Fate of lignin and substituted xylan during commercial cultivation of Agaricus bisporus

Wheat straw based compost is the substrate for commercial growth of Agaricus bisporus mushrooms, but it is unknown to which extent the carbohydrate-lignin matrix changes and how much is metabolized during commercial cultivation. In this paper we report yields and remaining structures of the major compost carbohydrates and lignin. Hereto, a mass balance was conducted in a tunnel-experiment at industrial scale, and total dry matter, cellulose, hemicellulosic xylan and lignin were quantified (Jurak et al., 2015a). Remaining (substituted) xylan structures were extracted and 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 units were 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 to degrade 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, as observed by an increase in the ratio of syringyl (S) to guaiacyl (G) units from 0.5 to 0.7 during growth, while fewer decorations on the phenolic skeleton of both S and G units remained (Jurak et al., 2015a). During mushroom formation, mainly endo-xylanase, endo-glucanase, β-xylosidase and β-glucosidase activities were determined in the compost extracts. Arabinofuranosidase activity able to remove arabinosyl residues from doubly substituted xylosyl residues nor α-glucuronidase activity were detected (Jurak et al., 2015b). The latter correlated with the observed accumulation of xylan fragments substituted with arabinosyl and glucuronic acid substituents in the compost towards the end of the cultivation (Jurak et al., 2015c). Hence, it was concluded that compost grown A. bisporus lacks the ability to degrade and consume highly substituted xylan fragments