Suchergebnisse

Hydrogen is a key intermediate element in microbial electrosynthesis as a mediator of the reduction of carbon dioxide (CO2) into added value compounds. In the present work we aimed at studying the biological production of hydrogen in biocathodes operated at − 1.0 V vs. Ag/AgCl, using a highly comparable technology and CO2 as carbon feedstock. Ten bacterial strains were chosen from genera Rhodobacter, Rhodopseudomonas, Rhodocyclus, Desulfovibrio and Sporomusa, all described as hydrogen producing candidates. Monospecific biofilms were formed on carbon cloth cathodes and hydrogen evolution was constantly monitored using a microsensor. Eight over ten bacteria strains showed electroactivity and H2 production rates increased significantly (two to eightfold) compared to abiotic conditions for two of them (Desulfovibrio paquesii and Desulfovibrio desulfuricans). D. paquesii DSM 16681 exhibited the highest production rate (45.6 ± 18.8 μM min−1) compared to abiotic conditions (5.5 ± 0.6 μM min−1), although specific production rates (per 16S rRNA copy) were similar to those obtained for other strains. This study demonstrated that many microorganisms are suspected to participate in net hydrogen production but inherent differences among strains do occur, which are relevant for future developments of resilient biofilm coated cathodes as a stable hydrogen production platform in microbial electrosynthesis ; This study has received funding from the European Union's Horizon 2020 research and innovation program under the grant agreement no 760431. LEQUIA and IEA have been recognized as a consolidated research groups by the Catalan Government (2017-SGR-1552 and 2017SGR-548, respectively). E. P.-V. is grateful for the Research Training grant from the University of Girona (IFUdG2018/52). S.P. is a Serra Hunter Fellow (UdG-AG-575) and acknowledges the funding from the ICREA Academia award

Gasification of organic wastes coupled to syngas fermentation allows the recovery of carbon in the form of commodity chemicals, such as carboxylates and biofuels. Acetogenic bacteria ferment syngas to mainly two-carbon compounds, although a few strains can also synthesize four-, and six-carbon molecules. In general, longer carbon chain products have a higher biotechnological (and commercial) value due to their higher energy content and their lower water solubility. However, de-novo synthesis of medium-chain products from syngas is quite uncommon in acetogenic bacteria. An alternative to de-novo synthesis is bioproduction of short-chain products (C2 and C4), and their subsequent elongation to C4, C6, or C8 through reversed β-oxidation metabolism. This two-step synergistic approach has been successfully applied for the production of up to C8 compounds, although the accumulation of alcohols in these mixed cultures remained below detection limits. The present work investigates the production of higher alcohols from syngas by open mixed cultures (OMC). A syngas-fermenting community was enriched from sludge of an anaerobic digester for a period of 109 days in a lab-scale reactor. At the end of this period, stable production of ethanol and butanol was obtained. C6 compounds were only transiently produced at the beginning of the enrichment phase, during which Clostridium kluyveri, a bacterium able to carry out carbon chain elongation, was detected in the community. Further experiments showed pH as a critical parameter to maintain chain elongation activity in the co-culture. Production of C6 compounds was recovered by preventing fermentation pH to decrease below pH 4.5–5. Finally, experiments showed maximal production of C6 compounds (0.8 g/L) and alcohols (1.7 g/L of ethanol, 1.1 g/L of butanol, and 0.6 g/L of hexanol) at pH 4.8. In conclusion, low fermentation pH is critical for the production of alcohols, although detrimental to C. kluyveri. Fine control of fermentation pH to final values around 4.8 could allow sustained production of higher alcohols ; The authors would like to thank the Spanish Ministry (BestEnergy, CTQ2011-23632 and Bescarbox, CTM2013-43454-R for its financial support in this study. LEQUIA and EcoAqua have been recognized as consolidated research groups by the Catalan Government (2014-SGR-1168 and 2014-SGR-0484). RG gratefully acknowledges support from Ghent University BOF postdoctoral fellowship (BOF15/PDO/068)

