Microbial analysis in biogas reactors suffering by foaming incidents.

Foam formation can lead to total failure of digestion process in biogas plants. In the present study, possible correlation between foaming and the presence of specific microorganisms in biogas reactors was elucidated. The microbial ecology of continuous fed digesters overloaded with proteins, lipids and carbohydrates before and after foaming incidents was characterized using 16S rRNA gene sequencing. Moreover, the microbial diversity between the liquid and foaming layer was assessed. A number of genera that are known to produce biosurfactants, contain mycolic acid in their cell wall, or decrease the surface tension of the media, increased their relative abundance after foam formation. Finally, a microorganism similar to widely known foaming bacteria (Nocardia and Desulfotomaculum) was found to increase its relative abundance in all reactors once foam was observed, regardless of the used substrate. These findings suggest that foaming and specific microorganisms might have direct association which requires to be further investigated

Microbial dynamics in mesophilic and thermophilic biogas upgrading systems investigated at genomic level.

Biogas upgrading is an emerging technology for fuel production that can be used in transportation sector or substitute natural gas. Nowadays, the biological route for biogas upgrading via hydrogen assisted methanogenesis is gaining increased attention. The success of this process is strongly dependent on the applied environmental parameters (e.g. hydrogen partial pressure) and their subsequent influence on the microbial consortium (e.g. efficiency of syntrophic interactions). The present work is the first deep study on the microbiome of mesophilic and thermophilic biogas upgrading reactors using genome-centric metagenomics. Considering the applied environmental conditions (i.e. mesophilic, thermophilic, without and with H2 addition), two distinct communities developed in the reactors strongly determined by the operational temperature. The dominant archaea found in both communities belonged to Methanoculleus species, while for bacteria, the most abundant population genomes were related to Rikenellaceae, Syntrophomonadaceae and Thermoanaerobacteraceae. Moreover, the H2 addition greatly influenced the communities resulting in a remarkable enhancement of the reactors\u2019 CH4 production rate. While the increase in abundance of hydrogenotrophic methanogens is straightforward, the plethora of bacterial species and the complexity of their metabolic properties hindered the provision of a thorough explanation regarding their abundance variation. Nevertheless, by comparing the results obtained from bioinformatics functional analysis with the abundance variation, the effect of H2 addition on bacterial species was clarified. Particular attention was given to species involved in H2 production/utilization, for example those encoding genes for Wood\u2013Ljungdahl pathway, propionate and butanoate metabolism. Finally, it was shown that during the biogas upgrading process, the increased H2 partial pressure selectively inhibited the bacterial population and more evidently in the thermophilic reactor (i.e. 71% of population genomes were decreased). On contrary, population genomes belonging to specific taxa such as Syntrophomonadaceae and Halothermothrix were favored by the new environmental conditions and increased in abundance after the addition of H2