Enhancement of fermentative hydrogen production from Spirogyra sp. by increased carbohydrate accumulation and selection of the biomass pretreatment under a biorefinery model

Abstract

In this work, hydrogen (H₂) was produced through the fermentation of Spirogyra sp. biomass by Clostridium butyricum DSM 10702. Macronutrient stress was applied to increase the carbohydrate content in Spirogyra, and a 36% (w/w) accumulation of carbohydrates was reached by nitrogen depletion. The use of wet microalga as fermentable substrate was compared with physically and chemically treated biomass for increased carbohydrate solubilisation. The combination of drying, bead beating and mild acid hydrolysis produced a saccharification yield of 90.3% (w/w). The H₂ production from Spirogyra hydrolysate was 3.9 L H₂ L^-1, equivalent to 146.3 mL H₂ g^-1 microalga dry weight. The presence of protein (23.2 ± 0.3% w/w) and valuable pigments, such as astaxanthin (38.8% of the total pigment content), makes this microalga suitable to be used simultaneously in both food and feed applications. In a Spirogyra based biorefinery, the potential energy production and food-grade protein and pigments revenue per cubic meter of microalga culture per year was estimated on 7.4 MJ, US $412 and US $15, respectively, thereby contributing to the cost efficiency and sustainability of the whole bioconversion process.

Keywords: Clostridium butyricum; Dark fermentation; Microalga pigments; Microalgae; Spirogyra biorefinery.