Alternative chemo-enzymatic hydrolysis strategy applied to different microalgae species for bioethanol production

Microalgae have been considered third generation feedstock for biofuel production based on the expectation that large amounts of algal biomass can be cultivated at an acceptable cost. Transformation of biomass into ethanol requires a saccharification step, where complex carbohydrates are broken down by hydrolysis into sugars that can be fermented to bioethanol. Carbohydrate mobilization is hampered by the recalcitrance of the cell envelope of microalgal cells, because complex structural polysaccharides are difficult to depolymerize and make internal carbohydrate reserves inaccessible to hydrolysis. Saccharification can be accomplished by either acidic hydrolysis, enzymatic treatment or a combination of both. The present work focused on the chemo-enzymatic hydrolysis of lyophilized biomass of different microalgae and subsequent fermentation of hydrolysates with higher reducing sugar content. A chemo-enzymatic hydrolysis strategy was defined, consisting of an acid pretreatment carried out at high pressure and temperature, followed by incubation with Amyloglucosidase and finally by incubation with alpha-Amylase, the opposite order of the conventional use of these enzymes. An increase of reducing sugar yield of about one third was observed, and this strategy was successfully applied to a broad group of microalgae, resulting in maximum release yields of at least 34.0 +/- 1.0 g total reducing sugar/100 g dry biomass. For bioethanol production studies, the microalgae hydrolysates of Chlorella sorokiniana, Tetraselmis sp. (Necton) and Skeletonema sp. were selected according to their high reducing sugar content. High ethanol production was achieved with all hydrolysates, with ethanol yields close to the theoretical maximum and the highest ethanol concentrations so far reported under comparable conditions. Chlorella sorokiniana stood out as the best hydrolysate for ethanol production, with an ethanol yield of 0.464 +/- 0.013 g/g reducing sugar and ethanol productivity of 0.344 +/- 0.020 g/L.h.