Response surface optimization for enhanced production of cellulases with improved functional characteristics by newly isolated Aspergillus niger HN-2

Fungi isolated from partially decayed wood log samples showing characteristic diversity for spore colour, colony morphology and arrangement of spores were assessed for cellulolytic enzyme production. Isolates showing a cellulolytic index of ≥2.0 were assayed for filter paper (FP) cellulase and β-glucosidase (BGL) production. Molecular characterization confirmed the identity of the selected cellulolytic isolate as a strain of Aspergillus niger (A. niger HN-2). Addition of 2 % (w/v) urea enhanced FP and BGL activity by about 20 and 60 %, respectively. Validation studies conducted at parameters (29 °C, pH 5.4, moisture content 72 % and 66 h) optimized through response surface methodology in a solid-state static tray fermentation resulted in FP, BGL, cellobiohydrolase I (CBHI), endoglucanase (EG), xylanase activity and protein content of 25.3 FPU/g ds, 750 IU/g ds, 13.2 IU/g ds, 190 IU/g ds, 2890 IU/g ds and 0.9 mg/ml, respectively. In comparison, A. niger N402 which is a model organism for growth and development studies, produced significantly lower FP, BGL, CBHI, EG, xylanase activity and protein content of 10.0 FPU/g ds, 100 IU/g ds, 2.3 IU/g ds, 50 IU/g ds, 500 IU/g ds and 0.75 mg/ml, respectively under the same process conditions as were used for A. niger HN-2. Process optimization led to nearly 1.8- and 2.2-fold increase in FP and BGL activity, respectively showing promise for cellulase production by A. niger HN-2 at a higher scale of operation. Zymogram analysis revealed two isoforms each for EG and cellobiohydrolase and three isoforms for BGL. Crude cellulase complex produced by A. niger HN-2 exhibited thermostability under acidic conditions showing potential for use in biofuel industry.