ methanogenesis rates in Hydrate Ridge (HR) sediments by coupling experimentally derived minimal rates of methanogenesis to methanogen biomass determinations for discrete locations in the sediment column. When starved in a biomass recycle reactor, Methanoculleus submarinus produced ca. 0.017 fmol methane/cell/day. Quantitative PCR (QPCR) directed at the methyl coenzyme M reductase subunit A gene (mcrA) indicated that 75% of the HR sediments analyzed contained <1,000 methanogens/g. The highest numbers of methanogens were found mostly from sediments <10 m below seafloor. By considering methanogenesis rates for starved methanogens (adjusted to account for in situ temperatures) and the numbers of methanogens at selected depths, we derived an upper estimate of <4.25 fmol methane produced/g sediment/day for the samples with fewer methanogens than the QPCR method could detect. The actual rates could vary depending on the real number of methanogens and various seafloor parameters that influence microbial activity. However, our calculated rate is lower than rates previously reported for such sediments and close to the rate derived using geochemical modeling of the sediments. These data will help to improve models that predict microbial gas generation in marine sediments and determine the potential influence of this source of methane on the global carbon cycle.

Links between archaeal community structure, vegetation type and methanogenic pathway in Alaskan peatlands

Although northern peatlands contribute significantly to natural methane emissions, recent studies of the importance and type of methanogenesis in these systems have provided conflicting results. Mechanisms controlling methanogenesis in northern peatlands remain poorly understood, despite the importance of methane as a greenhouse gas. We used 16S rRNA gene retrieval and denaturing gradient gel electrophoresis (DGGE) to analyse archaeal communities in 15 high-latitude peatland sites in Alaska and three mid-latitude peatland sites in Massachusetts. Archaeal community composition was analysed in the context of environmental, vegetation and biogeochemical factors characterized in a parallel study. Phylogenetic analysis revealed that Alaskan sites were dominated by a cluster of uncultivated crenarchaeotes and members of the families Methanomicrobiaceae and Methanobacteriaceae, which are not acetoclastic. Members of the acetoclastic family Methanosarcinaceae were not detected, whereas those of the family Methanosaetaceae were either not detected or were minor. These results are consistent with biogeochemical evidence that acetoclastic methanogenesis is not a predominant terminal decomposition pathway in most of the sites analysed. Ordination analyses indicated a link between vegetation type and archaeal community composition, suggesting that plants (and/or the environmental conditions that control their distribution) influence both archaeal community activity and dynamics.

Biogeochemistry and biodiversity of methane cycling in subsurface marine sediments (Skagerrak, Denmark)

This biogeochemical, molecular genetic and lipid biomarker study of sediments ( approximately 4 m cores) from the Skagerrak (Denmark) investigated methane cycling in a sediment with a clear sulfate-methane-transition zone (SMTZ) and where CH(4) supply was by diffusion, rather than by advection, as in more commonly studied seep sites. Sulfate reduction removed sulfate by 0.7 m and CH(4) accumulated below. (14)C-radiotracer measurements demonstrated active H(2)/CO(2) and acetate methanogenesis and anaerobic oxidation of CH(4) (AOM). Maximum AOM rates occurred near the SMTZ ( approximately 3 nmol cm(-3) day(-1) at 0.75 m) but also continued deeper, overall, at much lower rates. Maximum rates of H(2)/CO(2) and acetate methanogenesis occurred below the SMTZ but H(2)/CO(2) methanogenesis rates were x 10 those of acetate methanogenesis, and this was consistent with initial values of (13)C-depleted CH(4) (delta(13)C c.-80 per thousand). Areal AOM and methanogenic rates were similar ( approximately 1.7 mmol m(-2) day(-1)), hence, CH(4) flux is finely balanced. A 16S rRNA gene library from 1.39 m combined with methanogen (T-RFLP), bacterial (16S rRNA DGGE) and lipid biomarker depth profiles showed the presence of populations similar to some seep sites: ANME-2a (dominant), ANME-3, Methanomicrobiales, Methanosaeta Archaea, with abundance changes with depth corresponding to changes in activities and sulfate-reducing bacteria (SRB). Below the SMTZ to approximately 1.7 m CH(4) became progressively more (13)C depleted (delta(13)C -82 per thousand) indicating a zone of CH(4) recycling which was consistent with the presence of (13)C-depleted archaeol (delta(13)C -55 per thousand). Pore water acetate concentrations decreased in this zone (to approximately 5 microM), suggesting that H(2), not acetate, was an important CH(4) cycling intermediate. The potential biomarkers for AOM-associated SRB, non-isoprenoidal ether lipids, increased below the SMTZ but this distribution reflected 16S rRNA gene sequences for JS1 and OP8 bacteria rather than those of SRB. At this site peak rates of methane production and consumption are spatially separated and seem to be conducted by different archaeal groups. Also AOM is predominantly coupled to sulfate reduction, unlike recent reports from some seep and gassy sediment sites.

