The toxicity of model lignin monomers to Clostridium thermocellum metabolism

Deconstruction of lignocellulosic biomass in a consolidated bioprocessing platform creates the potential for the release of lignin-derived compounds that are detrimental to ethanol yield and titer. When using Clostridium thermocellum as a biocatalyst for ethanol production, these compounds have the potential to i) directly inhibit microbial growth; and/or ii) impede fermentation and ethanol formation. In this study, model aromatic compounds representing lignin degradation products with differing side-chain substitutions, length of aliphatic branching, and functional group chemistry have been investigated with regard to their toxic effects on C. thermocellum growth and metabolism. Phenolic aldehydes were more toxic than their corresponding alcohols, followed by the corresponding acids. Further, the extent of aliphatic branching associated with the aromatic ring significantly affects the level of toxicity towards growth. To begin elucidating the mechanisms for toxicity, cell-wide protein expression levels were determined after exposing C. thermocellum to the highly toxic compounds, coniferaldehyde and sinapaldehyde, along with uninhibited controls. Of the 1,561 proteins detected in C. thermocellum proteomes, 365 were differentially expressed (p≤0.05) with at least a 2-fold change in protein abundance. While stress and sporulation responses were common to both treatments, significant differences in physiological and protein abundance patterns were identified between the coniferaldehyde- and sinapaldehyde-exposed cells. In addition, distinct color changes in the medium were observed in the inhibitor-containing cultures suggesting transformation of the compounds is possible. The ability of phenolic compounds to be transformed by C. thermocellum will be presented.