Evaluating the compatibility of solvents used in reductive lignin depolymerization with microbial funneling using Novosphingobium aromaticivorans

Abstract

The industrial revolution has resulted in the intensive use and dependence of fossil fuels and their products around the world. Fossil fuel emissions and the accumulation of carbon dioxide in the atmosphere are causing global climate change that has negative implications for the environment and our planet. Researchers and industries worldwide are developing technologies to make fuels and chemicals from lignocellulosic biomass to reduce fossil fuel dependance. Lignocellulosic biomass, a carbon-neutral and renewable feedstock, is composed of carbohydrate and aromatic polymers that can be used to produce clean fuels and value-added products, respectively. The conversion of carbohydrate components from lignocellulosic biomass into biofuels has been developed in a number of methods and processes, but the viability of these processes is limited by the need to produce large volumes of low-value biofuels. Hence, lignocellulosic biorefineries may be able to improve their viability and long-term sustainability if value-added products are made from lignin fractions. Despite the lack of economic viability at large scale to date, process pipelines have been proposed. One pipeline developed at the Great Lakes Bioenergy Research Center (GLBRC) includes the catalytic depolymerization of lignin into aromatic monomers using hydrogenolysis, followed by the biological transformation of the diverse set of aromatic monomers produced from the lignin into a single product using engineered strains of Novosphingobium aromaticivorans. Technoeconomic analyses of this pipeline have shown that using methanol as a solvent during hydrogenolysis leads to a process that requires costly reactors that are resistant to high pressure. It is therefore desirable to search for alternative solvents that allow the catalytic reactions to take place at lower pressures. The alternative solvents ethanol and ethylene glycol were investigated in this project, which focused on the biological process downstream of lignin depolymerization. To evaluate the effect of the solvent on the downstream microbial funneling step, the products of hydrogenolysis reactions performed with methanol, ethanol, or ethylene glycol (provided by Dr. Canan Sener) were used to evaluate the production of 2-pyrone-4,6-dicarboxylic acid (PDC) by an engineered strain of N. aromaticivorans. The PDC yield, per gram of biomass entering the hydrogenolysis process, was highest with ethanol, followed by ethylene glycol and methanol. In addition, the tolerance of N. aromaticivorans to the different solvents was also investigated. No inhibition to microbial growth was observed with ethylene glycol concentrations as high as 20% (by volume), whereas inhibition to microbial growth was observed at 2% with methanol and 1% with ethanol. These thresholds are pertinent to the use of N. aromaticivorans to produce value-added products with different lignocellulosic biomass streams.