Uma das alternativas para a recuperação de energia de resíduos é o tratamento anaeróbio de efluentes com produção de metano e hidrogênio, utilizando-se reatores anaeróbios. Dentre as configurações possíveis de reatores, os descontínuos, como o ASBR e o AnSBBR, destacam-se ao permitem maior flexibilidade e estabilidade na operação, bem como maior controle dos efluentes do processo e menores tempos de partida. O objetivo deste trabalho foi avaliar a produção de metano a partir da digestão anaeróbia do soro em condição termofílica, avaliando também a adequação ambiental deste resíduo. O soro de queijo é o maior subproduto da indústria de laticínios, sendo gerado na proporção de 90% do volume de leite utilizado, com concentração de matéria orgânica entre 2 e 80g DQO.L-1, dependendo do processo produtivo. O reator foi operado de forma a permitir avaliar a influência da carga orgânica aplicada (limite de estabilidade), da estratégia de alimentação (tempo de enchimento batelada ou batelada alimentada), e da relação entre tempo de ciclo e concentração afluente (flexibilidade operacional). O reator operou a 55ºC, com tempo de ciclo de 8 horas e velocidade de agitação (100 rpm), sendo o volume alimentado por ciclo de 1,0 litro, com 1,5 litros de volume residual. O reator teve um período inicial de adaptação de 29 dias. Após este período, estudou-se a influência do aumento da carga orgânica pelo aumento da carga orgânica volumétrica aplicada, variando de 6,20 a 30,34 gDQO.L-1.d-1. A condição com carga orgânica volumétrica aplicada de 19,20 gDQO.L-1.d-1 apresentou os melhores resultados globais, alcançando eficiência de remoção de matéria orgânica na forma de DQO de 85,8% e 99,6% de carboidratos. O rendimento de metano gerado em função da matéria orgânica consumida foi de 13,07 mmolCH4.gDQO-1, sendo a produção de biogás de 6506 mL-CNTP.ciclo-1, com fração molar de metano equivalente a 73,7%. A produtividade molar de metano alcançada nesta condição foi de 324,0 molCH4.m-3.d-1 . Percebeu-se que as maiores cargas aplicadas acarretaram em perda de eficiência e instabilidade no reator. O modelo cinético de todas as condições foi ajustado corretamente, indicando que a rota de produção de metano preferencial foi a hidrogenotrófica em todo o período de estudo, porém, a via acetoclástica também foi presenciada em todas as condições. A mudança de estratégia de alimentação de batelada (2% do ciclo) para batelada alimentada (50% do ciclo) não melhorou os resultados de eficiência, estabilidade e produção de metano no reator. Realizou-se também a estimativa da recuperação energética estimando a produção de um pequeno, médio e grande produtor de queijo. A condição V apresentou os melhores resultados, possibilitando a recuperação de 826,10 MWh por dia tratando o volume de soro gerado numa indústria de grande porte (1.000.000 kg.mês-1) enquanto a melhor condição global (VIII) apresentou recuperação de 548,40 MWh. / One of the alternatives to recover energy from waste treatment is by the anaerobic treatment of effluents using discontinuous reactor configuration, such as ASBR and AnSBBR, aiming to produce methane and hydrogen. This reactors configuration allows greater flexibility and stability in the operation, a better controlled process, as well as shorter start-up. The objective of this project was to evaluate the methane production from the anaerobic digestion of cheese whey under thermophilic conditions. Cheese whey is the largest by-product of the dairy industry, it is generated in a proportion of 90% of the milk volume, its organic matter concentration can vary between 2 and 80 g COD.L-1 depending on the process. The evaluation of the environmental suitability of this residue was also analyzed. The reactor was operated to determinate the influence of the applied organic load (stability limit), feeding strategy (filling time - batch or fed-batch), and the relationship between cycle time and affluent concentration (operational flexibility). The reactor was operated at 55 ºC, with cycle time of 8 