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.

Reducing methane emissions in sheep by immunization against rumen methanogens

This work was conducted to determine if methane emissions from sheep immunized with an anti-methanogen vaccine were significantly lower than methane emissions from non-immunized sheep, to test the effectiveness of two different vaccine formulations (VF) on methane abatement, and to compare methane emissions measured using a closed-circuit respiration chamber and the sulphur-hexafluoride (SF6) tracer technique. Thirty mature wether sheep were randomly allocated to three treatment groups (n = 10). One group received an immunization of adjuvant only on days 0 and 153 (control), a second group received an immunization with a 3-methanogen mix on days 0 and 153 (VF3 + 3), and a third group received an immunization of a 7-methanogen mix on day 0 followed by a 3-methanogen mix on day 153 (VF7 + 3). Four weeks post-secondary immunization, there was a significant 7.7% reduction in methane production per kg dry matter intake in the VF7 + 3 group compared to the controls (P = 0.051). However, methane emissions from sheep immunized with VF7 + 3 were not significantly different when compared to the sheep in the control group (P = 0.883). The average IgG and IgA antibody titres in both plasma and saliva of the VF3 + 3 immunized sheep were four to nine times higher than those immunized with VF7 + 3 (P< 0.001) at both 3 and 6 weeks post-secondary immunization. Data also revealed that SF6 methane estimates were consistently higher than the respiration chamber estimates and that there was no significant correlation between the SF6 methane estimates and the respiration chamber methane estimates (R2 = 0.11).

Activity and diversity of methanogens in a petroleum hydrocarbon-contaminated aquifer

Methanogenic activity was investigated in a petroleum hydrocarbon-contaminated aquifer by using a series of four push-pull tests with acetate, formate, H(2) plus CO(2), or methanol to target different groups of methanogenic Archaea. Furthermore, the community composition of methanogens in water and aquifer material was explored by molecular analyses, i.e., fluorescence in situ hybridization (FISH), denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes amplified with the Archaea-specific primer set ARCH915 and UNI-b-rev, and sequencing of DNA from dominant DGGE bands. Molecular analyses were subsequently compared with push-pull test data. Methane was produced in all tests except for a separate test where 2-bromoethanesulfonate, a specific inhibitor of methanogens, was added. Substrate consumption rates were 0.11 mM day(-1) for methanol, 0.38 mM day(-1) for acetate, 0.90 mM day(-1) for H(2), and 1.85 mM day(-1) for formate. Substrate consumption and CH(4) production during all tests suggested that at least three different physiologic types of methanogens were present: H(2) plus CO(2) or formate, acetate, and methanol utilizers. The presence of 15 to 20 bands in DGGE profiles indicated a diverse archaeal population. High H(2) and formate consumption rates agreed with a high diversity of methanogenic Archaea consuming these substrates (16S rRNA gene sequences related to several members of the Methanomicrobiaceae) and the detection of Methanomicrobiaceae by using FISH (1.4% of total DAPI [4',6-diamidino-2-phenylindole]-stained microorganisms in one water sample; probe MG1200). Considerable acetate consumption agreed with the presence of sequences related to the obligate acetate degrader Methanosaeata concilii and the detection of this species by FISH (5 to 22% of total microorganisms; probe Rotcl1). The results suggest that both aceticlastic and CO(2)-type substrate-consuming methanogens are likely involved in the terminal step of hydrocarbon degradation, while methanogenesis from methanol plays a minor role. DGGE profiles further indicate similar archaeal community compositions in water and aquifer material. The combination of hydrogeological and molecular methods employed in this study provide improved information on the community and the potential activity of methanogens in a petroleum hydrocarbon-contaminated aquifer.

