Response of methanogenic archaea and sulphate reducing bacteria during the anaerobic digestion of vegetal biomasses

The increasingly growing demand for renewable energy is stimulating the adoption of anaerobic digestion for extracting energy from agricultural residues and biomasses in the forms of methane and/or hydrogen-based biogas. A laboratory-scale multi-batch system was used to compare the methane production and to study the response of the microbial community involved in the methanogenesis of different inoculum-substrate mixtures (ratios of 2:1 expressed as TS basis) based on different vegetal biomasses mixed with livestock wastewater. The highest methane yields were obtained with energy crops (mais silage, sorgum silage and triticale silage) and livestock wastes (pollina and dairy manure) with an average production respectively of 564.2 ± 122.5 Nm3CH4/tVS and 564.9 ± 119.7 Nm3CH4/tVS. The responses of different taxonomic and trophic groups of methanogenic archaea and of sulphate reducing bacteria were measured by PCR and denaturing gradient gel electrophoresis (PCR-DGGE) on 16S rRNA gene and quantitative real-time PCR. While the diversity of archaea and bacteria, as observed by PCR-DGGE remained stable in the different operative conditions of the reactors, the number of Archaea before and after the process moderately increased reflecting the functionality of the process. In the cases of the substrates with the highest methane yields the number of methanogens remained stable or increased, while sulfate-reducing bacteria were always observed at lower titres. The Methanosarcinales sp., that includes acetate-consuming methanogens was the predominant group within the methanogenic community. The overall data indicate that process efficiency is more driven by the quantitative relationships between the different groups of microorganisms in the reactor, rather than deep changes in the diversity.