Fungal Treatment for the Valorization of Technical Soda Lignin

Abstract

Technical lignins produced as a by-product in biorefinery processes represent a potential source of renewable carbon. In consideration of the possibilities of the industrial transformation of this substrate into various valuable bio-based molecules, the biological deconstruction of a technical soda lignin by filamentous fungi was investigated. The ability of three basidiomycetes (Polyporus brumalis, Pycnoporus sanguineus and Leiotrametes menziesii) to modify this material, the resultant structural and chemical changes, and the secreted proteins during growth on this substrate were investigated. The three fungi could grow on the technical lignin alone, and the growth rate increased when the media were supplemented with glucose or maltose. The proteomic analysis of the culture supernatants after three days of growth revealed the secretion of numerous Carbohydrate-Active Enzymes (CAZymes). The secretomic profiles varied widely between the strains and the presence of technical lignin alone triggered the early secretion of many lignin-acting oxidoreductases. The secretomes were notably rich in glycoside hydrolases and H2O2-producing auxiliary activity enzymes with copper radical oxidases being induced on lignin for all strains. The lignin treatment by fungi modified both the soluble and insoluble lignin fractions. A significant decrease in the amount of soluble higher molar mass compounds was observed in the case of P. sanguineus. This strain was also responsible for the modification of the lower molar mass compounds of the lignin insoluble fraction and a 40% decrease in the thioacidolysis yield. The similarity in the activities of P. sanguineus and P. brumalis in modifying the functional groups of the technical lignin were observed, the results suggest that the lignin has undergone structural changes, or at least changes in its composition, and pave the route for the utilization of filamentous fungi to functionalize technical lignins and produce the enzymes of interest for biorefinery applications.

A Putative Lignin Copper Oxidase from Trichoderma reesei

“…The soda technical lignin (Protobind 1000) used in this study was produced from a wheat straw and Sarkanda grass mix and purchased from GreenValue Enterprises LLC (Media, PA, USA). The sample was analyzed for Klason lignin (88.1%), carbohydrates (1.9%, of which 1.2% xylose, 0.3% arabinose, 0.1% galactose and 0.2% glucose), free phenolic monomers (1.4%) and ash contents (1.4%) [ 42 ].…”

“…From agar slants, T. reseei was transferred and grown on potato dextrose agar plates (Sigma-Aldrich, St. Louis, MO, USA). Cultures on minimal media plates containing 10 g/L technical soda lignin and in submerged cultures containing 100 mL of lignin-containing media (10 g/L technical soda lignin) were performed as previously described [ 42 ]. Control conditions were supplemented with 1 g/L glucose and 2.5 g/L maltose for plate and liquid cultures, respectively.…”

“…Fungal growth on plates was followed by measuring the radial expansion of the fungus (cm/day). Growth in liquid cultures was followed by quantifying the fungal DNA material and relating it to the mycelial dry weight as previously described [ 42 , 45 ].…”

“…During the published study on three basidiomycetes grown on a technical soda lignin, Protobind 1000, a number of enzymes were identified in the secretomes specific to growth of the fungi on lignin alone, including CROs [ 42 ]. As part of the same study, the ascomycete Trichoderma reesei was also examined, initially as an outlier as it is known as a cellulolytic fungus rather than a lignocellulolytic one [ 43 ].…”

MeRAV5 promotes drought stress resistance in cassava by modulating hydrogen peroxide and lignin accumulation

“…Lignin is a complex polymer with high molecular weight and low solubility (Boerjan et al ., 2003). Recently, technical soda lignin was added to liquid cultures to investigate its effect on fungal growth (Daou et al ., 2021). Similarly, pretreatment with soluble lignin alkali demonstrated the effect of lignin in cassava drought stress resistance.…”