Microbial diversity in a biogas-producing co-fermentation of maize silage and bovine manure

Abstract

The efficiency of biogas reactors strongly depends on the substrate utilization by the microbial biocoenosis. Up to now, the structure of this microbial community has not been sufficiently characterized. In this study, an overview of the most abundant species within a biogas-producing, completely-stirred tank reactor (CSTR) during the co- fermentation of maize silage and bovine manure was sur- veyed. The analysis based on two 16S rDNA libraries each enriched for methanogenic Archaea (81 clones) and fer- mentative Bacteria (118 clones). The CSTR showed a huge diversity within the methanogenic Archaea. The or- der Methanomicrobiales was present with eight opera- tional taxonomic units (OUT), the order Methanosarcina- les with three OTU and the order Methanobacteriales with two OTU. The order Methanococcales was not detected. One OTU could not be assigned to any order. Most OTU (71 %) were affiliated to hydrogenotrophic methanogens like Methanoculleus bourgensis, Methanobrevibacter ru- minantium, Methanospirilllum hungatei and Methanocor- pusculum parvum. Acetate-utilizing Archaea were detected with 3 OTU related to Methanosarcina barkeri and Methanosaeta concilii. Within the domain Bacteria a rich diversity was found within the members of the phyla Fir- micutes (27 OTU) and Bacterioidetes (13 OTU). Also cer- tain OTU of the phyla Proteobacteria (4 OTU), Spiro- chaetes (2 OTU), Fibrobacteres (1 OTU) and Chloroflexi (1 OTU) were found. Ten OTU could not be grouped with any cultivated type strain. The dominance of OTU related with H2-consuming methanogens in the Archaea-specific 16S rDNA library and their rich diversity point to major formation of methane by oxidation of CO2.