Direct conversion of Spirulina to ethanol without pretreatment or enzymatic hydrolysis processes

Abstract

Oxygenic photosynthetic microorganisms such as cyanobacteria and microalgae have attracted attention as feedstocks for next-generation biofuels. To date, however, there have been no reports on efficient bioethanol production from cyanobacterial glycogen by yeast fermentation. Additionally, multiple pretreatment and enzymatic hydrolysis steps of polysaccharides are required for conventional ethanol production from agricultural crops and microalgae. Here, we investigate direct ethanol production from Arthrospira (Spirulina) platensis, a fast-growing halophilic cyanobacterium that accumulates large amounts of glycogen, using lysozyme and a recombinant amylase-expressing yeast strain to eliminate the need for biomass pretreatment and amylase hydrolysis. In the direct conversion process from A. platensis to ethanol, 6.5 g L⁻¹ (ethanol productivity of 1.08 g per L per day) of ethanol was produced. The total ethanol yield based on glycogen consumption was 86% of theoretical yield, which to our knowledge, is the highest yield of bioethanol from an oxygenic photosynthetic microorganism. The present findings indicate that A. platensis is a remarkable carbohydrate feedstock in the form of glycogen, which is a promising material for the production of bioethanol and various other commercially valuable chemicals.