hours and 100 rpm of agitation speed. The volume fed per cycle was 1.0 liter with 1.5 liters of residual volume. The reactor had an initial adaptation period of 29 days. After this period, the influence of the organic load by increasing the applied volumetric organic load, ranging from 6.20 to 30.34 gDQO.L-1.d-1, was studied. The condition with volumetric organic load of 19.20 gDQO.L-1.d-1 presented the best overall results, reaching organic matter removal efficiency in the form of COD of 85.8 ± 2.0% and 99.6 ± 0.2% for carbohydrates. The yield of methane generated by organic matter consumed was 13.07 mmolCH4.gDQO-1, within biogas production of 6506 ± 185 mL-CNTP.cycle-1, with a methane molar fraction equivalent to 73.68 ± 0,43%. This condition molar productivity of methane achieved 324.0 molCH4.m-3.d-1. It was noticed that the higher organic loads applied lead to loss of efficiency and instability of the reactor. The kinetic model of all conditions was correctly adjusted, indicating that the preferred methane production route was hydrogenotrophic throughout the study period, but the acetoclastic pathway was also observed in all conditions. Changing from batch feeding strategy (2% cycle) to fed batch (50% cycle) did not improved the efficiency, stability and methane production results in the reactor. The energy recovery was also estimated by estimating the production of a small, medium and large cheese producer. Condition V presented the best results, allowing the recovery of 826.10 MW per day by treating the volume of cheese whey generated in a large industry (1.000.000 kg.month-1), while the best overall condition (VIII) recovered 548.40 MW.

Microbial nitrogen (N2) fixation supplies important nitrogen inputs to boreal peatlands, extremely oligotrophic ecosystems dominated by Sphagnum mosses. In this study, we coupled major and trace nutrient analyses and rate measurements to characterize interactions between N2 fixation and CH4 cycling at the S1 peat bog in Marcell Experimental Forest and the Zim bog (Minnesota, USA). Total dissolved inorganic nitrogen (NO3-+NO2-+NH4+) and phosphate were both consistently < 2 μM in the porewater of surface peat, indicating severe nutrient limitation. While dissolved Fe was fairly abundant (18-35 mM), Mo, V and Cu were scarce (2-40 nM), suggesting that alternative metalloenzymes containing Fe in place of other metals may be favored. Rates of diazotrophy measured by both 15N2 incorporation and the acetylene (C2H2) reduction assay (ARA) were 7-fold higher under anoxic vs. oxic incubations conducted at both 4°C and 25°C. No significant difference in N2 fixation rates measured by either method was observed with or without the amendment of 1% CH4 at 25 °C; however, a significant inhibitory effect by methane was seen at 4°C in material from the S1 bog hollows. Anoxic 15N2 incorporation was 3-4x higher in treatments lacking acetylene, suggesting that the ARA likely underestimates N2 fixation by inhibiting diazotrophs sensitive to C2H2. Aerobic methanotrophy was also inhibited by 1% C2H2 when incubated under oxic conditions. No observations for the production of ethane (C2H6) were detected during the ARA, a biomarker for alternative nitrogenase activity. Major differences in ARA rates were observed to vary locally within microhabitats and between two bogs. In June 2014, peat sampled from hollows incubated under anoxic conditions showed the highest ARA rates (94.9 ± 11.0 nmol C2H4 g-1 moss dry mass hr-1), while the lowest rates were observed in ix hummock samples incubated under oxic conditions (5.1 ± 0.8 nmol C2H4 g-1 moss dry mass hr-1) in the S1 bog (T3 site). Observed rates have the potential to be a function of oxygen concentrations and or water content. ARA rates in all microcosm treatments were significantly lower at Zim bog compared to the S1 bog. The developed conversion factor between the regression of 15N2 and ARA in this study was 3.9 and agrees with the theoretical conversion factor as well as previous studies of soils and forest mosses.

