Effect of model methanogens on the electrochemical activity, stability, and microbial community structure of Geobacter spp. dominated biofilm anodes.

Combining anaerobic digestion (AD) and microbial electrochemical technologies (MET) in AD-MET holds great potential. Methanogens have been identified as one cause of decreased electrochemical activity and deterioration of Geobacter spp. biofilm anodes. A better understanding of the different interactions between methanogenic genera/species and Geobacter spp. biofilms is needed to shed light on the observed reduction in electrochemical activity and stability of Geobacter spp. dominated biofilms as well as observed changes in microbial communities of AD-MET. Here, we have analyzed electrochemical parameters and changes in the microbial community of Geobacter spp. biofilm anodes when exposed to three representative methanogens with different metabolic pathways, i.e., Methanosarcina barkeri, Methanobacterium formicicum, and Methanothrix soehngenii. M. barkeri negatively affected the performance and stability of Geobacter spp. biofilm anodes only in the initial batches. In contrast, M. formicicum did not affect the stability of Geobacter spp. biofilm anodes but caused a decrease in maximum current density of ~37%. M. soehngenii induced a coloration change of Geobacter spp. biofilm anodes and a decrease in the total transferred charge by ~40%. Characterization of biofilm samples after each experiment by 16S rRNA metabarcoding, whole metagenome nanopore sequencing, and shotgun sequencing showed a higher relative abundance of Geobacter spp. after exposure to M. barkeri as opposed to M. formicicum or M. soehngenii, despite the massive biofilm dispersal observed during initial exposure to M. barkeri.

The microbiology of Power-to-X applications.

Power-to-X (P2X) technologies will play a more important role in the conversion of electric power to storable energy carriers, commodity chemicals and even food and feed. Among the different P2X technologies, microbial components form cornerstones of individual process steps. This review comprehensively presents the state-of-the-art of different P2X technologies from a microbiological standpoint. We are focusing on microbial conversions of hydrogen from water electrolysis to methane, other chemicals and proteins. We present the microbial toolbox needed to gain access to these products of interest, assess its current status and research needs, and discuss potential future developments that are needed to turn todays P2X concepts into tomorrow's technologies.

Combining Geobacter spp. Dominated Biofilms and Anaerobic Digestion Effluents─The Effect of Effluent Composition and Electrode Potential on Biofilm Activity and Stability.

The combination of anaerobic digestion (AD) and microbial electrochemical technologies (METs) offers different opportunities to increase the efficiency and sustainability of AD processes. However, methanogenic archaea and/or particles may partially hinder combining MET and AD processes. Furthermore, it is unclear if the applied anode potential affects the activity and efficiency of electroactive microorganisms in AD-MET combinations as it is described for more controlled experimental conditions. In this study, we confirm that 6-week-old Geobacter spp. dominated biofilms are by far more active and stable in AD-effluents than 3-week-old Geobacter spp. dominated biofilms. Furthermore, we show that the biofilms are twice as active at -0.2 V compared to 0.4 V, even under challenging conditions occurring in AD-MET systems. Paired-end amplicon sequencing at the DNA level using 16S-rRNA and mcrA gene shows that hydrogenotrophic methanogens incorporate into biofilms immersed in AD-effluent without any negative effect on biofilm stability and electrochemical activity.

A unified and simple medium for growing model methanogens.

Apart from their archetypic use in anaerobic digestion (AD) methanogenic archaea are targeted for a wide range of applications. Using different methanogenic archaea for one specific application requires the optimization of culture media to enable the growth of different strains under identical environmental conditions, e.g., in microbial electrochemical technologies (MET) for (bio)electromethanation. Here we present a new culture medium (BFS01) adapted from the DSM-120 medium by omitting resazurin, yeast extract, casitone, and using a low salt concentration, that was optimized for Methanosarcina barkeri, Methanobacterium formicicum, and Methanothrix soehngenii. The aim was to provide a medium for follow-up co-culture studies using specific methanogens and Geobacter spp. dominated biofilm anodes. All three methanogens showed growth and activity in the BFS01 medium. This was demonstrated by estimating the specific growth rates ( µ ) and doubling times ( t d ) of each methanogen. The µ and t d based on methane accumulation in the headspace showed values consistent with literature values for M. barkeri and M. soehngenii. However, µ and t d based on methane accumulation in the headspace differed from literature data for M. formicicum but still allowed sufficient growth. The lowered salt concentration and the omission of chemically complex organic components in the medium may have led to the observed deviation from µ and t d for M. formicicum as well as the changed morphology. 16S rRNA gene-based amplicon sequencing and whole genome nanopore sequencing further confirmed purity and species identity.

Benefits of Age-Improved Resistance of Mature Electroactive Biofilm Anodes in Anaerobic Digestion.

Anaerobic digestion (AD) and microbial electrochemical technologies (MET) can be combined in manifold ways. Recent studies show negative influences of AD effluents on the performance of pre-grown Geobacter spp.-dominated biofilm anodes. In this study, it was investigated how such biofilm anodes are affected by AD effluents. Therefore, experiments using AD effluents in different concentrations (0-100%) in combination with biofilms of different ages were performed. Furthermore, the activity of methanogens was inhibited and minimized by application of 2-bromoethanesulfonate (2-BES) and microfiltration, respectively. Biofilms pre-grown for 5 weeks show higher resistance against AD effluents compared to biofilms pre-grown for only 3 weeks. Nevertheless, adaptation of biofilms to AD effluents was not successful. Biofilm activity in terms of coulombic efficiency and maximum current density (jmax) dropped by factor 32.2 ± 3.2 and 38.9 ± 8.4, respectively. The application of 2-BES and microfiltration had positive effects on the biofilm activity. The results support the assumption that methanogens or further compounds not studied here, for example, protozoans, which may have been inhibited or removed by 2-BES application or microfiltration, have an immediate influence on the stability of Geobacter spp.-dominated biofilms and may limit their practical application in AD environments.

