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’ 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–Ljungdahl 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.