Bioprospecting for fungal enzymes for applications in microalgal biomass biorefineries

Microalgal biomass is a promising feedstock for biofuels, feed/food, and biomaterials. However, while production and commercialization of single-product commodities are still not economically viable, obtaining multiple products in a biomass biorefinery faces several techno-economic challenges. The aim of this study was to identify a suitable source of hydrolytic enzymes for algal biomass saccharification. Screening of twenty-six fungal isolates for secreted enzymes activity on Chlamydomonas reinhardtii biomass resulted in the identification of Aspergillus niger IB-34 as a candidate strain. Solid-state fermentation on wheat bran produced the most active enzyme preparations. From sixty-five proteins identified by liquid chromatography coupled to mass spectrometry (LC-MS) (ProteomeXchange, identifier PXD034998) from A. niger IB-34, the majority corresponded to predicted secreted proteins belonging to the Gene Ontology categories of catalytic activity/hydrolase activity on glycosyl and O-glycosyl compounds. Skimmed biomass of biotechnologically relevant strains towards the production of commodities, Chlorella sorokiniana and Scenedesmus obliquus, was fully saccharified after a mild pretreatment at 80 °C for 10 min, at a high biomass load of 10% (w/v). The soluble liquid stream, after skimming and saccharification of biomass of both strains, was further converted into ethanol by fermentation with Saccharomyces cerevisiae at a theoretical maximum efficiency, in a separated saccharification and fermentation assays. The resulting insoluble protein, after biomass skimming with an organic solvent and enzymatic saccharification, was highly digestible in an in vitro digestion assay. Proof of concept is presented for an enzyme-assisted biomass biorefinery recovering 81% of the main biomass fractions in a likely suitable form for the conversion of lipids and carbohydrates into biofuels and proteins into feed/food. KEY POINTS: • Twenty-six fungal extracts were analyzed for saccharification of microalgal biomass. • Skimmed biomass was fully enzymatically saccharified and fermented into ethanol. • Up to 81% recovery of biomass fractions suitable for biofuels and feed/food.