Coal beds are one of the most promising reservoirs for geologic carbon dioxide (CO₂) sequestration, as CO₂ can strongly adsorb onto organic matter and displace methane; however, little is known about the long-term fate of CO₂ sequestered in coal beds. The "2800' sand" of the Olla oil field is a coal-bearing, oil and gas-producing reservoir of the Paleocene–Eocene Wilcox Group in north-central Louisiana. In the 1980s, this field, specifically the 2800' sand, was flooded with CO₂ in an enhanced oil recovery (EOR) project, with 9.0×10⁷m³ of CO₂ remaining in the 2800' sand after injection ceased. This study utilized isotopic and geochemical tracers from co-produced natural gas, oil and brine from reservoirs located stratigraphically above, below and within the 2800' sand to determine the fate of the remaining EOR-CO₂, examining the possibilities of CO₂ migration, dissolution, mineral trapping, gas-phase trapping, and sorption to coal beds, while also testing a previous hypothesis that EOR-CO₂ may have been converted by microbes (CO₂-reducing methanogens) into methane, creating a microbial "hotspot". Reservoirs stratigraphically-comparable to the 2800' sand, but located in adjacent oil fields across a 90-km transect were sampled to investigate regional trends in gas composition, brine chemistry and microbial activity. The source field for the EOR-CO₂, the Black Lake Field, was also sampled to establish the δ¹³C-CO₂ value of the injected gas (0.9‰ +/- 0.9‰). Four samples collected from the Olla 2800' sand produced CO₂-rich gas with δ¹³C-CO₂ values (average 9.9‰) much lower than average (pre-injection) conditions (+15.9‰, average of sands located stratigraphically below the 2800' sand in the Olla Field) and at much higher CO₂ concentrations (24.9 mole %) than average (7.6 mole %, average of sands located stratigraphically below the 2800' sand in the Olla Field), suggesting the presence of EOR-CO₂ and gas-phase trapping as a major storage mechanism. Using δ¹³C values of CO₂ and dissolved organic carbon (DIC), CO₂ dissolution was also shown to be a major storage mechanism for 3 of the 4 samples from the Olla 2800' sand. Minor storage mechanisms were shown to be migration, which only affected 2 samples (from 1 well), and some EOR-CO₂ conversion to microbial methane for 3 of the 4 Olla 2800' sand samples. Since methanogenesis was not shown to be a major storage mechanism for the EOR-CO₂ in the Olla Field (CO₂ injection did not stimulate methanogenesis), samples were examined from adjacent oil fields to determine the cause of the Olla microbial "hot-spot". Microbial methane was found in all oil fields sampled, but indicators of methanogenesis (e.g. alkalinity, high δ¹³C-DIC values) were the greatest in the Olla Field, and the environmental conditions (salinity, pH, temperature) were most ideal for microbial CO₂ reduction in the Olla field, compared to adjacent fields.

This study utilizes new geochemical analyses of fluids (formation water and gas) collected predominately from Devonian organic-rich shales and reservoir sandstones from the northern Appalachian Basin margin to investigate basin scale hydrologic processes, water-rock reactions, microbial activity, and natural gas generation. Elemental and isotopic composition of co-produced formation waters and natural gas show that the majority of methane in Devonian organic-rich shales and reservoir sandstones is thermogenic in origin with localized accumulations of microbial gas. Microbial methanogenesis appears to be primarily limited by redox buffered conditions favoring microbial sulfate reduction. Thermal maturity (bioavailability) of shale organic matter and the paucity of formation waters may also explain the lack of extensive microbial methane accumulations. Iodine and strontium isotopes, coupled to elemental chemistry demonstrate basin scale fluid flow and clay mineral diagenesis. Evidence for this is based on anomalously high ¹²⁹I/I values sourced from uranium deposits (fissiogenic production of ¹²⁹I) at the structural front of the Appalachian Basin. Radiogenic ⁸⁷Sr/⁸⁶Sr (up to 0.7220), and depleted boron and potassium concentrations support smectite clay diagenesis at temperatures greater than 120 °C. The development of fissiogenic ¹²⁹I as a tracer of basin scale fluid flow is a novel application of iodine isotopes provided that the sources of cosmogenic and anthropogenic ¹²⁹I are reasonably well constrained. The anomalously high ¹²⁹I/I in Appalachian Basin brines may be alternatively explained by microbial fractionation based on a correlation with decreasing δ¹³C-DIC values and decreasing sulfate concentrations in the range of sulfate reduction. These results demonstrate that the microbial fractionation of iodine isotopes may be possible and an important consideration when interpreting ¹²⁹I/I, regardless of the source of ¹²⁹I. Results from this study have important implications for understanding the controls on and origins of natural gas production in sedimentary basins; tectonically and topographically driven basin scale fluid flow, including diagenetically induced waterrock reactions and mineral ore deposition related to orogenesis; and an improvement of the use of iodine isotopes for understanding large scale fluid flow, and possibly its use as a tracer of organic matter diagenesis and the distribution of radionuclides in the environment.

