Abstract
BackgroundLignin is a potential feedstock for microbial conversion into various chemicals. However, the microbial degradation rate of native or technical lignin is low, and chemical depolymerization is needed to obtain reasonable conversion rates. In the current study, nine bacterial strains belonging to the Pseudomonas and Rhodococcus genera were evaluated for their ability to grow on alkaline-treated softwood lignin as a sole carbon source.ResultsPseudomonas fluorescens DSM 50090 and Rhodococcus opacus DSM1069 showed the best growth of the tested species on plates with lignin. Further evaluation of P. fluorescens and R. opacus was made in liquid cultivations with depolymerized softwood Kraft lignin (DL) at a concentration of 1 g/L. Size-exclusion chromatography (SEC) showed that R. opacus consumed most of the available lower-molecular weight compounds (approximately 0.1–0.4 kDa) in the DL, but the weight distribution of larger fractions was almost unaffected. Importantly, the consumed compounds included guaiacol—one of the main monomers in the DL. SEC analysis of P. fluorescens culture broth, in contrast, did not show a large conversion of low-molecular weight compounds, and guaiacol remained unconsumed. However, a significant shift in molecular weight distribution towards lower average weights was seen after cultivation with P. fluorescens.ConclusionsRhodococcus opacus and P. fluorescens were identified as two potential microbial candidates for the conversion/consumption of base-catalyzed depolymerized lignin, acting on low- and high-molecular weight lignin fragments, respectively. These findings will be of relevance for designing bioconversion of softwood Kraft lignin.
Highlights
Lignin is one of the most abundant biopolymers on Earth and constitutes 18–32% [1] of woody biomass by weight and 40% by energy [2]
We explore the possibilities of utilizing depolymerized softwood Kraft lignin (Indulin AT) as a substrate for bacterial conversion
Bacterial screening on agar plates with depolymerized lignin To identify bacteria able to grow on depolymerized lignin (DL), growth on plates were tested for nine bacteria previously known to metabolize aromatic compounds or isolated on lignin-media [17, 29, 30, 35, 36]
Summary
Lignin is one of the most abundant biopolymers on Earth and constitutes 18–32% [1] of woody biomass by weight and 40% by energy [2]. Most large-scale industrial facilities that exploit plant polysaccharides (e.g., from wood) have almost exclusively incinerated the co-produced lignin to generate heat and power for biomass conversion and/or product drying, and have not aimed for higher-value products [5]. The emergence of biorefineries, which predominately convert the carbohydrate part of cellulosic biomass into liquid fuels, will generate substantially more lignin than needed for process heat or for power generation to the operation, which will add to the lignin volumes already generated in the pulp and paper industry. Lignin is a potential feedstock for microbial conversion into various chemicals. The microbial degradation rate of native or technical lignin is low, and chemical depolymerization is needed to obtain reasonable conversion rates. Nine bacterial strains belonging to the Pseudomonas and Rhodococcus genera were evaluated for their ability to grow on alkaline-treated softwood lignin as a sole carbon source
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