Gasification of organic wastes coupled to syngas fermentation allows the recovery of carbon in the form of commodity chemicals, such as carboxylates and biofuels. Acetogenic bacteria ferment syngas to mainly two-carbon compounds, although a few strains can also synthesize four-, and six-carbon molecules. In general, longer carbon chain products have a higher biotechnological (and commercial) value due to their higher energy content and their lower water solubility. However, de-novo synthesis of medium-chain products from syngas is quite uncommon in acetogenic bacteria. An alternative to de-novo synthesis is bioproduction of short-chain products (C2 and C4), and their subsequent elongation to C4, C6, or C8 through reversed β-oxidation metabolism. This two-step synergistic approach has been successfully applied for the production of up to C8 compounds, although the accumulation of alcohols in these mixed cultures remained below detection limits. The present work investigates the production of higher alcohols from syngas by open mixed cultures (OMC). A syngas-fermenting community was enriched from sludge of an anaerobic digester for a period of 109 days in a lab-scale reactor. At the end of this period, stable production of ethanol and butanol was obtained. C6 compounds were only transiently produced at the beginning of the enrichment phase, during which Clostridium kluyveri, a bacterium able to carry out carbon chain elongation, was detected in the community. Further experiments showed pH as a critical parameter to maintain chain elongation activity in the co-culture. Production of C6 compounds was recovered by preventing fermentation pH to decrease below pH 4.5–5. Finally, experiments showed maximal production of C6 compounds (0.8 g/L) and alcohols (1.7 g/L of ethanol, 1.1 g/L of butanol, and 0.6 g/L of hexanol) at pH 4.8. In conclusion, low fermentation pH is critical for the production of alcohols, although detrimental to C. kluyveri. Fine control of fermentation pH to final values around 4.8 could allow sustained production of higher alcohols ; The authors would like to thank the Spanish Ministry (BestEnergy, CTQ2011-23632 and Bescarbox, CTM2013-43454-R for its financial support in this study. LEQUIA and EcoAqua have been recognized as consolidated research groups by the Catalan Government (2014-SGR-1168 and 2014-SGR-0484). RG gratefully acknowledges support from Ghent University BOF postdoctoral fellowship (BOF15/PDO/068)

The current energy model based on fossil fuels is coming to an end due to the increase in global energy demand. Biofuels such as ethanol and butanol can be produced through the syngas fermentation by acetogenic bacteria. The present work hypothesizes that formate addition would positively impact kinetic parameters for growth and alcohol production in Clostridium ljungdahlii PETC and Clostridium carboxidivorans P7 by diminishing the need for reducing equivalents. Fermentation experiments were conducted using completely anaerobic batch cultures at different pH values and formate concentrations. PETC cultures were more tolerant to formate concentrations than P7, specially at pH 5.0 and 6.0. Complete growth inhibition of PETC occurred at sodium formate concentrations of 30.0 mM; however, no differences in growth rates were observed at pH 7.0 for the two strains. Incubation at formate concentrations lower than 2.0 mM resulted in increased growth rates for both strains. The most recognizable effects of formate addition on the fermentation products were the increase in the total carbon fixed into acids and alcohols at pH 5.0 and pH 6.0, as well as, a higher ethanol to total products ratio at pH 7.0. Taken all together, these results show the ability of acetogens to use formate diminishing the energy demand for growth, and enhancing strain productivity ; The authors thank the Autonomous Government of Catalonia (Generalitat de Catalunya) (2013 FI-DGR) and the Spanish Ministry of Science and Innovation (Best-Energy, CTQ2011-23632, CTM2013-43454-R) for their financial support in this study. LEQUIA and IEA have been recognized as consolidated research groups by the Catalan Government (2014-SGR-1168 and 2014-SGR-2016). RG gratefully acknowledges support from Beatriu de Pinos fellowship (BP-2011-B) and FP7 Marie Curie Career Integration Grants (PCIG13-GA-2013-618593)