Anaerobic Digestion in the 21st Century.

Despite being a mature biotechnological process, anaerobic digestion is still attracting considerable research attention, mainly due to its versatility both in substrate and product spectra, as well as being a perfect test system for the microbial ecology of anaerobes [...].

Determining incremental coulombic efficiency and physiological parameters of early stage Geobacter spp. enrichment biofilms.

Geobacter spp. enrichment biofilms were cultivated in batch using one-chamber and two-chamber bioelectrochemical reactors. Time-resolved substrate quantification was performed to derive physiological parameters as well as incremental coulombic efficiency (i.e., coulombic efficiency during one batch cycle, here every 6h) during early stage biofilm development. The results of one-chamber reactors revealed an intermediate acetate increase putatively due to the presence of acetogens. Total coulombic efficiencies of two-chamber reactors were considerable lower (19.6±8.3% and 49.3±13.2% for 1st and 2nd batch cycle, respectively) compared to usually reported values of mature Geobacter spp. enrichment biofilms presumably reflecting energetic requirements for biomass production (i.e., cells and extracellular polymeric substances) during early stages of biofilm development. The incremental coulombic efficiency exhibits considerable changes during batch cycles indicating shifts between phases of maximizing metabolic rates and maximizing biomass yield. Analysis based on Michaelis-Menten kinetics yielded maximum substrate uptake rates (vmax,Ac, vmax,I) and half-saturation concentration coefficients (KM,Ac,KM,I) based on acetate uptake or current production, respectively. The latter is usually reported in literature but neglects energy demands for biofilm growth and maintenance as well as acetate and electron storage. From 1st to 2nd batch cycle, vmax,Ac and KM,Ac, decreased from 0.0042-0.0051 mmol Ac- h-1 cm-2 to 0.0031-0.0037 mmol Ac- h-1 cm-2 and 1.02-2.61 mM Ac- to 0.28-0.42 mM Ac-, respectively. Furthermore, differences between KM,Ac/KM,I and vmax,Ac/vmax,I were observed providing insights into the physiology of Geobacter spp. enrichment biofilms. Notably, KM,I considerably scattered while vmax,Ac/vmax,I and KM,Ac remained rather stable indicating that acetate transport within biofilm only marginally affects reaction rates. The observed data variation mandates the requirement of a more detailed analysis with an improved experimental system, e.g., using flow conditions and a comparison with Geobacter spp. pure cultures.

Biofouling of membranes in microbial electrochemical technologies: Causes, characterization methods and mitigation strategies.

The scope of the review is to discuss the current state of knowledge and lessons learned on biofouling of membrane separators being used for microbial electrochemical technologies (MET). It is illustrated what crucial membrane features have to be considered and how these affect the MET performance, paying particular attention to membrane biofouling. The complexity of the phenomena was demonstrated and thereby, it is shown that membrane qualities related to its surface and inherent material features significantly influence (and can be influenced by) the biofouling process. Applicable methods for assessment of membrane biofouling are highlighted, followed by the detailed literature evaluation. Finally, an outlook on e.g. possible mitigation strategies for membrane biofouling in MET is provided.

Anaerobic Digestion.

The term anaerobic digestion usually refers to the microbial conversion of organic material to biogas, which mainly consists of methane and carbon dioxide. The technical application of the naturally-occurring process is used to provide a renewable energy carrier and - as the substrate is often waste material - to reduce the organic matter content of the substrate prior to disposal.Applications can be found in sewage sludge treatment, the treatment of industrial and municipal solid wastes and wastewaters (including landfill gas utilization), and the conversion of agricultural residues and energy crops.For biorefinery concepts, the anaerobic digestion (AD) process is, on the one hand, an option to treat organic residues from other production processes. Concomitant effects are the reduction of organic carbon within the treated substance, the conversion of nitrogen and sulfur components, and the production of an energy-rich gas - the biogas. On the other hand, the multistep conversion of complex organic material offers the possibility of interrupting the conversion chain and locking out intermediates for utilization as basic material within the chemical industry.

Does the addition of proteases affect the biogas yield from organic material in anaerobic digestion?

The aim of this study was to investigate the biochemical disintegration effect of hydrolytic enzymes in lab scale experiments. Influences of enzyme addition on the biogas yield as well as effects on the process stability were examined. The addition of proteases occurred with low and high dosages in batch and semi-continuous biogas tests. The feed mixture consisted of maize silage, chicken dung and cow manure. Only very high concentrated enzymes caused an increase in biogas production in batch experiments. In semi-continuous biogas tests no positive long-term effects (100 days) were observed. Higher enzyme-dosage led to a reduced biogas-yield (13% and 36% lower than the reference). Phenylacetate and -propionate increased (up to 372 mgl(-1)) before the other volatile fatty acids did. Volatile organic acids rose up to 6.8 gl(-1). The anaerobic digestion process was inhibited.