In current biorefineries, lignin is regarded as a by-product with little added value, whereas it could be considered as a renewable source of aromatic compounds for the production of biofuels and chemical compounds. The development of efficient lignin deconstruction systems would enable existing biorefinery processes to be improved and lignin to be valorized. Among lignin conversion routes, bioconversion by microbial species offers the advantage of being part of a green chemistry approach: low energy consumption, absence of solvents and toxic chemical reagents. As part of Louison Dumond's thesis, we highlighted the potential of microbial consortia derived from the digestive system of termites to degrade lignin. To achieve this, we combined bioreactor studies with cutting-edge analytical chemistry techniques and shotgun metagenomic and functional screening analyses. Our results highlighted the functional complementarity between the microbial species present in the microbial consortia and identified genetic targets involved in lignin deconstruction.
Background and challenges
Today, our societies are aware that fossil resources such as oil, natural gas and coal are finite, and that their intensive use has a major impact on the climate. This has led us to look for new ways to supply industry with energy and basic molecules and chemicals. In this context, lignocellulosic biomass can be a notable source. Of the polymers making up lignocellulose, lignin is undoubtedly the most difficult to convert into valuable products. However, the range of aromatic molecules that can be obtained by converting lignin makes setting up a lignin biorefinery worth the challenge of degrading it.
Termites are known to be the most efficient lignocellulose degraders in nature, feeding on lignocellulose polysaccharides and converting them into energy and termite biomass. Nevertheless, the exact role of the termite gut microbiome in lignin conversion has not been fully characterized. We therefore set out to investigate the ability of the termite digestive microbiome to degrade lignin.
Our studies in controlled bioreactors, combined with cutting-edge analytical chemistry techniques (HSQC NMR and 13C-Py-GC-MS), revealed the degradation of over 20% of the lignin present in the initial biomass, as well as structural modifications to the polymer with the appearance of metabolites indicative of lignin degradation. Shotgun metagenomic sequencing of the entire DNA of the microbial consortium enriched in the bioreactors has enabled us to reconstruct the genomes of the species present in the lignolytic microbial consortium and to identify the functional complementarities existing between the members of the community. Our results provide new insights into the underlying cooperation between bacteria to degrade lignocellulose and lignin.
Analysis of sequencing data coupled with functional screening metagenomics and enzyme activity measurements enabled us to identify genes encoding functions linked to plant wall deconstruction.
Our research has yielded microbial consortia capable of degrading lignin and new sequencing data for screening new enzymes of use to the biorefinery.
These results open up broad prospects for the use of microbial consortia and their enzymes to modify and degrade lignin. We will complete the analysis of our sequencing data to identify new enzymes of interest for the biorefinery.