Photosynthetic microorganisms are considered excellent feedstock for biofuel production in developing biomass production technologies. A study was conducted to evaluate ethanol production with the sequential enzymatic saccharification and fermentation of Arthrospira platensis (Spirulina) biomass with the metabolically engineered Escherichia coli strain MS04. A. platensis was cultivated semicontinuously in an open raceway pond, and the carbohydrate content was determined to be as high as 40%. The enzymatic saccharification was designed to release the maximum amount of glucose. After 40 h of enzymatic saccharification, 27 g L−1 of monosaccharides was obtained. These slurries were fermented with ethanologenic bacteria, achieving 12.7 g L−1 ethanol after 9 h of fermentation, which corresponds to 92% conversion yield of the glucose content in the hydrolysate, 0.13 g of ethanol per 1 g of Spirulina biomass and a volumetric productivity of 1.4 g of ethanol L−1 h−1. Therefore, we conclude that it is possible, in a short time, to obtain a high ethanol yield corresponding to 160 L per ton of dry biomass with a high productivity.
O emprego de microalgas para desenvolvimento de produtos está em pleno desenvolvimento considerando a composição de diversas espécies e a adaptação das mesmas ao meio de cultivo. Neste trabalho realizou-se o estudo da hidrólise enzimática da biomassa da microalga considerando a potencialidade de produção a partir de cepas de Chlorella sp., Scenedesmus sp., e Spirulina (Arthrospira platensis) empregando uma sequência de enzimas composta de duas amilases e dois complexos de celulases. Para tanto, foi realizado inicialmente a caracterização das espécies, seguindo-se o estudo com a Spirulina, que apresentou 40,02 % de carboidratos e mais que a metade do teor em amido (24,95 %) se mostrando promissor o seu uso para a otimização da hidrólise enzimática. Os melhores resultados de conversão foram encontrados com 12 FPU da enzima CTec2 enquanto que foi com 6,9 FPU da enzima HTec2, alcançando próximo a 100 % de rendimento. Desta forma pode-se concluir que a hidrólise empregando uma sequência de enzimas, alfa-amilase, glucoamilase e celulases mostrou-se eficiente para a microalga Spirulina, mostrando a viabilidade de aplicação da biomassa para o desenvolvimento de outros produtos, tornando a produção de matéria prima para etanol mais renovável.
Purpose
Dunaliella is a halophilic genus of microalgae with high potential in the global food market. The microalgal cultivation process contributes to not only economic impact but also environmental impact, especially regarding the artificial medium composition. In this context, a life cycle assessment was carried out to analyze the impacts associated with the components of the modified Johnson medium (MJM) and to predict the best scenarios to cultivate Dunaliella tertiolecta and Dunaliella salina for biomass, glycerol, and beta-carotene production.
Method
Two chains were analyzed separately: (1) Dunaliella salina (strain DF 15) cultivated in 8 scenarios combining different nitrogen (0.1 and 1.0 g L−1 KNO3) and magnesium (1.1–2.3 g L−1 MgCl2.6H2O) concentrations to produce biomass, glycerol, and beta-carotene and (2) Dunaliella tertiolecta (strain CCAP 19/30) cultivated in 5 scenarios combining different nitrogen (0.1 and 1.0 g L−1 KNO3) and salt (116.9–175.4 g L−1 sea salt) concentrations to produce biomass and glycerol. In addition, we evaluated the potential of cultivating these species to reduce the carbon footprint of the proposed scenarios.
Results and discussion
For D. salina, S5 (1 g L−1 KNO3, 1.1 g L−1 MgCl2.6H2O) had the lowest environmental damage for biomass (74.2 mPt) and glycerol production (0.95 Pt) and S3 (0.1 g L−1 KNO3, 1.9 g L−1 MgCl2.6H2O) for beta-carotene (3.88 Pt). T4 (1 g L−1 KNO3, 116.9 g L−1 sea salt) was the best for D. tertiolecta for biomass (74 mPt) and glycerol (0.49 Pt). “Respiratory inorganics,” “Non-renewable energy,” and “Global warming” were the most impacted categories. “Human health,” “Climate change,” and “Resources” had the highest share of all damage categories. All the scenarios presented negative carbon emission after proposing using brine as alternative salt source: S5 was the best scenario (− 157.5 kg CO2-eq) for D. salina and T4 for D. tertiolecta (− 213.6 kg CO2-eq).
Conclusion
The LCA proved its importance in accurately predicting the optimal scenarios for MJM composition in the analyzed bioproducts, as confirmed by the Monte Carlo simulation. Although the absolute values of impacts and productivity cannot be directly compared to large-scale cultivation, the validity of the LCA results at this scale remains intact. Productivity gains could outweigh the impacts of “surplus” MJM components. Our study showcased the potential of combining D. salina and D. tertiolecta cultivation with CO2 capture, leading to a more environmentally friendly cultivation system with a reduced carbon footprint.
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