This study investigated the changes in leachate composition and clogging evolution in leachate transmission pipes and the use of methanogenesis as a leachate treatment alternative for Bioreactor landfills, by using pilot-scale and laboratory studies. The pilot-scale study consisted of a research station built at Brady Road Landfill, housing sixteen HDPE pipes of three different diameters, conveying leachate intermittently at eight different Reynolds numbers, under reasonably controlled conditions. The pipes were tested for leachate degradation, clogging evolution and hydraulic impairment over time. The laboratory studies carried out tested (1) the effect of turbulence intensity and temperature on leachate degradation and clogging effects and (2) biological pretreatment of leachate prior to injection into a bioreactor cell. The pilot study results showed that under the conditions tested, pipes developed a significant amount of organic and inorganic clog material in less than a year of operation. Since limited quantities of fresh leachate (approx. 3 m3) were used during each leachate degradation analyses, the anticipated effects of clogging in a full scale injection system are expected to be more pronounced, which can negatively impact the long-term hydraulic performance, operation, and service life of a Bioreactor Landfill. The first laboratory study showed that increasing the turbulent energy dissipation rate caused greater amounts of CO2 evolution from the leachate, and temperature increase had an impact on dissolved Ca2+ under atmospheric conditions, affecting clog development. The second and third laboratory studies showed that performing leachate methanogenesis reduces organic (COD, VFA) and inorganic (Ca2+, ISS) clog constituents within the leachate However, the rate of methanogenesis was influenced by the ratio of acetate and propionate. It is suggested that if leachate undergoes methanogenesis in a separate leachate digester prior to re-injection into a bioreactor waste cell, it may protect the pipes and other engineered landfill systems against clogging and its detrimental effects, while allowing for CH4 recovery. However, blending of leachates from different wells or cells prior to the methanogenic digester may be needed to balance the variable concentrations and ratios of acetate and propionate over time from different landfill wells and cells.

This study investigated the changes in leachate composition and clogging evolution in leachate transmission pipes and the use of methanogenesis as a leachate treatment alternative for Bioreactor landfills, by using pilot-scale and laboratory studies. The pilot-scale study consisted of a research station built at Brady Road Landfill, housing sixteen HDPE pipes of three different diameters, conveying leachate intermittently at eight different Reynolds numbers, under reasonably controlled conditions. The pipes were tested for leachate degradation, clogging evolution and hydraulic impairment over time. The laboratory studies carried out tested (1) the effect of turbulence intensity and temperature on leachate degradation and clogging effects and (2) biological pretreatment of leachate prior to injection into a bioreactor cell. The pilot study results showed that under the conditions tested, pipes developed a significant amount of organic and inorganic clog material in less than a year of operation. Since limited quantities of fresh leachate (approx. 3 m3) were used during each leachate degradation analyses, the anticipated effects of clogging in a full scale injection system are expected to be more pronounced, which can negatively impact the long-term hydraulic performance, operation, and service life of a Bioreactor Landfill. The first laboratory study showed that increasing the turbulent energy dissipation rate caused greater amounts of CO2 evolution from the leachate, and temperature increase had an impact on dissolved Ca2+ under atmospheric conditions, affecting clog development. The second and third laboratory studies showed that performing leachate methanogenesis reduces organic (COD, VFA) and inorganic (Ca2+, ISS) clog constituents within the leachate However, the rate of methanogenesis was influenced by the ratio of acetate and propionate. It is suggested that if leachate undergoes methanogenesis in a separate leachate digester prior to re-injection into a bioreactor waste cell, it may protect the pipes and other engineered landfill systems against clogging and its detrimental effects, while allowing for CH4 recovery. However, blending of leachates from different wells or cells prior to the methanogenic digester may be needed to balance the variable concentrations and ratios of acetate and propionate over time from different landfill wells and cells.