The current energy model based on fossil fuels is coming to an end due to the increase in global energy demand. Biofuels such as ethanol and butanol can be produced through the syngas fermentation by acetogenic bacteria. The present work hypothesizes that formate addition would positively impact kinetic parameters for growth and alcohol production in Clostridium ljungdahlii PETC and Clostridium carboxidivorans P7 by diminishing the need for reducing equivalents. Fermentation experiments were conducted using completely anaerobic batch cultures at different pH values and formate concentrations. PETC cultures were more tolerant to formate concentrations than P7, specially at pH 5.0 and 6.0. Complete growth inhibition of PETC occurred at sodium formate concentrations of 30.0 mM; however, no differences in growth rates were observed at pH 7.0 for the two strains. Incubation at formate concentrations lower than 2.0 mM resulted in increased growth rates for both strains. The most recognizable effects of formate addition on the fermentation products were the increase in the total carbon fixed into acids and alcohols at pH 5.0 and pH 6.0, as well as, a higher ethanol to total products ratio at pH 7.0. Taken all together, these results show the ability of acetogens to use formate diminishing the energy demand for growth, and enhancing strain productivity ; The authors thank the Autonomous Government of Catalonia (Generalitat de Catalunya) (2013 FI-DGR) and the Spanish Ministry of Science and Innovation (Best-Energy, CTQ2011-23632, CTM2013-43454-R) for their financial support in this study. LEQUIA and IEA have been recognized as consolidated research groups by the Catalan Government (2014-SGR-1168 and 2014-SGR-2016). RG gratefully acknowledges support from Beatriu de Pinos fellowship (BP-2011-B) and FP7 Marie Curie Career Integration Grants (PCIG13-GA-2013-618593)

The current energy model based on fossil fuels is coming to an end due to the increase in global energy demand. Biofuels such as ethanol and butanol can be produced through the syngas fermentation by acetogenic bacteria. The present work hypothesizes that formate addition would positively impact kinetic parameters for growth and alcohol production in Clostridium ljungdahlii PETC and Clostridium carboxidivorans P7 by diminishing the need for reducing equivalents. Fermentation experiments were conducted using completely anaerobic batch cultures at different pH values and formate concentrations. PETC cultures were more tolerant to formate concentrations than P7, specially at pH 5.0 and 6.0. Complete growth inhibition of PETC occurred at sodium formate concentrations of 30.0 mM; however, no differences in growth rates were observed at pH 7.0 for the two strains. Incubation at formate concentrations lower than 2.0 mM resulted in increased growth rates for both strains. The most recognizable effects of formate addition on the fermentation products were the increase in the total carbon fixed into acids and alcohols at pH 5.0 and pH 6.0, as well as, a higher ethanol to total products ratio at pH 7.0. Taken all together, these results show the ability of acetogens to use formate diminishing the energy demand for growth, and enhancing strain productivity ; The authors thank the Autonomous Government of Catalonia (Generalitat de Catalunya) (2013 FI-DGR) and the Spanish Ministry of Science and Innovation (Best-Energy, CTQ2011-23632, CTM2013-43454-R) for their financial support in this study. LEQUIA and IEA have been recognized as consolidated research groups by the Catalan Government (2014-SGR-1168 and 2014-SGR-2016). RG gratefully acknowledges support from Beatriu de Pinos fellowship (BP-2011-B) and FP7 Marie Curie Career Integration Grants (PCIG13-GA-2013-618593)

Electroactive microorganisms can exchange electrons with other cells or conductive interfaces in their extracellular environment. This property opens the way to a broad range of practical biotechnological applications, from manufacturing sustainable chemicals via electrosynthesis, to bioenergy, bioelectronics or improved, low-energy demanding wastewater treatments. Besides, electroactive microorganisms play key roles in environmental bioremediation, significantly impacting process efficiencies. This review highlights our present knowledge on microbial interactions promoting the communication between electroactive microorganisms in a biofilm on an electrode in bioelectrochemical systems (BES). Furthermore, the immediate knowledge gaps that must be closed to develop novel technologies will also be acknowledged ; C.P acknowledges the funding from FCT– Fundaçao para a Ciência e a Tecnologia, I.P. (FCT), Project MOSTMICRO-ITQB with refs UIDB/04612/2020 and UIDP/04612/2020. M. A. R. has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant agreement No.864669). A.E.R is funded by the Novo Nordisk Foundation (Ascending Investigator grant no. NNF21OC0067353) and the Danish Research Council (DFF 1026-00159). S.P is a Serra Húnter Fellow (UdG-AG-575) and acknowledges the funding from the ICREA Academia award and the Spanish Ministry of Science and Innovation (RTI2018-098360-B-I00 and PLEC2021-007802). LEQUiA [2017-SGR-1552] and Ecoaqua [2017SGR- 548] have been recognized as consolidated research groups by the Catalan Government. C.P and S.P acknowledge support of CA19123 COST ACTION PHOENIX

