Valorization of Cotton Seed Hulls as a Potential Feedstock for the Production of Thermostable and Alkali-Tolerant Bacterial Xylanase

Herein, the potential of cotton seed hulls (CSH) as prospective feedstock for producing thermostable and alkaliphilic xylanase by Bacillus pumilus and Bacillus licheniformis during submerged fermentation was demonstrated. Results revealed that CSH predominantly contains holo-cellulose (65.6%), alpha-cellulose (38.8%), hemicelluloses (24.5%), and lignin (18.5%). Compared to the control, 0.5% CSH-supplemented growth media better supported bacterial growth (OD600 nm). Moreover, the extracellular xylanase production increased from 16 h and peaked at 24 h for both B. pumilus (359.2 U/mL) and B. licheniformis (360.8 U/mL). Bacterial xylanase demonstrated considerable stability at a higher temperature (55 °C). Likewise, significant xylanase activity was detected at pH 7.0. However, the enzyme also showed remarkable activity at alkaline pH (At pH 9.0, 84.61%, and 81.89% activity for B. pumilus and B. licheniformis, respectively), indicating alkali tolerance nature. These xylanolytic bacteria potentially depolymerized lignocellulosic fraction with a considerable release of fermentable monosaccharides (1175.10 ± 5.69 and 1278 ± 2.22 ug/mL for B. pumilus and B. lichenformis treated CSH, respectively). The scanning electron micrograph revealed the surface smoothness, cleanliness, and presence of cracks/grooves on xylanase-treated CSH surfaces. FTIR studies confirmed the stretching (at 3318 cm−1 corresponding to the O–H peak of lignin and hemicelluloses) and deformation of inter and intermolecular bonds (at 2950–2850 cm−1 related to aromatic C–H bonds present in cellulose, hemicellulose, and lignin) of lignocellulosic components of CSH. Briefly, the study highlighted the potentiality of CSH as an alternative fermentative substrate for the biosynthesis of xylanase in a biorefinery approach.