The structural and functional contributions of β-glucosidase-producing microbial communities to cellulose degradation in composting

Background: Compost habitats sustain a vast ensemble of microbes that engender the degradation of cellulose, which is an important part of global carbon cycle. beta-Glucosidase is the rate-limiting enzyme of degradation of cellulose. Thus, analysis of regulation of beta-glucosidase gene expression in composting is beneficial to a better understanding of cellulose degradation mechanism. Genetic diversity and expression of beta-glucosidase-producing microbial communities, and relationships of cellulose degradation, metabolic products and the relative enzyme activity during natural composting and inoculated composting were evaluated. Results: Compared with natural composting, adding inoculation agent effectively improved the degradation of cellulose, and maintained high level of the carboxymethyl cellulose (CMCase) and beta-glucosidase activities in thermophilic phase. Gene expression analysis showed that glycoside hydrolase family 1 (GH1) family of beta-glucosidase genes contributed more to beta-glucosidase activity in the later thermophilic phase in inoculated compost. In the cooling phase of natural compost, glycoside hydrolase family 3 (GH3) family of beta-glucosidase genes contributed more to beta-glucosidase activity. Intracellular beta-glucosidase activity played a crucial role in the regulation of beta-glucosidase gene expression, and upregulation or downregulation was also determined by extracellular concentration of glucose. At sufficiently high glucose concentrations, the functional microbial community in compost was altered, which may contribute to maintaining beta-glucosidase activity despite the high glucose content. Conclusion: This research provides an ecological functional map of microorganisms involved in carbon metabolism in cattle manure-rice straw composting. The performance of the functional microbial groups in the two composting treatments is different, which is related to the cellulase activity and cellulose degradation, respectively.