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

Extracellular beta-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 degrees C for Gilbertella and Mucor , and 30 degrees 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 degrees 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 beta-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.