Fate of Lignin and Substituted Xylan during Commercial Cultivation of Agaricus bisporus

Abstract

Wheat straw based compost is the substrate for commercial growth of Agaricus bisporusmushrooms, but it is unknown to which extent the carbohydrate-lignin matrix changes and howmuch is metabolized during commercial cultivation. In this paper we report yields and remainingstructures of the major compost carbohydrates and lignin. Hereto, a mass balance was conducted ina tunnel-experiment at industrial scale, and total dry matter, cellulose, hemicellulosic xylan andlignin were quantified (Jurak et al., 2015a). Remaining (substituted) xylan structures were extractedand analysed in detail by enzymatic fingerprinting with use of HPAEC and MALDI-TOF MS.Remaining lignin structures were subjected to analytical pyrolysis GC/MS and typical lignin unitswere determined. In addition, to determine why certain fractions remain unused in the compost,activities of water extracted carbohydrate degrading enzymes were analysed for their ability todegrade a range of polysaccharides (Jurak et al., 2015b,c). During growth of A. bisporus,carbohydrates were only slightly consumed and xylan was partially degraded. At the same time,lignin was metabolized for 45% based on pyrolysis GC/MS. Remaining lignin was modified, asobserved by an increase in the ratio of syringyl (S) to guaiacyl (G) units from 0.5 to 0.7 duringgrowth, while fewer decorations on the phenolic skeleton of both S and G units remained (Jurak etal., 2015a). During mushroom formation, mainly endo-xylanase, endo-glucanase, β-xylosidase andβ-glucosidase activities were determined in the compost extracts. Arabinofuranosidase activity ableto remove arabinosyl residues from doubly substituted xylosyl residues nor α-glucuronidase activitywere detected (Jurak et al., 2015b). The latter correlated with the observed accumulation of xylanfragments substituted with arabinosyl and glucuronic acid substituents in the compost towards theend of the cultivation (Jurak et al., 2015c). Hence, it was concluded that compost grown A. bisporuslacks the ability to degrade and consume highly substituted xylan fragments