[Prevalence of nonmevalonate and mevalonate pathways for isoprenoid biosynthesis among bacteria of different systematic groups]

The effect of fosmidomycin and mevinoline, inhibitors of the nonmevalonate and the mevalonate pathway of isoprenoid biosynthesis, respectively, on the growth of 34 anaerobic and 10 aerobic prokaryotic strains was studied. Fosmidomycin at the concentrations used was shown to inhibit the growth of 9 (of 10) representatives of the family Microbacteriaceae, 4 (of 5) strains of Thermoanaerobacter, and 11 (of 12) strains of Clostridium, whereas mevinoline inhibited the growth of lactobacilli (Carnobacterium), methanogenic and sulfate-reducing bacteria insensitive to fosmidomycin. During the late growth phase, four strains of actinobacteria (of nine) accumulate the compound, which, upon oxidation, generates a long-lived free radical; three strains synthesize 2-C-methyl-D-erythritol-2,4-cyclopyrophosphate (MEC). It was concluded that the difference in the sensitivity of the organisms to fosmidomycin and mevinoline might serve as a test to differentiate several representatives of the family Microbacteriaceae. The use of mevinoline for inhibiting methanogens in ecological investigations seems to be promising.

Effects of several inhibitors on pure cultures of ruminal methanogens

AIMS

To examine the effects of five inhibitors of methanogenesis, 2-bromoethanesulphonate (BES), 3-bromopropanesulphonate (BPS), lumazine, propynoic acid and ethyl 2-butynoate, on CH4 production of the ruminal methanogens Methanobrevibacter ruminantium, Methanosarcina mazei and Methanomicrobium mobile.

METHODS AND RESULTS

Methanogens were grown in MS medium including 25% (v/v) clarified ruminal fluid. Methane production was measured after 4 and 6 days of incubation. Methanobrevibacter ruminantium was the most sensitive species to BES, propynoic acid and ethyl 2-butynoate. Methanosarcina mazei was the least sensitive species to those chemical additives, and Mm. mobile was intermediate. BPS failed to inhibit any of the methanogens. All three species were almost completely inhibited by 50- and 100%-lumazine saturated media, but the inhibition was somewhat lower with a 25%-lumazine saturated media.

CONCLUSIONS

There were important differences among species of methanogens regarding their sensitivity to the different inhibitors. In general, Ms. mazei was the most resistant to inhibitors, Mb. ruminantium the least resistant, and Mm. mobile was intermediate.

SIGNIFICANCE AND IMPACT OF THE STUDY

Differences among methanogens regarding their resistance to chemical inhibitors should be considered when designing strategies of inhibition of ruminal methanogenesis, as selection of resistant species may result.

Acetoclastic and hydrogenotrophic methane production and methanogenic populations in an acidic West-Siberian peat bog

Sites in the West Siberian peat bog 'Bakchar' were acidic (pH 4.2-4.8), low in nutrients, and emitted CH4 at rates of 0.2-1.5 mmol m(-2) h(-1). The vertical profile of delta13CH4 and delta13CO2 dissolved in the porewater indicated increasing isotope fractionation and thus increasing contribution of H2/CO2-dependent methanogenesis with depth. The anaerobic microbial community at 30-50 cm below the water table produced CH4 with optimum activity at 20-25 degrees C and pH 5.0-5.5 respectively. Inhibition of methanogenesis with 2-bromo-ethane sulphonate showed that acetate, phenyl acetate, phenyl propionate and caproate were important intermediates in the degradation pathway of organic matter to CH4. Further degradation of these intermediates indicated that 62-72% of the CH4 was ultimately derived from acetate, the remainder from H2/CO2. Turnover times of [2-14C]acetate were on the order of 2 days (15, 25 degrees C) and accounted for 60-65% of total CH4 production. Conversion of 14CO2 to 14CH4 accounted for 35-43% of total CH4 production. These results showed that acetoclastic and hydrogenotrophic methanogenesis operated closely at a ratio of approximately 2 : 1 irrespective of the incubation temperature (4, 15 and 25 degrees C). The composition of the archaeal community was determined in the peat samples by terminal restriction fragment length polymorphism (T-RFLP) analysis and sequencing of amplified SSU rRNA gene fragments, and showed that members of Methanomicrobiaceae, Methanosarcinaceae and Rice cluster II (RC-II) were present. Other, presumably non-methanogenic archaeal clusters (group III, RC-IV, RC-V, RC-VI) were also detected. Fluorescent in situ hybridization (FISH) showed that the number of Bacteria decreased (from 24 x 10(7) to 4 x 10(7) cells per gram peat) with depth (from 5 to 55 cm below the water table), whereas the numbers of Archaea slightly increased (from 1 x 10(7) to 2 x 10(7) cells per gram peat). Methanosarcina spp. accounted for about half of the archaeal cells. Our results show that both hydrogenotrophic and acetoclastic methanogenesis are an integral part of the CH4-producing pathway in acidic peat and were represented by appropriate methanogenic populations.

