Power-to-Methane as one part of Power-to-Gas has been recognized globally as one of the key elements for the transition towards a sustainable energy system. While plants that produce methane catalytically have been in operation for a long time, biological methanation has just reached industrial pilot scale and near-term commercial application. The growing importance of the biological method is reflected by an increasing number of scientific articles describing novel approaches to improve this technology. However, these studies are difficult to compare because they lack a coherent nomenclature. In this article, we present a comprehensive set of parameters allowing the characterization and comparison of various biological methanation processes. To identify relevant parameters needed for a proper description of this technology, we summarized existing literature and defined system boundaries for Power-to-Methane process steps. On this basis, we derive system parameters providing information on the methanation system, its performance, the biology and cost aspects. As a result, three different standards are provided as a blueprint matrix for use in academia and industry applicable to both, biological and catalytic methanation. Hence, this review attempts to set the standards for a comprehensive description of biological and chemical methanation processes.

The archaellum is the macromolecular machinery that Archaea use for propulsion or surface adhesion, enabling them to proliferate and invade new territories. The molecular composition of the archaellum and of the motor that drives it appears to be entirely distinct from that of the functionally equivalent bacterial flagellum and flagellar motor. Yet, the structure of the archaellum machinery is scarcely known. Using combined modes of electron cryo-microscopy (cryoEM), we have solved the structure of the Pyrococcus furiosus archaellum filament at 4.2 Å resolution and visualise the architecture and organisation of its motor complex in situ. This allows us to build a structural model combining the archaellum and its motor complex, paving the way to a molecular understanding of archaeal swimming motion.

Archaea use a molecular machine, called the archaellum, to swim. The archaellum consists of an ATP-powered intracellular motor that drives the rotation of an extracellular filament composed of multiple copies of proteins named archaellins. In many species, several archaellin homologs are encoded in the same operon; however, previous structural studies indicated that archaellum filaments mainly consist of only one protein species. Here, we use electron cryo-microscopy to elucidate the structure of the archaellum from Methanocaldococcus villosus at 3.08 Å resolution. The filament is composed of two alternating archaellins, suggesting that the architecture and assembly of archaella is more complex than previously thought. Moreover, we identify structural elements that may contribute to the filament’s flexibility.

We have described previously that the flagella of the Euryarchaeon Pyrococcus furiosus are multifunctional cell appendages used for swimming, adhesion to surfaces and formation of cell-cell connections. Here, we characterize these organelles with respect to their biochemistry and transcription. Flagella were purified by shearing from cells followed by CsCl-gradient centrifugation and were found to consist mainly of a ca. 30 kDa glycoprotein. Polymerization studies of denatured flagella resulted in an ATP-independent formation of flagella-like filaments. N-terminal sequencing of the main flagellin revealed an unexpected N-terminus. Therefore, we resequenced the respective region of the genome, thereby discovering that the published genome sequence is not correct. A total of 771 bp are missing in the data base, resulting in the fact that a total of three flagellin genes are present. To keep in line with the earlier nomenclature we call these flaB0, flaB1, and flaB2. Very interestingly, the previously not identified flaB0 codes for the major flagellin. Transcriptional analyses of the newly defined flagellar operon identified various different transcripts depending on the growth phase.

A novel methanogenic strain, CaP3V-MF-L2A^T, was isolated from an exploratory oil well from Cahuita National Park, Costa Rica. The cells were irregular cocci, 0.8–1.8 μm in diameter, stained Gram-negative and were motile. The strain utilized H₂/CO₂, formate and the primary and secondary alcohols 1-propanol and 2-propanol for methanogenesis, but not acetate, methanol, ethanol, 1-butanol or 2-butanol. Acetate was required as carbon source. The novel isolate grew at 25–40 °C, pH 6.0–7.5 and 0–2.5% (w/v) NaCl. 16S rRNA gene sequence analysis revealed that the strain is affiliated to the genus Methanofollis. It shows 98.8% sequence similarity to its closest relative Methanofollis ethanolicus. The G + C content is 60.1 mol%. Based on the data presented here type strain CaP3V-MF-L2A^T (= DSM 113321^T = JCM 39176^T) represents a novel species, Methanofollis propanolicus sp. nov.

A novel interdomain consortium composed of a methanogenic Archaeon and a sulfate-reducing bacterium was isolated from a microbial biofilm in an oil well in Cahuita National Park, Costa Rica. Both organisms can be grown in pure culture or as stable co-culture. The methanogenic cells were non-motile rods producing CH₄ exclusively from H₂/CO₂. Cells of the sulfate-reducing partner were motile rods forming cell aggregates. They utilized hydrogen, lactate, formate, and pyruvate as electron donors. Electron acceptors were sulfate, thiosulfate, and sulfite. 16S rRNA sequencing revealed 99% gene sequence similarity of strain CaP3V-M-L2A^T to Methanobacterium subterraneum and 98.5% of strain CaP3V-S-L1A^T to Desulfomicrobium baculatum. Both strains grew from 20 to 42 °C, pH 5.0–7.5, and 0–4% NaCl. Based on our data, type strains CaP3V-M-L2A^T (= DSM 113354 ^T = JCM 39174 ^T) and CaP3V-S-L1A^T (= DSM 113299 ^T = JCM 39179 ^T) represent novel species which we name Methanobacterium cahuitense sp. nov. and Desulfomicrobium aggregans sp. nov.