Production and Optimization of Fungal Silicase Using Rice Straw under Solid State Fermentation

Abstract

Aim: The present study aimed to convert agricultural residue, specifically rice straw, into a valuable enzymatic product through solid-state fermentation (SSF), using three fungal strains—Penicillium limosum (WF1), Bipolaris sorokiniana (BF1), and Pleurotus ostreatus (P1). The objective was to optimize the production of the silica-degrading enzyme silicase, which catalyzes the transformation of silica into plant-available silicic acid. Study Area: Rice straw used in this study was sourced from agricultural fields in Haryana, India. The experimental work was carried out at the Department of Biotechnology, Kurukshetra University, Kurukshetra. Study Design and Methodology: A one-variable-at-a-time (OVAT) strategy was applied to optimize process variables such as substrate form (crude vs. powdered), incubation period, temperature, pH, rice straw-to-water ratio, and the use of nutrient additives. Silicase production was compared between submerged fermentation (SmF) and solid-state fermentation (SSF). Under SSF, the optimized medium consisted of crude rice straw with a 1:5 straw-to-water ratio, incubated at 30 °C and pH 7 for 8 days. Silicase activity was measured spectrophotometrically using a silicate assay kit. Results: Solid-state fermentation significantly outperformed submerged fermentation for enzyme production. Under optimal SSF conditions, silicase activity reached 0.98 U/mL/min for WF1, 0.96 U/mL/min for BF1, 1.00 U/mL/min for P1, and 0.99 U/mL/min for the fungal consortium (WF1+BF1+P1). Supplementation with jaggery and soy protein further enhanced enzyme yield. Crude rice straw supported higher enzyme activity than powdered forms, and incubation beyond 8 days or deviations in pH and temperature resulted in decreased activity. Conclusion: The study demonstrates that rice straw can be efficiently bioconverted into a high-value enzymatic product using solid-state fermentation with silicate-solubilizing fungi. The optimized SSF system, supplemented with cost-effective nutrient additives, provides an eco-friendly and scalable strategy for silicase production. These findings have potential applications in sustainable agriculture, silica bioremediation, and industrial processing of silica-rich biomass.