Stable-isotope probing of microorganisms thriving at thermodynamic limits: syntrophic propionate oxidation in flooded soil

Propionate is an important intermediate of the degradation of organic matter in many anoxic environments. In methanogenic environments, due to thermodynamic constraints, the oxidation of propionate requires syntrophic cooperation of propionate-fermenting proton-reducing bacteria and H(2)-consuming methanogens. We have identified here microorganisms that were active in syntrophic propionate oxidation in anoxic paddy soil by rRNA-based stable-isotope probing (SIP). After 7 weeks of incubation with [(13)C]propionate (<10 mM) and the oxidation of approximately 30 micromol of (13)C-labeled substrate per g dry weight of soil, we found that archaeal nucleic acids were (13)C labeled to a larger extent than those of the bacterial partners. Nevertheless, both terminal restriction fragment length polymorphism and cloning analyses revealed Syntrophobacter spp., Smithella spp., and the novel Pelotomaculum spp. to predominate in "heavy" (13)C-labeled bacterial rRNA, clearly showing that these were active in situ in syntrophic propionate oxidation. Among the Archaea, mostly Methanobacterium and Methanosarcina spp. and also members of the yet-uncultured "rice cluster I" lineage had incorporated substantial amounts of (13)C label, suggesting that these methanogens were directly involved in syntrophic associations and/or thriving on the [(13)C]acetate released by the syntrophs. With this first application of SIP in an anoxic soil environment, we were able to clearly demonstrate that even guilds of microorganisms growing under thermodynamic constraints, as well as phylogenetically diverse syntrophic associations, can be identified by using SIP. This approach holds great promise for determining the structure and function relationships of further syntrophic or other nutritional associations in natural environments and for defining metabolic functions of yet-uncultivated microorganisms.

Coenzyme binding in F420-dependent secondary alcohol dehydrogenase, a member of the bacterial luciferase family

F(420)-dependent secondary alcohol dehydrogenase (Adf) from methanogenic archaea is a member of the growing bacterial luciferase family which are all TIM barrel enzymes, most of which with an unusual nonprolyl cis peptide bond. We report here on the crystal structure of Adf from Methanoculleus thermophilicus at 1.8 A resolution in complex with a F(420)-acetone adduct. The knowledge of the F(420) binding mode in Adf provides the molecular basis for modeling F(420) and FMN into the other enzymes of the family. A nonprolyl cis peptide bond was identified as an essential part of a bulge that serves as backstop at the Re-face of F(420) to keep it in a bent conformation. The acetone moiety of the F(420)-acetone adduct is positioned at the Si-face of F(420) deeply buried inside the protein. Isopropanol can be reliably modeled and a hydrogen transfer mechanism postulated. His39 and Glu108 can be identified as key players for binding of the acetone or isopropanol oxygens and for catalysis.

Anaerobic 2-Propanol Degradation in Anoxic Paddy Soil and the Possible Role of Methanogens in Its Degradation

The anaerobic degradation of 2-propanol in anoxic paddy soil was studied with soil cultures and a 2-propanol-utilizing methanogen. Acetone was the first and the major intermediate involved in the methanogenic degradation of 2-propanol. Analyses with a methanogenesis inhibitor, bacteria antibiotics, and the addition of H2 to the gas phase revealed that 2-propanol oxidation to acetone directly occurred using 2-propanol-utilizing methanogens, but not with H2-producing syntrophic bacteria, for which the removal of acetone is required for complete 2-propanol oxidation. The 2-propanol-utilizing strain IIE1, which is phylogenetically closely related to Methanoculleus palmolei, was isolated from paddy soil, and the potential role of the strain in 2-propanol degradation was investigated. 2-Propanol is one of the representative fermentation intermediates in anaerobic environments. This is the first report on the anaerobic 2-propanol degradation process.