The deep-sea hypersaline lakes in the Red Sea are among the most challenging, extreme, and unusual environments on the planet Earth. Despite their harshness to life, they are inhabited by diverse and novel members of prokaryotes. Methanogenesis was proposed as one of the main metabolic pathways that drive microbial colonization in similar habitats. However, not much is known about the identities of the methane-producing microbes in the Red Sea, let alone the way in which they could adapt to such poly extreme environments. Combining a range of microbial community assessment, cultivation and omics (genomics, transcriptomics, and single amplified genomics) approaches, this dissertation seeks to fill these gaps in our knowledge by studying archaeal composition, particularly methanogens, their genomic capacities and transcriptomic characteristics in order to elucidate their diversity, function, and adaptation to the deep-sea brines of the Red Sea. Although typical methanogens are not abundant in the samples collected from brine pool habitats of the Red Sea, the pilot cultivation experiment has revealed novel halophilic methanogenic species of the domain Archaea. Their physiological traits as well as their genomic and transcriptomic features unveil an interesting genetic and functional adaptive capacity that allows them to thrive in the unique deep-sea hypersaline environments in the Red Sea.

Uma das alternativas para a recuperação de energia de resíduos é o tratamento anaeróbio de efluentes com produção de metano e hidrogênio, utilizando-se reatores anaeróbios. Dentre as configurações possíveis de reatores, os descontínuos, como o ASBR e o AnSBBR, destacam-se ao permitem maior flexibilidade e estabilidade na operação, bem como maior controle dos efluentes do processo e menores tempos de partida. O objetivo deste trabalho foi avaliar a produção de metano a partir da digestão anaeróbia do soro em condição termofílica, avaliando também a adequação ambiental deste resíduo. O soro de queijo é o maior subproduto da indústria de laticínios, sendo gerado na proporção de 90% do volume de leite utilizado, com concentração de matéria orgânica entre 2 e 80g DQO.L-1, dependendo do processo produtivo. O reator foi operado de forma a permitir avaliar a influência da carga orgânica aplicada (limite de estabilidade), da estratégia de alimentação (tempo de enchimento batelada ou batelada alimentada), e da relação entre tempo de ciclo e concentração afluente (flexibilidade operacional). O reator operou a 55ºC, com tempo de ciclo de 8 horas e velocidade de agitação (100 rpm), sendo o volume alimentado por ciclo de 1,0 litro, com 1,5 litros de volume residual. O reator teve um período inicial de adaptação de 29 dias. Após este período, estudou-se a influência do aumento da carga orgânica pelo aumento da carga orgânica volumétrica aplicada, variando de 6,20 a 30,34 gDQO.L-1.d-1. A condição com carga orgânica volumétrica aplicada de 19,20 gDQO.L-1.d-1 apresentou os melhores resultados globais, alcançando eficiência de remoção de matéria orgânica na forma de DQO de 85,8% e 99,6% de carboidratos. O rendimento de metano gerado em função da matéria orgânica consumida foi de 13,07 mmolCH4.gDQO-1, sendo a produção de biogás de 6506 mL-CNTP.ciclo-1, com fração molar de metano equivalente a 73,7%. A produtividade molar de metano alcançada nesta condição foi de 324,0 molCH4.m-3.d-1 . Percebeu-se que as maiores cargas aplicadas acarretaram em perda de eficiência e instabilidade no reator. O modelo cinético de todas as condições foi ajustado