Lignocellulosic biomass to biobutanol: Toxic effects and response mechanism of the combined stress of lignin-derived phenolic acids and phenolic aldehydes to Clostridium acetobutylicum

Lignocellulosic biomass is the most abundant bioresource to consider as an alternative feedstock to produce environmentally friendlier biobutanol as an advanced biofuel. However, the pretreatment for lignocellulose depolymerization suffers from toxic inhibitors formation and wastewater generation. The phenolic compounds, including phenolic aldehydes (Phe-Ad) and phenolic acids (Phe-Ac), derived from lignin degradation during the acid pretreatment of lignocellulose such as corn stover, are the main inhibitors for butanol fermentation. However, the overall impacts of the co-existence of Phe-Ad and Phe-Ac (Com-Phe) on solventogenic clostridia metabolism were not investigated thus far. Here, toxic effects of Com-Phe on butanol fermentation were firstly evaluated by analyzing apparent kinetics, reducing power, and reactive oxygen species (ROS). The lower intracellular reducing power level and higher ROS result in a 56.3 % decrease of butanol titer after 12 h stress. Subsequently, the response mechanism of Clostridium acetobutylicum to Com-Phe stress was elucidated by RNA-seq focusing on physiological processes with carbon metabolism and product biosynthesis. Some key genes related to heat shock proteins (HSP) and antioxidative activity were up-regulated under Com-Phe stress, which might be an adaptive mechanism of cells to alleviate its toxicity. Phenolics also altered two-component systems, sporulation, and cell division processes. This is the first report on systematic evaluation of the toxicity of Com-Phe on solventogenic clostridia. The proposed response mechanism provides valuable guidance (such as regulation of sugar transport, HSP-related genes, glutathione metabolism, etc.) for developing robust strains and promotes cleaner biobutanol production from lignocellulosic biomass.