Exploring and integrating cellulolytic systems of insects to advance biofuel technology

In line with the requirements for sustainable economies and clean environments, cellulose-based biofuels have recently received tremendous attention both in industry and academic communities worldwide. Alternative and renewable fuels derived from lignocellulosic biomass offer the potential to reduce our dependence on fossil fuels and mitigate global climate change. The world, therefore, is on the verge of an unprecedented increase in the production and use of biofuels. However, in industry, breakthrough technologies to overcome barriers to developing cost-effective processes for converting biomass to fuels and chemicals are not yet fully realized. It is worthy to note that, over the past two decades, industrial bioethanol technology has mainly been based on biocatalysis and fermentation technologies from bacterial and fungal cellulolytic systems, in combination with breakthroughs in molecular genetics, enzyme engineering and metabolic engineering. In practice, the current state of technology with respect to biomass conversion is still far away from being mature for large scale application due to its efficiency and processing economics. To improve upon our current technology, it seems that we need to review our ongoing strategy and explore/learn from other sound cellulolytic systems in nature, such as wood-feeding termites or other insects. Such insects can process lignocellulosic biomass much more efficiently with their highly specialized gut systems, which can truly be considered as highly efficient natural bioreactors. With some of the most intractable issues facing the world regarding efficient and economic conversion of cellulosic biomass, this special publication comes at a critical and timely moment. Most insects are unable to use lignocellulosic substrates as their main food sources, but some insects subsist on lignocellulosic biomass as their only foods. The types of biomass fed upon by cellulolytic insects range from agricultural crops to forest woody substrates, such as in the case of termites (all seven families), wood-feeding roaches (Blattidae, Cryptoceridae), beetles (Anobiidae, Buprestidae, Cerambycidae, Scarabaeidae), wood wasps (Siricidae), leaf-shredding aquatic insects (Pteronarcidae, Limnephilidae, Tipulidae), silverfish (Lepismatidae), leaf-cutting ants (Formicidae), and so on. Cellulose digestion has been demonstrated in more than 20 insect families representing ten distinct insect orders, for example, Thysanura, Plecoptera, Dictyoptera, Orthoptera, Isoptera, Coleoptera, Trichoptera, Hymenoptera, Phasmida and Diptera. The ability of these insects to feed on wood, foliage and detritus has recently stimulated extensive investigation into the mechanisms of how these insects digest the structural and recalcitrant lignocellulose in their foods, as well as their potential to advance current biofuel technologies and processing. Recent studies using advanced molecular biotechnologies, such as metagenomics, proteomics, transcriptomics, and so on, have brought new insights into the mechanisms of biomass deconstruction within these small, but complicated insect gut systems. It has been reported that the digestion efficiency of wood-feeding termites is 74%–99% for cellulose and 65%–87% for hemicellulose, which mainly function via a collaboration between two catalyst systems: (i) termite endogenous catalyst systems, and (ii) catalysts from a variety of gut symbiotic microorganisms, including cellulolytic protozoa and bacteria. The number of the novel cellulases and hemicellulases, as well as the associated encoding genes from a variety of cellulose-feeding insects has been continuously updated in recent years. Descriptions of lignase enzymes and the genes that code them have been lacking; however, recent findings have suggested several viable candidates in both