[Effect of iron hydroxide on the phosphate elimination during anaerobic digestion of active sludge]

Addition of iron (III) hydroxide during methanogenic digestion of active sludge by anaerobic sludge displaying an iron-reducing activity resulted in a microbial reduction of iron (III) with formation of iron (II), capable of precipitating phosphates. Feasibility of eliminating 66.6 to 99.6% of dissolved phosphate at initial concentrations of 1000 to 3500 mg PO4(3-)/l by adding 6420 mg/l iron (III) hydroxide into a reactor for anaerobic fermentation of active sludge. The optimal ratio of iron (III) added to dissolved phosphate eliminated (mg) providing a 95% elimination amounted to 2:1. These results may be used in a new technology for anaerobic wastewater treatment with phosphate elimination.

[CH4 and N2O emissions from phaeozem rice field and their mitigative measures]

Determinations on the CH4 and N2O emissions from phaeozem rice field showed that their emission amount in rice growing season was much lower than any other regions in China. There was trade-off relationship between CH4 and N2O emissions(r = -0.513, P < 0.05). Under the same fertilization, compared with continual irrigation, intermittent irrigation could reduce significantly CH4 emission and increase N2O emission, but the overall warming potential of greenhouse effect was reduced greatly, while rice yield was not affected. Therefore, intermittent irrigation was an effective irrigation measures to reduce greenhouse gas emissions from rice field. In addition, investigations on CH4 and N2O emissions and their related microbial process showed a positive relation between methanogens number and CH4 emission(R2 = 0.82, P < 0.05), and the important relationship between the numbers of nitrifiers and denitrifiers and N2O emissions.

Methanoculleus bourgensis, Methanoculleus olentangyi and Methanoculleus oldenburgensis are subjective synonyms

Methanoculleus bourgensis, Methanoculleus olentangyi and Methanoculleus oldenburgensis are subjective synonyms on the basis of phenotypic, genotypic and phylogenetic characteristics. Methanoculleus bourgensis must be the name of the united species because it is the type of the genus METHANOCULLEUS:

The role of sulphidogenesis in anaerobic treatment phase of tannery wastewater treatment in advanced integrated wastewater pond system

An outdoor experiment was conducted to study the competition between Sulphate Reducing Bacteria (SRB) and Methanogenic Archaea (MA) in anaerobic treatment phase of tannery wastewater treatment in pilot-scale Advanced Facultative Pond (AFP). The relative electron flow towards sulphate reduction was higher (59-83%) than towards methanogenesis (17-41%), although the COD recovery within the reactor varied between 15 and 90%. The results also demonstrated that the flow of electrons towards SRB increased with increase of the sulphate concentration and decrease of the COD:SO4= ratio.

Use of sulfate reducing cell suspension bioreactors for the treatment of SO2 rich flue gases

This paper describes a novel bioscrubber concept for biological flue gas desulfurization, based on the recycling of a cell suspension of sulfite/sulfate reducing bacteria between a scrubber and a sulfite/sulfate reducing hydrogen fed bioreactor. Hydrogen metabolism in sulfite/sulfate reducing cell suspensions was investigated using batch activity tests and by operating a completely stirred tank reactor (CSTR). The maximum specific hydrogenotrophic sulfite/sulfate reduction rate increased with 10% and 300%, respectively, by crushing granular inoculum sludge and by cultivation of this sludge as cell suspension in a CSTR. Operation of a sulfite fed CSTR (hydraulic retention time 4 days; pH 7.0; sulfite loading rate 0.5-1.5 g SO3(2-) l(-1) d(-1)) with hydrogen as electron donor showed that high (up to 1.6 g l(-1)) H2S concentrations can be obtained within 10 days of operation. H2S inhibition, however, limited the sulfite reducing capacity of the CSTR. Methane production by the cell suspension disappeared within 20 days reactor operation. The outcompetition of methanogens in excess of H2 can be attributed to CO2 limitation and/or to sulfite or sulfide toxicity. The use of cell suspensions opens perspectives for monolith or packed bed reactor configurations, which have a much lower pressure drop compared to air lift reactors, to supply H2 to sulfite/sulfate reducing bioreactors.