corretamente, indicando que a rota de produção de metano preferencial foi a hidrogenotrófica em todo o período de estudo, porém, a via acetoclástica também foi presenciada em todas as condições. A mudança de estratégia de alimentação de batelada (2% do ciclo) para batelada alimentada (50% do ciclo) não melhorou os resultados de eficiência, estabilidade e produção de metano no reator. Realizou-se também a estimativa da recuperação energética estimando a produção de um pequeno, médio e grande produtor de queijo. A condição V apresentou os melhores resultados, possibilitando a recuperação de 826,10 MWh por dia tratando o volume de soro gerado numa indústria de grande porte (1.000.000 kg.mês-1) enquanto a melhor condição global (VIII) apresentou recuperação de 548,40 MWh. / One of the alternatives to recover energy from waste treatment is by the anaerobic treatment of effluents using discontinuous reactor configuration, such as ASBR and AnSBBR, aiming to produce methane and hydrogen. This reactors configuration allows greater flexibility and stability in the operation, a better controlled process, as well as shorter start-up. The objective of this project was to evaluate the methane production from the anaerobic digestion of cheese whey under thermophilic conditions. Cheese whey is the largest by-product of the dairy industry, it is generated in a proportion of 90% of the milk volume, its organic matter concentration can vary between 2 and 80 g COD.L-1 depending on the process. The evaluation of the environmental suitability of this residue was also analyzed. The reactor was operated to determinate the influence of the applied organic load (stability limit), feeding strategy (filling time - batch or fed-batch), and the relationship between cycle time and affluent concentration (operational flexibility). The reactor was operated at 55 ºC, with cycle time of 8 hours and 100 rpm of agitation speed. The volume fed per cycle was 1.0 liter with 1.5 liters of residual volume. The reactor had an initial adaptation period of 29 days. After this period, the influence of the organic load by increasing the applied volumetric organic load, ranging from 6.20 to 30.34 gDQO.L-1.d-1, was studied. The condition with volumetric organic load of 19.20 gDQO.L-1.d-1 presented the best overall results, reaching organic matter removal efficiency in the form of COD of 85.8 &#177; 2.0% and 99.6 &#177; 0.2% for carbohydrates. The yield of methane generated by organic matter consumed was 13.07 mmolCH4.gDQO-1, within biogas production of 6506 &#177; 185 mL-CNTP.cycle-1, with a methane molar fraction equivalent to 73.68 &#177; 0,43%. This condition molar productivity of methane achieved 324.0 molCH4.m-3.d-1. It was noticed that the higher organic loads applied lead to loss of efficiency and instability of the reactor. The kinetic model of all conditions was correctly adjusted, indicating that the preferred methane production route was hydrogenotrophic throughout the study period, but the acetoclastic pathway was also observed in all conditions. Changing from batch feeding strategy (2% cycle) to fed batch (50% cycle) did not improved the efficiency, stability and methane production results in the reactor. The energy recovery was also estimated by estimating the production of a small, medium and large cheese producer. Condition V presented the best results, allowing the recovery of 826.10 MW per day by treating the volume of cheese whey generated in a large industry (1.000.000 kg.month-1), while the best overall condition (VIII) recovered 548.40 MW.

