The scarab gut: A potential bioreactor for bio-fuel production

Cellulose and hemicelluloses are the most prevalent sources of carbon in nature. Currently many approaches employ micro-organisms and their enzyme products to degrade plant feedstocks for production of bioenergy. Scarab larvae are one such model. They consume celluloses from a variety of sources including plant roots, soil organic matter and decaying wood, and are able to extract nutrients and energy from these sources. In this paper, we review the physicochemical properties of the scarab larval gut, the diversity and digestive role that microflora play in the scarab gut and discuss the potential for applying these digestive processes in bioreactors for improving bio-fuel production. Scarab larvae are characterised by their highly alkaline midgut which is dominated by serine proteinase enzymes, and a modified hindgut which harbors the majority of the intestinal microbiota under anaerobic conditions. Evidence suggests that digestion of recalcitrant organic matter in scarab larvae likely results from a combination of endogenous gut proteinases and cellulolytic enzymes produced by symbiotic micro-organisms. Most of the easily digestible proteins are mobilized and absorbed in the midgut by endogenous proteinases. The hindgut contents of scarab larvae are characterized by high concentrations of volatile fatty acids, the presence of fermenting bacteria, and typical anaerobic activities, such as methanogenesis. The hindgut typically contains a wide diversity of micro-organisms, some of which appear to be obligate symbionts with cellulolytic potential. As a result, the scarab larval gut can be regarded as a small bioreactor resembling the rumen of sheep or cattle, where solid food particles composed of cellulose, hemicellulose, pectin and polysaccharides are degraded through enzymatic and fermentation processes. Together these observations suggest scarab larvae have potential to assist the bio-fuel industry by providing new sources of (hemi)cellulolytic bacteria and bacterial (hemi)cellulolytic enzymes.