Abstract
Abstract 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 cellulose-degrading community in biogas digesters, but there were no major differences in the digester performance and gas yield.
Highlights
Wheat straw is one of the most abundant crop residues produced in the world, making it highly interesting as a substrate for biogas production
terminal-restriction fragment length polymorphism (T-RFLP) glycoside hydrolase families 48 (Cel48) The cellulose-degrading community structure in the different digesters, managed with different operating strategies and substrates, was examined by T-RFLP analysis combined with the construction of clone libraries
In summary, the quantitative information on the abundance of cel48 obtained by quantitative polymerase chain reaction (qPCR) assay revealed that straw in the substrate stimulated a tenfold increase in the numbers of this gene copy
Summary
Wheat straw is one of the most abundant crop residues produced in the world, making it highly interesting as a substrate for biogas production. The microbial degradation of organic material to biogas consists of four steps: (1) hydrolysis of complex organic polymers to soluble compounds; (2) fermentation of the products of hydrolysis into intermediate compounds such as fatty acids and alcohols; (3) anaerobic oxidation of these intermediate products to produce acetate, H2 and CO2; and (4) methane production by methanogenic Archaea [12]. Numerous studies have examined anaerobic cellulosedegrading bacteria and their enzymatic capabilities in order to clarify the degradation mechanisms and identify ways to enhance degradation rates. Most of these studies have been performed on samples from gut and soil ecosystems [3,13,15,16] and only a few have examined cellulose-degrading bacteria in biogas digesters [17,18,19]
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