Elucidating Thermothielavioides terrestris secretome changes for improved saccharification of mild steam-pretreated spruce

Background The efficient use of softwood in biorefineries is hampered by its recalcitrance to enzymatic saccharification. In the present study, the fungus Thermothielavioides terrestris LPH172 was cultivated on three steam-pretreated spruce materials (STEX180 degrees C/auto, STEX210 degrees C/auto, and STEX210 degrees C/H2SO4), characterized by different hemicellulose content and structure, as well as on untreated biomass. The aim of the study was to map substrate-induced changes in the secretome of T. terrestris grown on differently treated spruce materials and to evaluate the hydrolytic efficiency of the secretome as supplement for a commercial enzyme mixture. Results The cultivation of T. terrestris was monitored by endo-cellulase, endo-xylanase, endo-mannanase, laccase, and peroxidase activity measurements. Proteomic analysis was performed on the secretomes induced by the spruce materials to map the differences in enzyme production. Growth of T. terrestris on STEX180 degrees C/auto and STEX210 degrees C/auto induced higher expression level of mannanases and mannosidases of the GH5_7 CAZy family compared to cultivation on the other materials. Cultivation on untreated biomass led to overexpression of GH47, GH76, and several hemicellulose debranching enzymes compared to the cultivation on the pretreated materials. T. terrestris grown on untreated, STEX180 degrees C/auto and STEX210 degrees C/auto induced three arabinofuranosidases of the GH43 and GH62 families; while growth on STEX210 degrees C/H2SO4 induced a GH51 arabinofuranosidase and a GH115 glucuronidase. All secretomes contained five lytic polysaccharide monooxygenases of the AA9 family. Supplementation of Celluclast (R) + Novozym188 with the secretome obtained by growing the fungus grown on STEX180 degrees C/auto achieved a twofold higher release of mannose from spruce steam-pretreated with acetic acid as catalyst, compared to the commercial enzyme cocktail alone. Conclusions Minor changes in the structure and composition of spruce affect the composition of fungal secretomes, with differences in some classes explaining an increased hydrolytic efficiency. As demonstrated here, saccharification of spruce biomass with commercial enzyme cocktails can be further enhanced by supplementation with tailor-made secretomes.