Methanoculleus palmolei sp. nov., an irregularly coccoid methanogen from an anaerobic digester treating wastewater of a palm oil plant in north-Sumatra, Indonesia

Strain INSLUZT (= DSM 4273T) was isolated from a biogas-producing bioreactor treating wastewater of a palm oil mill on North-Sumatra (Indonesia). Cells of strain INSLUZT were highly irregularly coccoid, 1.25-2.0 microns in diameter, had a cell envelope consisting of the cytoplasmic membrane and an S-layer of hexagonally arranged glycoprotein subunits with an M(r) of 120,000, and were flagellated (motility was not observed). Cells were mesophilic and grew most rapidly at 40 degrees C on H2/CO2' formate, 2-propanol/CO2 2-butanol/CO2 and cyclopentanol/CO2 to give methane. Tungstate promoted growth on H2/CO2 with acetate as the solely required organic medium supplement. The G + C content of DNA was 59 mol% (Tm method) and 59.5 mol% (HPLC method). 16S rDNA analysis revealed a phylogenetic relationship to Methanoculleus species; the name Methanoculleus palmolei sp. nov. is therefore proposed for strain INSLUZT (= DSM 4273T).

Long-term competition between sulfate reducing and methanogenic bacteria in UASB reactors treating volatile fatty acids

The competition between acetate utilizing methane-producing bacteria (MB) and sulfate-reducing bacteria (SRB) was studied in mesophilic (30 degrees C) upflow anaerobic sludge bed (UASB) reactors (upward velocity 1 m h-1; pH 8) treating volatile fatty acids and sulfate. The UASB reactors treated a VFA mixture (with an acetate:propionate:butyrate ratio of 5:3:2 on COD basis) or acetate as the sole substrate at different COD:sulfate ratios. The outcome of the competition was evaluated in terms of conversion rates and specific methanogenic and sulfidogenic activities. The COD:sulfate ratio was a key factor in the partitioning of acetate utilization between MB and SRB. In excess of sulfate (COD:sulfate ratio lower than 0.67), SRB became predominant over MB after prolonged reactor operation: 250 and 400 days were required to increase the amount of acetate used by SRB from 50 to 90% in the reactor treating, respectively, the VFA mixture or acetate as the sole substrate. The competition for acetate was further studied by dynamic simulations using a mathematical model based on the Monod kinetic parameters of acetate utilizing SRB and MB. The simulations confirmed the long term nature of the competition between these acetotrophs. A high reactor pH (+/-8), a short solid retention time (<150 days), and the presence of a substantial SRB population in the inoculum may considerably reduce the time required for acetate-utilising SRB to outcompete MB.

Kinetic evidence for pentachlorophenol-dependent growth of a dehalogenating population in a pentachlorophenol- and acetate-fed methanogenic culture

A method was developed to evaluate growth of a reductively dechlorinating bacterial population within a pentachlorophenol (PCP)- and acetate-fed, mixed, methanogenic culture. In 6- to 12-day experiments, a computer-monitored/feedback-controlled bioreactor was used to maintain constant pH, temperature, and acetate concentration, while transformation of multiple PCP additions was monitored. The potential at a platinum electrode, EPt, was not controlled externally, but was maintained constant at -0.25 +/- 0.002 V (vs. SHE) by iron sulfides in the medium and the activity of the culture. PCP was reductively dechlorinated at the ortho position, yielding 3, 4,5-trichlorophenol (3,4,5-TCP) via 2,3,4,5-tetrachlorophenol (2,3,4, 5-TeCP). Below an initial PCP concentration of 0.5 microM, PCP was transformed to 3,4,5-TCP within 3 to 6 h. Biomass concentration changes were small during this period, and PCP and 2,3,4,5-TeCP transformations were modeled as pseudo-first-order reactions. Increases in pseudo-first-order rate constants for PCP and 2,3,4, 5-TeCP were directly related to the amount of PCP transformed to 3,4, 5-TCP, suggesting enrichment of a PCP-catabolizing population. Moreover, rate constant increases were independent of the amount of acetate consumed, changes in the overall volatile suspended solids (VSS) concentration, and the experimental duration. When PCP was added to the reactor at increasingly shorter time intervals in an exponential pattern, pseudo-first-order rate constants increased exponentially. An average rate constant doubling time of 1.7 days (1. 4 to 2.3 d) was estimated. While the VSS concentration of the culture increased 60% in an 8-day period, pseudo-first-order rate constants increased by a factor of approximately 6. This large increase in transformation rate constants suggests growth of a bacterial population capable of using PCP and 2,3,4,5-TeCP as terminal electron acceptors.