Volatile silicon compounds present in the biogas of anaerobic digesters can cause severe problems in the energy recovery systems, inducing costly damages. Herein, the microbial community of a lab-scale biotrickling filter (BTF) was studied while testing its biodegradation capacity on octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), in the presence of toluene, limonene and hexane. The reactor performance was tested at different empty bed residence times (EBRT) and packing materials. Community structure was analysed by bar-coded amplicon sequencing of the 16S rRNA gene. Microbial diversity and richness were higher in the inoculum and progressively decreased during BTF operation (Simpson's diversity index changing from 0.98–0.90 and Richness from 900 to 200 OTUs). Minimum diversity was found when reactor was operated at relatively low EBRT (7.3 min) using a multicomponent feed. The core community was composed of 36 OTUs (accounting for 55% of total sequences). Packing material played a key role in the community structure. Betaproteobacteriales were dominant in the presence of lava rock and were partially substituted by Corynebacteriales and Rhizobiales when activated carbon was added to the BTF. Despite these changes, a stable and resilient core microbiome was selected defining a set of potentially degrading bacteria for siloxane bioremoval as a complementary alternative to non-regenerative adsorption onto activated carbon ; This work was funded by MINECO – Spain (CTQ2014-53718R) co-funded by FEDER and University of Girona. E. B. thanks SENESCYT – Ecuador for her predoctoral grant. IEA and LEQUIA have been recognized as consolidated research groups by the Catalan Government (2017SGR-548 and 2017SGR-1552, respectively). E.S.C. thanks Universitat de Girona for his predoctoral grant (IFUdG-2015/51). The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 712949 ...

The benefits of plant-microbe interactions have been exploited extensively for nutrient removal. Radial oxygen loss in aquatic macrophytes potentially promotes nitrification and accelerates nitrogen removal through coupled nitrification-denitrification process. Nitrification is likely the limiting activity for an effective nitrogen removal in wetlands. In this work, we have quantified the effect of radial oxygen losses in Typha angustifolia plants in environments of contrasting salinities, including a temporary lagoon, a constructed wetland, and a river estuary. In all sites, radial oxygen diffusion occurred mainly at a narrow band, from 1 to 5 cm from the root tip, and were almost absent at the tip and basal sections of the root (> 5 cm). Root sections with active oxygen diffusion tended to show higher bacterial and archaeal densities in the rhizoplane according to 16S rRNA gene abundance data, except at higher salinities. Archaeal amoA /bacterial amoA gene ratios were highly variable among sites. Archaeal nitrifiers were only favoured over bacteria on the root surface of Typha collected from the constructed wetland. Collectively, radial oxygen loss had little effect on the nitrifying microbial community at the smaller scale (differences according to root-section), and observed differences were more likely related to prevailing physicochemical conditions of the studied environments or to long-term effects of the root microenvironment (root vs sediment comparisons) ; EHdA received a grant from the University of Girona (IF-UDG2013). This work was funded by University of Girona (Projects MPCUdG2016/121 and UdG PONT2019/09). IEA has been recognised as a consolidated research group by the Catalan Government (2017SGR-548)

Electromethanogenesis is the bioreduction of carbon dioxide (CO2) to methane (CH4) utilizing an electrode as electron donor. Some studies have reported the active participation of Methanobacterium sp. in electron capturing, although no conclusive results are available. In this study, we aimed at determining short-time changes in the expression levels of [NiFe]-hydrogenases (Eha, Ehb and Mvh), heterodisulfide reductase (Hdr), coenzyme F420-reducing [NiFe]-hydrogenase (Frh), and hydrogenase maturation protein (HypD), according to the electron flow in independently connected carbon cloth cathodes poised at– 800 mV vs. standard hydrogen electrode (SHE). Amplicon massive sequencing of cathode biofilm confirmed the presence of an enriched Methanobacterium sp. population (>70% of sequence reads), which remained in an active state (78% of cDNA reads), tagging this archaeon as the main methane producer in the system. Quantitative RT-PCR determinations of ehaB, ehbL, mvhA, hdrA, frhA, and hypD genes resulted in only slight (up to 1.5 fold) changes for four out of six genes analyzed when cells were exposed to open (disconnected) or closed (connected) electric circuit events. The presented results suggested that suspected mechanisms for electron capturing were not regulated at the transcriptional level in Methanobacterium sp. for short time exposures of the cells to connected-disconnected circuits. Additional tests are needed in order to confirm proteins that participate in electron capturing in Methanobacterium sp ; This work was funded by Spanish Government (MINECO, CTQ2014-53718-R) and the Universitat de Girona (MPCUdG2016/139 and MCPUdG2016/121). Sebastià Puig is a Serra Hunter Fellow (UdG-AG-575). Ramiro Blasco-Gómez is grateful for the Research Personnel Training (FPI) grant (BES-2015-074229). Elisabet Perona-Vico received a Research Training grant from the University of Girona (IFUdG2018/52). LEQUiA and IEA have been recognized as consolidated research groups by the Generalitat de Catalunya (2017SGR-1552, and ...

