Bioscience Evidence 2024, Vol.14, No.4, 143-153 http://bioscipublisher.com/index.php/be 146 Pathway Engineering: Introducing and optimizing synthetic pathways like the CETCH cycle and the MOG pathway in model organisms such as Escherichia coli and yeast can lead to higher yields of target organic acids. These pathways are designed to be more efficient than natural CO2 fixation routes (Wang et al., 2023). Electrochemical Systems: Utilizing microbial electrosynthesis (MES) systems can improve the conversion efficiency of CO2 to organic acids. By optimizing the bioelectrochemical conditions, such as cathodic potential and electron transfer mechanisms, the production of acetic acid and other organic acids can be significantly enhanced (Liu et al., 2018; Mateos et al., 2019) (Table 1). Mixed-Culture Biocathodes: Employing mixed microbial cultures in bioelectrochemical systems can stabilize reactor performance and enhance CO2 reduction. For example, a mixed-culture biocathode containing Sporomusa and Clostridium species has been shown to effectively convert CO2 to acetate (Mateos et al., 2019). By integrating these approaches, the microbial fixation of CO2 and its conversion into valuable organic acids can be optimized, contributing to sustainable and economically viable bioprocesses. 4 Process Optimization for Industrial Applications 4.1 Fermentation process design Designing an effective microbial fermentation process for CO2 fixation and organic acid production involves optimizing several key parameters. These include substrate utilization, microbial community enrichment, and operational parameters such as pH, temperature, and partial pressure of gases. For instance, dark fermentation processes have shown that optimizing these parameters can significantly enhance biohydrogen yield and the production of valuable by-products like volatile fatty acids (Ghimire et al., 2015). Additionally, maintaining a sufficient CO2 transfer rate and optimizing pH levels can improve succinate yield in bioreactors (Wu et al., 2012). 4.2 Bioreactor technologies Various bioreactor types and configurations have been developed to optimize microbial CO2 fixation. Continuous stirred-tank reactors (CSTR) and immobilized cell reactors are commonly used, with the latter showing advantages in mass transfer and cell density (Klasson et al., 1991). Hollow fiber membrane bioreactors (L/G MBR) have also been developed to combine biohydrogen production, in situ liquid-gas separation, and bacteria retention, which simplifies the fermentation process and enhances hydrogen yield (Renaudie et al., 2021). Additionally, bioreactors equipped with self-inducing agitators have been shown to reduce CO2 waste and improve succinate production (Wu et al., 2012). 4.3 Scale-up challenges Scaling up microbial CO2 fixation processes from lab to industrial scale presents several challenges. One major issue is the mass transfer limitation due to low gas solubilities, which can hinder the efficiency of the process (Klasson et al., 1991). Additionally, maintaining consistent microbial activity and product yield at larger scales can be difficult. For example, the performance of syngas fermentation processes can be significantly affected by mass transfer rates and gas flow profiles, which need to be carefully managed in large-scale bioreactors (Benalcázar et al., 2020). Furthermore, the integration of gasification and fermentation processes remains an underdeveloped area that requires further research to achieve technological breakthroughs (Pacheco et al., 2023). Table 1 Results of hydrogen autotrophic fermentation and microbial electrosynthesis experiments (Adopted from Liu et al., 2018) OD600,max Formate (g/L, max) Acetate (g/L,max) Butyrate (g/L,max) Ethanol (g/L, max) Rate of acetate (g/L/d) H2 (%) 80%H2–10%CO2 0.120 0.023 1.250 0.320 0.192 0.170 80 10%H2–10%CO2–80%N2 0.110 0.008 0.362 0.145 0.083 0.050 10 −0.6V 0.052 – 0.030 0.010 0 0.001 0 −0.8V 0.055 – 0.095 0.051 0.010 0.012 4.7972 −1.05V 0.059 – 0.301 0.059 0.013 0.041 9.7412 −1.2V 0.063 – 0.440 0.085 0.015 0.060 13.1777
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