Direct and highly productive conversion of cyanobacteria Arthrospira platensis to ethanol with CaCl2 addition

Jo-Shu Chang,Chiaki Ogino,Shimpei Aikawa,Satoshi Wakai,Tomohisa Hasunuma,Kentaro Inokuma,Kengo Sasaki,Akihiko Kondo

doi:10.1186/s13068-018-1050-y

Abstract

BackgroundThe cyanobacterium Arthrospira platensis shows promise as a carbohydrate feedstock for biofuel production. The glycogen accumulated in A. platensis can be extracted by lysozyme-degrading the peptidoglycan layer of the bacterial cell walls. The extracted glycogen can be converted to ethanol through hydrolysis by amylolytic enzymes and fermentation by the yeast Saccharomyces cerevisiae. Thus, in the presence of lysozyme, a recombinant yeast expressing α-amylase and glucoamylase can convert A. platensis directly to ethanol, which would simplify the procedure for ethanol production. However, the ethanol titer and productivity in this process are lower than in ethanol production from cyanobacteria and green algae in previous reports.ResultsTo increase the ethanol titer, a high concentration of A. platensis biomass was employed as the carbon source for the ethanol production using a recombinant amylase-expressing yeast. The addition of lysozyme to the fermentation medium increased the ethanol titer, but not the ethanol productivity. The addition of CaCl2 increased both the ethanol titer and productivity by causing the delamination of polysaccharide layer on the cell surface of A. platensis. In the presence of lysozyme and CaCl2, ethanol titer, yield, and productivity improved to 48 g L−1, 93% of theoretical yield, and 1.0 g L−1 h−1 from A. platensis, corresponding to 90 g L−1 of glycogen.ConclusionsWe developed an ethanol conversion process using a recombinant amylase-expressing yeast from A. platensis with a high titer, yield, and productivity by adding both lysozyme and CaCl2. The direct and highly productive conversion process from A. platensis via yeast fermentation could be applied to multiple industrial bulk chemicals.

Highlights

The cyanobacterium Arthrospira platensis shows promise as a carbohydrate feedstock for biofuel production
We examined the effect of 1 g L−1 (6.7 mg (g dry-cell weight)−1) of lysozyme on ethanol production from a high concentration of A. platensis (150 g dry-cell weight L−1) using S. cerevisiae strain BY4741 AASS/GASS
The addition of lysozyme increased ethanol titer and ethanol yield to 40 g L−1 and 0.27 g-ethanol (g drycell weight)−1 at both 38 and 40 °C. These results indicate that lysozyme addition effectively enhances ethanol production from a higher concentration of A. platensis biomass

Summary

Introduction

The cyanobacterium Arthrospira platensis shows promise as a carbohydrate feedstock for biofuel production. Cyanobacteria and green algae show promise as a carbohydrate feedstock for biorefinery and for the production of fuels and chemicals from biomass [1, 2]. They have a high carbohydrate content (> 50% of dry-cell weight) in nutrient-depleted conditions [3,4,5]. Cyanobacteria and microalgae, which primarily grow in aquatic environments, have the benefit of year-round cultivation using non-arable lands [6]. Acid or enzymatic hydrolysis is commonly employed to obtain fermentable sugars, such as glucose, from cyanobacteria

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