Microbial electrosynthesis (MES) for bioelectro carbon dioxide (CO2) recycling is an interesting and sustainable opportunity to exploit off gases from industrial facilities and convert them into valuable energy sources. In the present study, a two-step process based on coupling a bioelectrochemical system (BES) and heterotrophic microalgae Auxenochlorella protothecoides is proposed to convert carbon dioxide into a biodiesel compatible oil. The MES effluent was further processed in a heterotrophic microalgae batch reactor, where the acetate previously synthesized from CO2 was converted into bio-oil in a subsequent, extraction-free step. Two MES reactors were operated in batch mode at an applied cathodic potential of 0.8 V vs. SHE (standard hydrogen electrode) for 95 days. The system reached a concentration of up to 13 g L 1 of acetate (at a maximum production rate of 0.29 g L 1 d 1). Microbial community analysis revealed the presence of Clostridium spp. in both reactors. In a second stage, the effluent from the biocathode was transferred to microalgae reactors containing A. protothecoides to assess oil production. The bio-oil content was up to 22% w/w (dry weight), sufficient to further explore the feasibility of microalgae-to-oil recovery in the future. According to our estimations, 7.59 kg CO2 can be converted into 1 kg acetate, which can be used to grow heterotrophically 1.11 kg dry algae; an overall balance of 0.03 kg bio-oil produced per kg CO2 captured was assessed. The oil obtained can be further processed to produce a biodiesel compatible with EU requirements for biofuels ; This research was funded by the Spanish Ministry of Science through the grant RTI2018-098360-B-I00 and the Agency for Business Competitiveness of the Government of Catalonia (ACCIO) through the DigesTake Project (COMRDI16-1-0061). E. ´ P.-V. is grateful for the Research Training grant from the University of Girona (IFUdG2018/52). S. P. is a Serra Hunter ´ Fellow (UdG-AG-575) and acknowledges the funding from the ICREA Acad`emia award. ...

Hydrogen-oxidizing bacteria (HOBs) are prime candidates for the sustainable production of microbial protein because of their high nutritional value and simple metabolic requirements, which could allow a simple resource recycling of NH4+ and CO2 into a valuable product. The modality of hydrogen production/supply to HOBs represents a challenge. In this work, bioelectrochemical hydrogen and oxygen productions were coupled with HOBs cultivation in the same reactor. Using this approach, the protein content was 59 ± 8% with Hydrogenophaga and Xanthobacter as the most abundant HOBs. Biomass concentration was found to be dependent of the current density with bulk liquid biomass increasing between 101 and 124 mg TSS·L−1 for an increase of 0.1 mA·cm−2. Electrodes directly immersed in the bioreactor promoted biofilm formation on the electrode surface, which could become an attractive alternative for biomass harvesting. The results of this work establish a novel proof-of-concept for bio-electro production of microbial protein ; This research was carried out in the project "RITA – urban cycle ResIlient To pAndemics) funded by AGAUR (ref. 2020PANDE00176). The authors acknowledge the technical support of Ms. Sandra Cano and Mr. Juan José Murga. S.P. is a Serra Hunter Fellow (UdG-AG-575) and acknowledges the funding from the ICREA Academia award. LEQUIA (2017-SGR-1552) and Ecoaqua (2017-SGR-548) have been recognized as consolidated research groups by the Catalan Government

