Termite Gut Microbiota Contribution to Wheat Straw Delignification in Anaerobic Bioreactors

Lignin is a major lock for lignocellulose valorization in biorefineries, prompting a need to find new ligninolytic systems. Termites are efficient lignocellulose degraders, and a large part of this ability comes from its anaerobic gut microbiome. However, the potential of termite gut microbiomes to degrade lignin under anaerobic conditions has yet to be elucidated. By applying wet chemistry, multidimensional NMR spectroscopy, and quantitative 13C-IS py-GC-MS, we determined the chemical and structural characteristics of wheat straw digested by the gut microbiomes of higher termites Nasutitermes ephratae, Nasutitermes lujae, Microcerotermes parvus, and Termes hospes implemented in anaerobic bioreactors. Interestingly, all gut microbiomes managed to remove lignin (up to 37%), although hemicellulose (mean 51%) and cellulose (mean 41%) were degraded more efficiently. Important structural differences, indicative of ligninolytic action, were discerned in the residual lignin. For the studied termite gut microbiomes, a slight decrease of S/G ratio was observed upon digestion, whereas lignin's interunit linkages and Cα-oxidized moieties, including benzaldehyde and hydroxypropiovanillone/syringone substructures, accumulated. Additionally, tricin terminal units were clearly depleted, along with decreases in p-coumarate and ferulate pendant units. At least 80% of the observed delignification is estimated to be due to the removal of "true"lignin, with the remainder being explained by the removal of hydroxycinnamic acids and tricin. Collectively, our findings suggest a partial deconstruction of lignin, with "peripheral"lignin subunits being preferentially targeted. The present work thus provides new insights into the anaerobic deconstruction of lignin by termite gut microbiomes.