AEROBIC BACTERIAL TRANSFORMATIONS OF LIGNIN-DERIVED AROMATIC COMPOUNDS

Lignin, the most abundant aromatic polymer on earth, has been estimated to contribute ~20% of the total carbon deposited in nature and thus imparts a large influence on carbon cycling in the environment. The extraordinary abundance of carbon stored in this material renders it a desirable source of renewable carbon for a variety of applications including hydrocarbon fuels and industrial chemicals. Due to its incredibly stable architecture and entanglement with cell wall polysaccharides, however, efforts toward the conversion of lignin to high value commodities have historically been impeded. Despite this obstacle, many microbes in nature are capable of degrading lignin for use as a carbon and energy source. Microbial lignin depolymerization is typically initiated by the activity of extracellular peroxidases produced by fungi and a limited number of bacteria. This deconstruction liberates a pool of lower molecular weight aromatic compounds that can be subsequently catabolized by bacteria in the environment. The work presented here investigates the diverse reactions involved in the bacterial transformation of lignin-derived compounds, with a strong focus on conversions with potential to deliver valuable products. Insights into the bacterial transformation of ferulic acid, an abundant lignin-derived compound, are provided through mutagenesis studies with the model marine roseobacter strain, Sagittula stellata E-37. This study specifically interrogates the role of two annotated feruloyl-CoA synthase genes in the catabolism of ferulic acid. Results unveil the possible misannotation of genes and incite intrigue concerning substrate promiscuity across aromatic acyl-CoA synthases. Additional evidence is provided for the utilization and transformation of a pretreated organosolv lignin by another roseobacter species, Citreicella sp. SE45. This work highlights the potential application of lignolytic bacteria to upgrade residual lignin from a biorefinery. Finally, biotransformation studies with bacterial ring-hydroxylating dioxygenases present evidence for the transformation of lignin model compounds to a highly valuable cis-dihydrodiol intermediate that can be chemical converted to an array of synthetic chemicals. Collectively, this work provides an enhanced understanding of bacterial reactions with lignin-derived compounds and offers new insights and suggestions for continued studies in this field.