Lignocellulose-Adapted Endo-Cellulase Producing Streptomyces Strains for Bioconversion of Cellulose-Based Materials

Abstract

Twenty-four <i>Actinobacteria</i> strains, isolated from <i>Arundo donax, Eucalyptus camaldulensis</i> and <i>Populus nigra</i> biomass during natural biodegradation and with potential enzymatic activities specific for the degradation of lignocellulosic materials, were identified by a polyphasic approach. All strains belonged to the genus <i>Streptomyces</i> (<i>S</i>.) and in particular, the most highly represented species was <i>Streptomyces argenteolus</i> representing 50% of strains, while 8 strains were identified as <i>Streptomyces flavogriseus</i> (synonym <i>S. flavovirens</i>) and <i>Streptomyces fimicarius</i> (synonyms <i>Streptomyces acrimycini, Streptomyces baarnensis, Streptomyces caviscabies</i>, and <i>Streptomyces flavofuscus</i>), and the other four strains belonged to the species <i>Streptomyces drozdowiczii, Streptomyces rubrogriseus, Streptomyces albolongus</i>, and <i>Streptomyces ambofaciens</i>. Moreover, all <i>Streptomyces</i> strains, tested for endo and exo-cellulase, cellobiase, xylanase, pectinase, ligninase, peroxidase, and laccase activities using qualitative and semi-quantitative methods on solid growth medium, exhibited multiple enzymatic activities (from three to six). The 24 strains were further screened for endo-cellulase activity in liquid growth medium and the four best endo-cellulase producers (<i>S. argenteolus</i> AE58P, <i>S. argenteolus</i> AE710A, <i>S. argenteolus</i> AE82P, and <i>S. argenteolus</i> AP51A) were subjected to partial characterization and their enzymatic crude extracts adopted to perform saccharification experiments on <i>A. donax</i> pretreated biomass. The degree of cellulose and xylan hydrolysis was evaluated by determining the kinetics of glucose and xylose release during 72 h incubation at 50°C from the pretreated biomass in the presence of cellulose degrading enzymes (cellulase and β-glucosidase) and xylan related activities (xylanase and β-xylosidase). The experiments were carried out utilizing the endo-cellulase activities from the selected <i>S. argenteolus</i> strains supplemented with commercial β-gucosidase and xylanase preparations from Genencore (Accellerase BG and Accellerase XY). Cellulose and xylan conversion, when conducted using commercial (hemi)cellulases, gave glucose and xylose yields of 30.17 and 68.9%, respectively. The replacement of the cellulolytic preparation from Genencor (Accellerase 1500), with the endo-cellulase from <i>S. argenteolus</i> AE58P resulted in almost 76% of the glucose yield obtained in the presence of the commercial counterpart. Due to the promising results obtained by using the enzymatic crude extracts from <i>S. argenteolus</i> AE58P in the pretreated <i>A. donax</i> saccharification experiments, the proteins putatively responsible for endo-cellulase activity in this strain were identified by proteomics. Several proteins were confidently identified in different <i>Streptomyces</i> spp., eight of which belong to the class of Carbohydrate active enzymes. Overall results highlighted the biotechnological potential of <i>S. argenteolus</i> AE58P being an interesting candidate biocatalyst-producing bacterium for lignocellulose conversion and production of biochemicals and bioenergy.