Characterization of the Proteolytic Activity of a Halophilic Aspergillus reticulatus Strain SK1-1 Isolated from a Solar Saltern

Abstract

Salterns are hypersaline environments that are inhabited by diverse halophilic microorganisms, including fungi. In this study, we isolated a fungal strain SK1-1 from a saltern in the Republic of Korea, which was identified as <i>Asperillus reticulatus</i>. This is the first reported saline-environment-derived <i>A. reticulatus</i> that belongs to the <i>Aspergillus penicillioides</i> clade and encompasses xerophilic fungi. SK1-1 was halophilic, obligately requiring NaCl for growth, with a maximum radial growth of 6%-9% (<i>w</i>/<i>v</i>) NaCl. To facilitate the biotechnological application of halophilic fungi, we screened the SK1-1 strain for proteolytic activity. Proteases have widespread applications in food processing, detergents, textiles, and waste treatment, and halophilic proteases can enable protein degradation in high salt environments. We assessed the proteolytic activity of the extracellular crude enzyme of SK1-1 using azocasein as a substrate. The crude protease exhibited maximum activity at 40-50 °C, pH 9.5-10.5, and in the absence of NaCl. It was also able to retain up to 69% of its maximum activity until 7% NaCl. Protease inhibitor assays showed complete inhibition of the proteolytic activity of crude enzymes by Pefabloc^® SC. Our data suggest that the halophilic <i>A. reticulatus</i> strain SK1-1 produces an extracellular alkaline serine protease.