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

Abstract

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 66h) 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/gds, 750IU/gds, 13.2IU/gds, 190IU/gds, 2890IU/gds and 0.9mg/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/gds, 100IU/gds, 2.3IU/gds, 50IU/gds, 500IU/gds and 0.75mg/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.