Journal of Energy Bioscience 2024, Vol.15, No.2, 96-107 http://bioscipublisher.com/index.php/jeb 99 increase the carbohydrate content of microalgae, thereby enhancing the feedstock quality for bioethanol production (Kim et al., 2014). Additionally, the use of mixed microalgae cultures and the integration of various pretreatment strategies, such as acidic and enzymatic hydrolysis, can further improve sugar extraction and ethanol yield (Shokrkar et al., 2017). Post-treatment processes, such as neutralization and electrodialysis, can also play a significant role in enhancing bioethanol yield by removing inhibitory compounds and optimizing fermentation conditions (Seon et al., 2020). Overall, a comprehensive approach that combines these optimization strategies can significantly enhance the efficiency and yield of bioethanol production from microalgae (Ho et al., 2013; Hernández et al., 2015; Hemalatha et al., 2019). 4 Cultivation Conditions for Microalgae 4.1 Factors affecting microalgal growth Microalgal growth is influenced by several key factors including light, temperature, pH, and nutrients. Light is essential for photosynthesis, and its intensity and wavelength can significantly impact growth rates. Optimal light conditions vary among species, but generally, higher light intensities promote faster growth up to a certain threshold beyond which photoinhibition can occur (Singh and Dhar, 2011; Hossain and Mahlia, 2019). Temperature also plays a crucial role, with most microalgae thriving in a range of 20 ℃~30 ℃. Deviations from this range can lead to reduced growth rates or even cell death (Hossain and Mahlia, 2019). The pH of the culture medium affects nutrient availability and cellular processes, with most microalgae preferring a pH range of 7~8 (Hossain and Mahlia, 2019). Nutrients such as nitrogen, phosphorus, and trace elements are vital for cellular functions and biomass production. Nutrient limitation, particularly nitrogen, can induce lipid accumulation, which is beneficial for biofuel production (Yang et al., 2016; Ho et al., 2017). 4.2 Photobioreactors vs. open pond systems Photobioreactors (PBRs) and open pond systems are the two primary cultivation systems for microalgae. PBRs are closed systems that offer better control over environmental conditions, leading to higher biomass productivity and reduced contamination risks. They can be designed in various configurations such as tubular, flat-plate, and vertical columns (Muñoz and Guieysse, 2006; Yang et al., 2016; Hossain and Mahlia, 2019). However, PBRs are generally more expensive to construct and operate compared to open pond systems. Open ponds, including raceway ponds, are simpler and cheaper but are more susceptible to contamination and environmental fluctuations (Singh and Dhar, 2011; Arcigni et al., 2019). Hybrid systems combining the advantages of both PBRs and open ponds have been proposed to optimize productivity and cost-efficiency (Singh and Dhar, 2011). 4.3 Strategies for maximizing biomass production Several strategies can be employed to maximize biomass production in microalgal cultivation. Optimizing light distribution and intensity is crucial, as it enhances photosynthetic efficiency. Techniques such as light dilution and the use of reflective surfaces can improve light penetration in dense cultures (Doucha and Lívanský, 2009; Liu et al., 2013). Temperature control is also important, and maintaining optimal temperatures can be achieved through the use of temperature-regulated systems or selecting strains adapted to local climatic conditions (Ho et al., 2017; Hossain and Mahlia, 2019). Nutrient optimization, including the use of wastewater as a nutrient source, can reduce costs and enhance growth rates (Muñoz and Guieysse, 2006; Yang et al., 2016). Additionally, employing genetic and metabolic engineering to develop high-yielding strains can significantly boost biomass production (Singh and Dhar, 2011). 4.4 Case studies of successful cultivation practices Several case studies highlight successful microalgal cultivation practices. For instance, the cultivation of Chlorella vulgaris and Scenedesmus obliquus in a vertical flat-plate PBR using municipal wastewater demonstrated high growth rates and efficient nutrient removal, making it a cost-effective approach for biomass production (Yang et al., 2016). Another study on the outdoor cultivation of Scenedesmus obliquus in tubular PBRs in Taiwan showed high carbohydrate productivity and CO2 fixation rates, particularly during the summer, indicating the feasibility of year-round cultivation (Figure 2) (Ho et al., 2017). The use of an attached cultivation method, where microalgae grow on vertical surfaces, has also shown promise in achieving high biomass productivity while saving water and
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