Identification of acid- and thermotolerant extracellular β-glucosidase activities in Zygomycetes fungi

Extracellular β-glucosidase activity of 94 strains, representing 24 species of the genera Gilbertella, Mucor, Rhizomucor, and Rhizopus was evaluated in submerged culture and under solid state fermentation on wheat bran. Gilbertella persicaria G1 isolate showed the highest activity (70.9 U ml–1) followed by other Gilbertella (58.6–59.0 U ml–1) and Rhizomucor miehei isolates (29.2–42.0 U ml–1). Optimum temperature for enzyme production was 25 °C for Gilbertella and Mucor, and 30 °C for Rhizomucor and Rhizopus strains. Enzymes of R. miehei strains proved to be thermotolerant preserving up to 92.8% residual activity after heating to 75 °C in the presence of cellobiose substrate. Enzymes of Mucor racemosus f. chibinensis, R. miehei and Rhizopus microsporus var. oligosporus strains were activated at acidic condition (pH 4). Glucose was a strong inhibitor for each fungal β-glucosidase tested but some of them showed ethanol tolerance up to 20% (v/v). Ethanol also activated the enzyme in these strains suggesting glycosyl transferase activity