Im Rahmen des BioPara Netzwerkes: „Gesamterfassung von biochemischen und metagenomischen Parametern in Biogasanlagen und deren Korrelation zur Produkteffizienz“ erfolgte die in dieser Arbeit durchgeführte Isolierung und Charakterisierung von prozessrelevanten methanogenen Archaeen aus frischen Schlammproben einer kommerziellen Biogasanlage. Insgesamt wurden sechs verschiedene Arten der Gattungen Methanobacterium, Methanoculleus sowie Methanosarcina isoliert und als Reinkultur kultiviert. Darunter wurden mit Methanobacterium aggregans und Methanosarcina flavescens bis dahin unbekannte Spezies identifiziert und umfassend charakterisiert. Außerdem erfolgte mit Methanoculleus bourgensis und M. flavescens die Anreicherung der beiden abundantesten methanoarchaealen Mikroorganismen des beprobten Biogasreaktors. Weiterführend wurde ein anaerobes Testsystem etabliert, um die Methanproduktivität der analysierten Biogasanlagen im Labormaßstab abzubilden. Mit diesem Testsystem wurden die hydrogenotrophen und methylotrophen Wege der Methanogenese als potentiell raten-limitierende Schritte des Biogasprozesses in NawaRo-Anlagen analysiert. Die hydrogenotrophe Methanogenese verlief in allen beprobten Biogasreaktoren nicht an der maximalen Kapazitätsgrenze und stellte daher keinen raten-limitierenden Schritt dar. Weiterführend wurde die methylotrophe Methanogenese bei hohen Methanosarcina-Abundanzen als nicht-ratenlimitierender Prozess identifiziert und durch die Zugabe von Isolat E03.2 zu inaktivierten Schlammproben der untersuchten Biogasanlagen, ergaben sich Hinweise auf die Anwesenheit von Methoxygruppen. Aufgrund der fundamentalen Bedeutung des Wasserstoff-Stoffwechsels in NawaRo-Biogasanlagen, wurden u.a. Wasserstoff-abhängige Gesamt- sowie methanoarchaeale Enzymreaktionen aus frischen Schlammproben quantifiziert und als mögliche Parameter zur Prozessüberwachung analysiert. Dabei konnte eine positive Korrelation der Gesamt-Hydrogenaseaktivität mit der Methanproduktivität in den untersuchten Biogasanlagen gezeigt werden. Die Gesamt-Hydrogenaseaktivität ist somit einen aussagekräftiger Parameter zur Prozesskontrolle.

Microbial electrochemical technologies (MET) exploit the bioelectrocatalytic activity of microorganisms, with a main focus on waste-to-resource recovery. Electromethanogenesis, a type of MET, describes the process of CO2 reduction specifically to methane, catalyzed by methanogens that utilize the cathode directly as an electron donor or through H2 evolving from the cathode surface. Applications are mainly in the direction of bioelectrochemical power-to-gas, as well as biogas upgrading and carbon capture and utilization. As the cathode and its associated microbial consortia are key to the process, larger scale applications require improvements especially in terms of optimal operational parameters, cathode materials and the dynamics of the effect of electron transfer within the cathodic biofilm. The focus of this dissertation is to improve the understanding of the dynamics and function of methaneproducing biofilms grown on cathodes in electromethanogenic reactors in the presence of two different electron donors: the cathode and the H2 evolving from the cathode surface. The spatial homogeneity of the microbial communities across the area of the cathode was demonstrated, which is relevant for large scale applications where reproducibility is required for predictable engineered systems. Metagenomic and metatranscriptomic methods were applied to elucidate the short-term changes in the actively transcribed methanogenesis and central carbon assimilation pathways in response to varying the availability of electrons by changing the set cathode potential in a novel Methanobacterium species enriched from electromethanogenic biocathodes. Although changes in functional performance were evident with varying potential, no significant differential expression was observed and genes from the methanogenesis and carbon assimilation pathways were highly expressed throughout. Indium tin oxide (ITO) as a potentially hydrogen evolution reaction (HER) – inert cathode material was evaluated using the mixotrophic Methanosarcina barkeri in an attempt to develop a simplified material-science driven approach to future electron transfer studies. It was found to be electrochemically unstable under the tested conditions, losing its conductivity over time. Overall, the findings from these studies provide new knowledge on the effects of electron donor availability on the functional performance and the biocathode community dynamics. The understandings derived from the study are relevant to methanogenic processes and should aid in system scaleup design.