Exploring the potential of Rhodopseudomonas palustris for the utilization of lignin-derived carbon sources

Lignin-derived products are common by-products of several industrial processes and are often used for low-value energy recovery through combustion. Their microbial conversion could represent a more sustainable strategy to obtain higher-value products; however, lignin recalcitrance and heterogeneity make its efficient degradation and valorization challenging. Rhodopseudomonas palustris is a metabolically versatile bacterium of interest for lignin valorization because of its ability to grow on lignin-derived aromatic compounds and lignocellulosic hydrolysates. This thesis aimed to investigate the utilization of selected lignin-derived carbon sources by R. palustris, focusing on guaiacyl-related aromatic compounds and Kraft lignin. Vanillin, vanillic acid, vanillyl alcohol and protocatechuate were supplied at increasing carbon concentrations to evaluate the effects of substrate type and concentration on bacterial growth and substrate utilization. Targeted bioinformatic analyses were also performed to identify candidate enzymes potentially related to lignin depolymerization and vanillyl alcohol–vanillin interconversion. Bacterial growth was evaluated under aerobic and anaerobic batch and semicontinuous conditions. The response to aromatic monomers depended on substrate type, concentration and oxygen availability: 1.00 gC L⁻¹ was generally the most favorable aerobic concentration, while 1.56 gC L⁻¹ reduced biomass formation, suggesting a possible concentration-dependent inhibitory effect. Protocatechuate and vanillyl alcohol showed the strongest responses, whereas soluble organic carbon removal was not always associated with proportional biomass formation. Semicontinuous cultivation supported culture maintenance with selected monomers and Kraft lignin, further supporting the ability of R. palustris to interact with lignin-derived carbon sources.