Journal of Energy Bioscience 2024, Vol.15, No.2, 96-107 http://bioscipublisher.com/index.php/jeb 98 2.3 Specific strains of microalgae with high bioethanol potential Several strains of microalgae have been identified as having high potential for bioethanol production. Chlorella vulgaris, for instance, is known for its rapid growth and high carbohydrate content, making it a suitable candidate for bioethanol feedstock (Kim et al., 2014). Under nutrient stress conditions, such as nitrogen limitation, the carbohydrate content of C. vulgaris can be significantly increased, enhancing its bioethanol yield (Kim et al., 2014). Other promising strains include those identified in China, which have shown potential for integrated biorefinery applications, producing both bioethanol and high-value co-products (Li et al., 2015). The optimization of cultivation conditions and genetic manipulation of these strains can further enhance their bioethanol production capabilities (Li et al., 2015; Simas-Rodrigues et al., 2015). The continuous research and development in this field aim to identify and optimize microalgal strains that can provide high bioethanol yields while maintaining economic and environmental sustainability (Lee et al., 2015; Simas-Rodrigues et al., 2015; Khan et al., 2018). In summary, microalgae present a viable and sustainable alternative to traditional bioethanol feedstocks. Their rapid growth, high carbohydrate content, and adaptability to various cultivation conditions make them an attractive option for bioethanol production. By optimizing cultivation techniques and exploring specific high-potential strains, the bioethanol yield from microalgae can be significantly enhanced, contributing to the development of a sustainable biofuel industry. 3 Biochemical Pathways for Bioethanol Production 3.1 Overview of biochemical conversion processes The production of bioethanol from microalgae involves several biochemical conversion processes, including pretreatment, enzymatic hydrolysis, and fermentation. These processes are designed to break down the complex carbohydrates in microalgal biomass into fermentable sugars, which are then converted into ethanol by microbial fermentation. The efficiency of these processes is crucial for maximizing bioethanol yield and making the production economically viable (Ho et al., 2013; Kim et al., 2014; Hernández et al., 2015). 3.2 Enzymatic hydrolysis of microalgal biomass Enzymatic hydrolysis is a critical step in the conversion of microalgal biomass to bioethanol. This process involves the use of specific enzymes to break down the polysaccharides in the biomass into simple sugars. Various enzymes, such as cellulases and pectinases, have been studied for their effectiveness in hydrolyzing microalgal carbohydrates. For instance, pectinase from Aspergillus aculeatus has shown a high saccharification yield of 79% after 72 hours at 50 ℃ (Kim et al., 2014). Similarly, cellulase from Trichoderma reesei has been effective in hydrolyzing Chloroccum sp., achieving a glucose yield of 64.2% under optimal conditions (Harun and Danquah, 2011). The combination of physical, chemical, and enzymatic pretreatments can further enhance the hydrolysis efficiency, as demonstrated by the combination of acid hydrolysis followed by enzymatic hydrolysis, which produced high monosaccharide concentrations in various microalgal species (Hernández et al., 2015). 3.3 Fermentation processes: yeast and bacterial fermentation Fermentation is the process by which the fermentable sugars obtained from hydrolysis are converted into ethanol by microorganisms. Yeast, particularly Saccharomyces cerevisiae, is commonly used for this purpose due to its high ethanol tolerance and efficiency. For example, continuous immobilized yeast fermentation of microalgal hydrolysate has achieved an ethanol yield of 89% (Kim et al., 2014). Additionally, the use of simultaneous saccharification and fermentation (SSF) processes has been shown to enhance ethanol yield, with repeated-batch SSF using immobilized yeast cells achieving a bioethanol yield of 88.2% of the theoretical yield (El-Dalatony et al., 2016). Bacterial fermentation, although less common, also holds potential for bioethanol production from microalgae, particularly when combined with yeast fermentation to optimize the overall process (Shokrkar et al., 2017). 3.4 Optimization of biochemical pathways for increased yield Optimizing the biochemical pathways involved in bioethanol production from microalgae is essential for improving yield and process efficiency. This includes optimizing pretreatment methods, enzyme selection and conditions, and fermentation processes. For instance, nutrient stress cultivation, such as nitrogen limitation, can
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