How mushrooms feed on compost: conversion of carbohydrates and linin in industrial wheat straw based compost enabling the growth of Agaricus bisporus

Abstract In this thesis, the fate of carbohydrates and lignin was studied in industrial wheat straw based compost during composting and growth of Agaricus bisporus. The aim was to understand the availability and degradability of carbohydrates in order to help improve their utilization in the compost. The wheat straw based compost was characterized as being composed mainly of cellulose and lowly substituted xylan. During the first phase of composting, ester-bound substituents were removed from the xylan backbone and during the second phase of composting 50% of carbohydrates present in the original material where metabolized in a uniform manner. Lignin structure, however, remained unaltered during these composting stages. Over the period of A. bisporus mycelium growth, 20% of the original xylan became water soluble while xylan structures remained rather similar and the remaining water insoluble xylan was partially degraded. In addition, 40% of lignin was metabolized during mycelium growth with an increase in the ratio of syringyl to guaiacyl lignin units from 0.5 to 0.7 in mycelium grown compost compared to the basic compost mixture. During the fruiting body formation minor changes in lignin structure occurred, while accumulation of xylan substituents was observed for arabinosyl residues and glucuronic acid substituents. Finally, putative genes encoding carbohydrate degrading enzymes were identified in A. bisporus’ genome. Genes involved in the pentose and hexose catabolic pathway were found to be upregulated in A. bisporus mycelium. A. bisporus was found to produce both xylan and cellulose degrading enzymes and maximum activity was observed during the formation of the 1st flush of mushrooms. But, as observed from the remaining xylan structures analyzed, A. bisporus lacks the enzymatic activity to degrade xylan substituted with two arabinosyl- residues and glucuronic acid substituted xylan. Edita Jurak How Mushrooms feed on compost: Conversion of carbohydrates and linin in industrial wheat straw based compost enabling the growth of Agaricus bisporus