Bioscience Evidence 2024, Vol.14, No.4, 143-153 http://bioscipublisher.com/index.php/be 144 Chen et al., 2023). Microbial electrosynthesis (MES) systems have shown promise in enhancing the conversion efficiency of CO2 to organic acids by utilizing bioelectrochemical processes. These systems leverage the metabolic capabilities of microorganisms to produce organic acids with high selectivity and yield (Song et al., 2011; Liu et al., 2018; Mateos et al., 2019; Wang, 2024). The primary objective of this study is to investigate the microbial fixation of CO2 and its subsequent conversion into organic acids. This research aims to explore the various microbial pathways involved in CO2 fixation, evaluate the efficiency of different microbial systems, and identify potential strategies to enhance the production of organic acids. By understanding and optimizing these processes, this study hopes to contribute to the development of sustainable and economically viable methods for reducing CO2 emissions and producing valuable biochemicals. 2 Mechanisms of Microbial CO2 Fixation 2.1 Overview of CO2 fixation pathways Microbial CO2 fixation involves several distinct biochemical pathways that convert atmospheric CO2 into organic compounds. The most well-known pathway is the Calvin-Bassham-Benson (CBB) cycle, which is prevalent in many autotrophic organisms and involves the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) (Dangel and Tabita, 2015; Xiao et al., 2020; Asplund-Samuelsson and Hudson, 2021). Other pathways include the reductive citric acid cycle, the reductive acetyl-CoA pathway (Wood-Ljungdahl pathway), the 3-hydroxypropionate bicycle, the dicarboxylate/4-hydroxybutyrate cycle, and the 3-hydroxypropionate/4-hydroxybutyrate cycle (Sánchez-Andrea et al., 2020; Xiao et al., 2020; Chen et al., 2021). Recently, a seventh pathway, the reductive glycine pathway, has been identified in Desulfovibrio desulfuricans, which is highly ATP-efficient (Sánchez-Andrea et al., 2020). 2.2 Key microorganisms involved Various microorganisms are involved in CO2 fixation, including both photoautotrophic and chemoautotrophic bacteria. Cyanobacteria are well-known for their role in the CBB cycle, while other bacteria such as Clostridium ljungdahlii utilize the Wood-Ljungdahl pathway (Schuchmann and Müller, 2014; Zhang et al., 2020). Sulfate-reducing bacteria like Desulfovibrio desulfuricans employ the reductive glycine pathway (Sánchez-Andrea et al., 2020). Additionally, heterotrophic microorganisms have been engineered to enhance CO2 fixation capabilities, leveraging their fast growth and ease of genetic modification (Hu et al., 2022). 2.3 Genetic and enzymatic basis The genetic basis for CO2 fixation involves a variety of genes encoding enzymes that catalyze the key steps in these pathways. For instance, the cbbL genes encode the large subunit of RubisCO in the CBB cycle, which is a critical enzyme for CO2 fixation (Xiao et al., 2020; Asplund-Samuelsson and Hudson, 2021). Other important enzymes include formate dehydrogenase in the reductive glycine pathway and acetyl-CoA synthase in the Wood-Ljungdahl pathway (Sánchez-Andrea et al., 2020; Zhang et al., 2020). Regulatory proteins such as CbbR play a crucial role in controlling the expression of these genes, ensuring the efficient operation of the CO2 fixation pathways (Dangel and Tabita, 2015). 2.4 Challenges in CO2 fixation Despite the potential of microbial CO2 fixation, several challenges remain. One major issue is the efficiency of CO2 fixation, which can be limited by the availability of energy and reducing equivalents (Gong et al., 2019; Hu et al., 2022). Additionally, the integration of CO2 fixation pathways into heterotrophic microorganisms requires careful optimization to balance metabolic fluxes and avoid the accumulation of toxic intermediates (Hu et al., 2022). Environmental factors such as the presence of pollutants can also impact the efficiency of microbial CO2 fixation (Chen et al., 2021). Addressing these challenges through metabolic engineering and synthetic biology approaches is crucial for enhancing the viability of microbial CO2 fixation as a sustainable solution for carbon capture and conversion (Gong et al., 2019; Salehizadeh et al., 2020; Hu et al., 2022).
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