A green process for lignin extraction and lignocellulose degrading enzyme production from rice straw by solid state fermentation with Streptomyces thermoviolaceous strains

Actinobacteria belonging to genus Streptomyces are a versatile group actively involved in global C cycle with abilities to degrade several recalcitrant substrates. Inhabiting diverse ecological niches, they are active in different pH and temperature regimes thus a source of robust enzymes for exploitation in bioprocessing. Compost is one such habitat supporting huge microbial diversity, lignocellulolytic actinobacteria being predominant in community. In this study, two actinobacterial strains isolated from compost through enrichment culture, identified as Streptomyces thermoviolaceous S1 and S2, showed lignocellulolose degrading enzyme production. When grown on rice straw under solid state fermentation they disrupted lignocellulose matrix. Structural changes in solid substrate were observed by non-invasive techniques SEM, XRD and FTIR. Alkali extraction of fermented solids removed about ∼ 33 % lignin from rice straw while buffer extracts showed high specific activities of all three components of cellulases, xylanase (84 IU/ mL), laccase (59 IU/ mL) and lignin peroxidase (26 IU/ mL).

S. thermoviolaceous S2 showed better enzyme activities, lignin removal and cellulose enrichment than S1 (53.03 % and 49.01 % by S. thermoviolaceous S2 and S1 respectively). Alkali extraction led to efficient lignin removal than buffer extraction as evident from higher absorbance of alkali extracts (@205 nm) which was corroborated by higher recovery of acid precipitable lignin. Better cellulose enrichment enabled higher glucan loading and higher sugar yields upon enzymatic saccharification than uninoculated substrate. This study outlined a green like process involving biological treatment of rice straw with S. thermoviolaceous strains for delignification, lignin recovery and simultaneous lignocellulose degrading enzyme production for biomass processing.