Biogas production from wheat straw: community structure of cellulose-degrading bacteria

Background: Wheat straw is one of the most abundant crop residues produced in the world, making it highly interesting as a substrate for biogas production. However, due to the complex structure, its degradability and gas yield are low. The degradability can be improved by pre-treatment, making the material more accessible to microbial degradation. Methods: To investigate the microbial response to straw as a feed stock for biogas production, this study examined the community structure of cellulose-degrading bacteria in lab-scale biogas digesters operating with manure, alone or in co-digestion with straw, with and without pre-treatment (steam-explosion) at different temperatures. The community was studied by targeting the functional gene encoding glycoside hydrolases of families 5 and 48 using T-RFLP, clone libraries and qPCR. Results: In general, bacteria belonging to the phyla Firmicutes and Bacteroidetes dominated the cellulose-degrading bacteria community in all digesters. The degree of similarity to the characterised bacteria was often low, and some clones were more closely related to the uncultured bacteria. The addition of straw, pre-treatment of straw and increasing operating temperature all affected the cellulose-degrading community structure, with differing responses in the cel48 and cel5 communities. Both communities changed in response to temperature, while only the cel5 community was affected by the addition of straw and cel48 community by straw pre-treatment. Conclusions: The addition of straw, pre-treatment of straw and operating temperature all affected the cellulosedegrading community in biogas digesters, but there were no major differences in the digester performance and gas yield.