Screening of Phytophagous and Xylophagous Insects Guts Microbiota Abilities to Degrade Lignocellulose in Bioreactor

Microbial consortia producing specific enzymatic cocktails are present in the gut of phytophagous and xylophagous insects; they are known to be the most efficient ecosystems to degrade lignocellulose. Here, the ability of these consortia to degrade ex-vivo lignocellulosic biomassin anaerobic bioreactors was characterized in term of bioprocessperformances, enzymatic activities and bacterial community structure. In a preliminary screening, guts of Ergates faber (beetle), Potosiacuprea (chafer), Gromphadorrhina portentosa ( cockroach), Locusta migratoria ( locust), and Gryllus bimaculatus(cricket) were inoculated in anaerobic batch reactors, in presence of grounded wheat straw at neutral pH. A short duration fermentation of less than 8 days was observed and was related to a drop of pH from 7 to below 4.5, leading to an interruption of gas and metabolites production. Consistently, a maximum of 180mgeq.COD of metabolites accumulated in the medium, which was related to a low degradation of the lignocellulosic biomass, with a maximum of 5 and 2.2% observed for chafer and locustgut consortia. The initial cell-bound and extracellular enzyme activities, i.e. xylanaseand β-endoglucanase, were similar to values observed in the literature. Wheat straw fermentation in bioreactors leads to an increase of cell-bounded enzyme activities, with an increase of 145% for cockroach xylanaseactivity. Bacterial community structures were insect dependent and mainly composed of Clostridia, Bacteroidiaand Gammaproteobacteria. Improvement of lignocellulose biodegradation was realized in successive batch mode at pH 8 using the most interesting consortia, i.e. locust, cockroachesand chafer gut consortia. In these conditions, lignocellulose degradation increased significantly: 8.4, 10.5 and 21.0% of the initial COD was degraded for chafer, cockroachesand locusts, respectively in 15 days. Consistently, xylanaseactivity tripled for the three consortia, attesting the improvement of the process. Bacteroidiawas the major bacterial class represented in the bacterial community for all consortia, followed by Clostridiaand Gammaproteobacteriaclasses. This work demonstrates the possibility to maintain apart of insect gut biological activity ex-vivo and shows that lignocellulose biodegradation can be improved by using a biomimetic approach. These results bring new insights for the optimisation of lignocellulose degradation in bioreactors.