Bioscience Evidence 2024, Vol.14, No.4, 143-153 http://bioscipublisher.com/index.php/be 145 3 Conversion of Fixed CO2 into Organic Acids 3.1 Pathways for organic acid production Microbial fixation of CO2 and its subsequent conversion into organic acids involves several metabolic pathways. Key pathways include the Calvin cycle, the reduced tricarboxylic acid (rTCA) cycle, the Wood-Ljungdahl (WL) pathway, the 3-hydroxypropionate/4-hydroxybutyrate (HP/HB) cycle, the dicarboxylate/4-hydroxybutyrate (DC/HB) cycle, and the 3-hydroxypropionate (3HP) cycle. Additionally, synthetic pathways such as the CETCH cycle, the MOG pathway, the acetyl-CoA bicycle, and the POAP cycle have been designed to enhance CO2 fixation and conversion efficiency (Salehizadeh et al., 2020; Wang et al., 2023). Acetic Acid: Acetic acid production is prominently facilitated by acetogenic bacteria such as Clostridium scatologenes and Moorella thermoacetica through the Wood-Ljungdahl pathway. These bacteria can convert CO2 into acetic acid under anaerobic conditions, often using H2 as an electron donor (Song et al., 2011; Liu et al., 2018). Microbial electrosynthesis (MES) systems have also been shown to enhance acetic acid production by improving CO2 availability and electron transfer efficiency (Mateos et al., 2019) (Figure 1). Lactic Acid: Lactic acid production from CO2 is less common but can be achieved through engineered microbial strains. Genetic modifications in lactic acid bacteria can enable the utilization of CO2 as a carbon source, although this area requires further research and development (Salehizadeh et al., 2020). Succinic Acid: Succinic acid is another valuable product derived from CO2 fixation. The production of succinic acid involves the reductive branch of the TCA cycle. Enhancing CO2 utilization efficiency through genetic and metabolic engineering has been a focus to improve succinic acid yields. Strategies include optimizing CO2 supply methods and employing advanced biotechnological approaches such as micro-nano bubbles and CO2 adsorption materials (Liebal et al., 2018; Chen et al., 2023). 3.2 Microbial engineering for enhanced production Genetic and metabolic engineering play crucial roles in enhancing the microbial conversion of CO2 into organic acids. Key strategies include: Enzyme Optimization: Improving the efficiency of carbon fixation enzymes such as ribulose-1,5-diphosphate carboxylase/oxygenase (RuBisCO), pyruvate carboxylase, and formate dehydrogenase (FDH) can significantly boost CO2 fixation rates. For instance, the regulation of FDH1 by lysine acetylation and transcriptional factors in Clostridium ljungdahlii has been shown to enhance CO2 metabolism (Zhang et al., 2020). Figure 1 Image and diagram of reactor setup (Adopted from Mateos et al., 2019) Image caption: CE: counter electrode; RE: reference electrode; WE: working electrode (Adopted from Mateos et al., 2019)
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