Improvement of Direct Interspecies Electron Transfer via Adding Conductive Materials in Anaerobic Digestion: Mechanisms, Performances, and Challenges

Genomic Insights into Syntrophic Lifestyle of 'Candidatus Contubernalis alkaliaceticus' Based on the Reversed Wood-Ljungdahl Pathway and Mechanism of Direct Electron Transfer

The anaerobic oxidation of fatty acids and alcohols occurs near the thermodynamic limit of life. This process is driven by syntrophic bacteria that oxidize fatty acids and/or alcohols, their syntrophic partners that consume the products of this oxidation, and the pathways for interspecies electron exchange via these products or direct interspecies electron transfer (DIET). Due to the interdependence of syntrophic microorganisms on each other's metabolic activity, their isolation in pure cultures is almost impossible. Thus, little is known about their physiology, and the only available way to fill in the knowledge gap on these organisms is genomic and metabolic analysis of syntrophic cultures. Here we report the results of genome sequencing and analysis of an obligately syntrophic alkaliphilic bacterium 'Candidatus Contubernalis alkaliaceticus'. The genomic data suggest that acetate oxidation is carried out by the Wood-Ljungdahl pathway, while a bimodular respiratory system involving an Rnf complex and a Na+-dependent ATP synthase is used for energy conservation. The predicted genomic ability of 'Ca. C. alkaliaceticus' to outperform interspecies electron transfer both indirectly, via H2 or formate, and directly, via pili-like appendages of its syntrophic partner or conductive mineral particles, was experimentally demonstrated. This is the first indication of DIET in the class Dethiobacteria.

Effects of Iron, Lime, and Porous Ceramic Powder Additives on Methane Production from Brewer's Spent Grain in the Anaerobic Digestion Process

The process of anaerobic digestion used for methane production can be enhanced by dosing various additive materials. The effects of these materials are dependent on various factors, including the processed substrate, process conditions, and the type and amount of the additive material. As part of the study, three different materials-iron powder, lime, and milled porous ceramic-were added to the 30-day anaerobic digestion of the brewer's spent grain to improve its performance. Different doses ranging from 0.2 to 2.3 gTS × L-1 were tested, and methane production kinetics were determined using the first-order model. The results showed that the methane yield ranged from 281.4 ± 8.0 to 326.1 ± 9.3 mL × gVS-1, while substrate biodegradation ranged from 56.0 ± 1.6 to 68.1 ± 0.7%. The addition of lime reduced the methane yield at almost all doses by -6.7% to -3.3%, while the addition of iron powder increased the methane yield from 0.8% to 9.8%. The addition of ceramic powder resulted in a methane yield change ranging from -2.6% to 4.6%. These findings suggest that the use of additive materials should be approached with caution, as even slight changes in the amount used can impact methane production.

Inter-kingdom interactions and stability of methanogens revealed by machine-learning guided multi-omics analysis of industrial-scale biogas plants

Multi-omics analysis is a powerful tool for the detection and study of inter-kingdom interactions, such as those between bacterial and archaeal members of complex biogas-producing microbial communities. In the present study, the microbiomes of three industrial-scale biogas digesters, each fed with different substrates, were analysed using a machine-learning guided genome-centric metagenomics framework complemented with metatranscriptome data. This data permitted us to elucidate the relationship between abundant core methanogenic communities and their syntrophic bacterial partners. In total, we detected 297 high-quality, non-redundant metagenome-assembled genomes (nrMAGs). Moreover, the assembled 16 S rRNA gene profiles of these nrMAGs showed that the phylum Firmicutes possessed the highest copy number, while the representatives of the archaeal domain had the lowest. Further investigation of the three anaerobic microbial communities showed characteristic alterations over time but remained specific to each industrial-scale biogas plant. The relative abundance of various microorganisms as revealed by metagenome data was independent from corresponding metatranscriptome activity data. Archaea showed considerably higher activity than was expected from their abundance. We detected 51 nrMAGs that were present in all three biogas plant microbiomes with different abundances. The core microbiome correlated with the main chemical fermentation parameters, and no individual parameter emerged as a predominant shaper of community composition. Various interspecies H₂/electron transfer mechanisms were assigned to hydrogenotrophic methanogens in the biogas plants that ran on agricultural biomass and wastewater. Analysis of metatranscriptome data revealed that methanogenesis pathways were the most active of all main metabolic pathways.

Magnetite Nanoparticles and Carbon Nanotubes for Improving the Operation of Mesophilic Anaerobic Digesters

Anaerobic waste processing contributes to the development of the bioenergy sector and solves environmental problems. To date, many technologies have been developed for increasing the rate of the anaerobic digestion process and yield of methane. However, new technological advancements are required to eliminate biogas production inefficiencies. The performance of anaerobic digesters can be improved by adding conductive materials. In this study, the effects of the separate and shared use of magnetite nanoparticles and carbon nanotubes in anaerobic digesters converting high-nitrogen-containing waste, chicken manure, were investigated. The tested nanomaterials accelerated the methane production and increased the decomposition of products from the acidogenesis and acetogenesis stages. The combined use of magnetite nanoparticles and carbon nanotubes gavae better results compared to using them alone or without them. Members of the bacterial classes Bacteroidia, Clostridia, and Actinobacteria were detected at higher levels in the anaerobic digesters, but in different proportions depending on the experiment. Representatives of the genera Methanosarcina, Methanobacterium, and Methanothrix were mainly detected within the methanogenic communities in the anaerobic digesters. The present study provides new data for supporting the anaerobic treatment of substrates with a high content of inhibitory compounds, such as chicken wastes.

Mechanism of dielectric barrier discharge plasma technology to improve the quantity of short-chain fatty acids in anaerobic fermentation of waste active sludge

The mechanism of improving the anaerobic fermentation performance of waste active sludge by using dielectric barrier discharge (DBD) plasma pretreatment technology was investigated. The maximum accumulation of short-chain fatty acids (SCFAs) was observed on the 7th day of anaerobic fermentation when the DBD power was 76.50 W, which was 1726.70 mg COD/L, 1.50 times of the control group. The ratio of acetic acid in DBD group was 9.30% higher than that in the control. Further mechanism research indicated that DBD pretreatment can destroy the structure of extracellular polymer substances and release organic substances such as protein and polysaccharide. The dissolved organic matter analysis indicated that the DBD technique could increase the release of biodegradable organics (eg., tyrosine proteins, soluble microbial by-products), thus accelerate the biotransformation of organic substance. Bacterial community structure analysis showed that the increase in the abundance of Firmicutes and Bacteroidetes and the decrease in the abundance of Proteobacteria in DBD group were beneficial to the accumulation of SCFAs. Besides, further archaeal analysis indicated that the decrease of Methanosaeta sp. and Methanosarcina sp. abundance in the DBD group facilitate acetic acid accumulation. This study demonstrated that the DBD technique can be used as an effective and potential pretreatment method to improve sludge anaerobic fermentation performance.