Diversity of cellulolytic microorganisms and microbial cellulases

Lignocellulosic biomass is a geographically abundant source of renewable energy, showing enormous potential to contribute to escalating energy demands. Improving the industrial economics of cellulase production remains a significant hurdle, thus enhancing the yield of cellulase production and the catalytic efficiency of cellulases would be the key targets for research and development. The isolation of Trichoderma reesei in the 1940s marked the beginning of cellulase research. Subsequently, fungal cellulases have been extensively studied to understand the mechanisms of cellulose degradation. Over the years, continuous efforts have been devoted to discovering novel cellulases with diverse physiochemical properties. The attention has been gradually shifted to bacterial sources due to its high specific activity and heterologous production efficiency. Recently, cellulases identified from archaea are considered promising candidates for industrial applications under harsh conditions. As sequencing technology develops, the approaches to discover cellulases have gone through rapid changes. In this review, we show the sequence and structural diversity of non-homologous cellulases and summarize cellulolytic enzyme systems used by cellulose degrading microorganisms. We also discuss recent progress in culture-independent metagenomic approaches as a powerful tool for the bioprospecting of novel cellulases, as well as the advantages and limitations of these methods. • Cellulases are highly diverse in sequence, structure and function. • Distribution of microbial cellulases is widespread but dispersed in specific lineages. • Metagenomics has been and will remain a powerful tool to discover novel cellulases.