Microbial electricity-driven anoxic ammonium removal could remove ammonium from wastewater without the presence of oxygen (aeration) using electricity. This study aims at unveiling the potential biologic pathways for the bioelectrochemical oxidation of ammonium to dinitrogen gas in an anaerobic bioelectrochemical system (BES). Known intermediate metabolites of this process (hydroxylamine, nitrite and nitrate) were monitored in two BES replicates. Ammonium was fully oxidized to dinitrogen gas without intermediates accumulation in the anodic chamber. Achromobacter sp. was the most abundant microorganism (up to 60%, according to sequence reads) in the mixed community. Hydroxylamine and nitrite oxidation were electroactive processes, reinforcing the role of the anodic electrode as the electron acceptor for ammonium oxidation. Taking it all together, ammonium can be removed in BES by a combination of different bio/electrochemical processes. A deeper understanding on how the different metabolisms are coupled together is required for increasing the current ammonium removal rates ; This work was funded through the European Union's Horizon 2020 project ELECTRA [no. 826244]. M. O-A. was supported by a grant from University of Girona (IFUdG2018/50). S.P is a Serra Húnter Fellow (UdG-AG-575) and acknowledges the funding from the ICREA Academia award. LEQUiA [2017-SGR-1552] and Ecoaqua [2017SGR- 548] have been recognized as consolidated research groups by the Catalan Governmen

Many industrial combustion processes produce carbon dioxide (CO2) at high temperature, which may be electrically recycled into valuable chemicals using microorganisms as catalysts. However, little attention has been paid to handle the remaining heat of these processes as an alternative to increase CO2 fixation and production rates. Thus, this study was aimed at steering electro bio-CO2 recycling into organic compounds under thermophilic conditions. A mesophilic anaerobic sludge was adapted in lab-scale reactors at 50 °C, developing a resilient biocathode. High amounts of acetate (5250 mg L−1) were accumulated during a long-term operation period (150 days). The maximum production rate was 28 g acetate per m2 per d, with columbic efficiencies over 80%. In terms of carbon (C) conversion, 0.31 kg of C as acetate were obtained per 1 kg of C as CO2 inlet, with an energy demand of 24 kW h per 1 kg of acetate. Thermoanaerobacterales appeared to dominate the cathodic chambers, though they were compartmentalized by distinct bacterial communities in the electrode biofilm compared to the bulk liquid. This research delves into the sustained ability of a mixed microbial culture to electrochemically produce organic compounds at 50 °C and considers the possibility of using CO2-saturated effluents from industrial heated point sources to bring the technology closer to its scale-up. ; This study has received funding from the European Union's Horizon 2020 research and innovation program under the grant agreement no. 760431. L. R.-A. acknowledges the support from the Catalan Government (2018 FI-B 00347) in the European FSE program (CCI 2014ES05SFOP007). E. P.-V. is grateful for the Research Training grant from the University of Girona (IFUdG2018/52). S. P. is a Serra Húnter Fellow (UdG-AG-575) and acknowledges the funding from the ICREA Acadèmia award. LEQUIA and IEA have been both recognized as consolidated research groups by the Catalan Government (2017-SGR-1552 and 2017-SGR-548)

The biocathodic reduction of nitrate in Microbial Fuel Cells (MFCs) is an alternative to remove nitrogen in low carbon to nitrogen wastewater and relies entirely on microbial activity. In this paper the community composition of denitrifiers in the cathode of a MFC is analysed in relation to added electron acceptors (nitrate and nitrite) and organic matter in the cathode. Nitrate reducers and nitrite reducers were highly affected by the operational conditions and displayed high diversity. The number of retrieved species-level Operational Taxonomic Units (OTUs) for narG, napA, nirS and nirK genes was 11, 10, 31 and 22, respectively. In contrast, nitrous oxide reducers remained virtually unchanged at all conditions. About 90% of the retrieved nosZ sequences grouped in a single OTU with a high similarity with Oligotropha carboxidovorans nosZ gene. nirS-containing denitrifiers were dominant at all conditions and accounted for a significant amount of the total bacterial density. Current production decreased from 15.0 A·m−3 NCC (Net Cathodic Compartment), when nitrate was used as an electron acceptor, to 14.1 A·m−3 NCC in the case of nitrite. Contrarily, nitrous oxide (N2O) accumulation in the MFC was higher when nitrite was used as the main electron acceptor and accounted for 70% of gaseous nitrogen. Relative abundance of nitrite to nitrous oxide reducers, calculated as (qnirS+qnirK)/qnosZ, correlated positively with N2O emissions. Collectively, data indicate that bacteria catalysing the initial denitrification steps in a MFC are highly influenced by main electron acceptors and have a major influence on current production and N2O accumulation ; A. V-S. and A. G-L. are recipients of pre-doctoral grants from the Universitat de Girona and the Ministerio de Ciencia y Educacion, respectively. This research was funded through grants from the Spanish Government (CGL2009-08338, CSD2007-00055 and CTQ2011-23632)