Abstract
AbstractLignin is the second most abundant carbon polymer on earth and despite having more fuel value than cellulose, it currently is considered a waste byproduct in many industrial lignocellulose applications. Valorization of lignin relies on effective and green methods of delignification, with a growing interest in the use of microbes. Here we investigate the physiology and lignin biotransformation mechanisms of the novel facultative anaerobic bacterium, Tolumonas lignolytica BRL6-1, under anoxic conditions. Physiological and biochemical changes were compared between cells grown anaerobically in either lignin-amended or unamended conditions. In the presence of lignin, BRL6-1 had a higher biomass and shorter lag phase compared to unamended conditions, and 14% of the proteins determined to be significantly higher in abundance by log2 fold-change of 2 or greater were related to Fe(II) transport in early exponential phase. Ferrozine assays of the supernatant (<10 kDa fraction) confirmed that Fe(III) was bound to lignin and reduced to Fe(II) only in the presence of BRL6-1, suggesting redox activity by the cells. LC-MS/MS analysis of the secretome showed an extra band at 20 kDa in lignin-amended conditions. Protein sequencing of this band identified a protein of unknown function with homology to enzymes in the radical SAM superfamily. Expression of this protein in lignin-amended conditions suggests its role in radical formation. From our findings, we suggest that BRL6-1 is using a protein in the radical SAM superfamily to interact with the Fe(III) bound to lignin and reducing it to Fe(II) for cellular use, increasing BRL6-1 yield under lignin-amended conditions. This interaction potentially generates organic free radicals and causes a radical cascade which could modify and depolymerize lignin. Further research should clarify the extent to which this mechanism is similar to previously described aerobic chelator-mediated Fenton chemistry or radical producing lignolytic enzymes, such as lignin peroxidases, but under anoxic conditions.
Publisher
Cold Spring Harbor Laboratory
Reference55 articles.
1. Lignin-degrading enzymes;FEBS Journal,2015
2. Towards lignin consolidated bioprocessing: simultaneous lignin depolymerization and product generation by bacteria
3. Promising Techniques for Depolymerization of Lignin into Value-added Chemicals;ChemCatChem,2019
4. Mapping the diversity of microbial lignin catabolism: experiences from the eLignin database
5. Holladay JE , White JF , Bozell JJ , Johnson D. Top Value-Added Chemicals from Biomass - Volume II—Results of Screening for Potential Candidates from Biorefinery Lignin [Internet]. 2007 Oct [cited 2019 Jul 29] p. PNNL-16983, 921839. Report No.: PNNL-16983, 921839. Available from: http://www.ost.govMservletsMpurlM921839M