Improved saccharification of Chlorella vulgaris biomass by fungal secreted enzymes for bioethanol production

Abstract

Microalgal carbohydrates can serve as a potential source of bioethanol after saccharification and fermentation. This study aimed to assess the utility of fungal crude enzyme extracts isolated from Penicillium chrysogenum, Aspergillus tubingensis, Aspergillus fumigatus, and Thermomyces lanuginosus in the degradation of intact or lipid-extracted Chlorella vulgaris biomass. All fungal extracts were able to release higher concentrations of sugars from lipid-extracted biomass than intact biomass. Notably, A. fumigatus crude enzyme extract produced the most efficient saccharification reaction at 37 °C, which released 67% and 94% of C. vulgaris carbohydrates from intact and lipid-extracted biomass, respectively. Moreover, the A. fumigatus extract could provide complete saccharification from intact biomass at 50 °C. Proteomic characterisation of the A. fumigatus secretome identified enzymes that were up-regulated or uniquely expressed in the presence of C. vulgaris biomass and included cellulases, glucanases, hemicellulases, chitinases, pectinases, amylases, lipases and proteases. These are therefore candidates for the degradation of complex carbohydrates, lipids and proteins in the microalgae biomass. However, the expression of much of the secretome was independent of carbon source as found by a comparison between microalgae biomass and glucose, indicating constitutive expression of key enzymes. These findings should facilitate rational design of enzyme cocktails for bioethanol production from microalgae.