Bioscience Evidence 2024, Vol.14, No.4, 143-153 http://bioscipublisher.com/index.php/be 147 4.4 Economic and environmental implications The economic viability of microbial CO2 fixation processes depends on several factors, including the cost of substrates, bioreactor design, and operational efficiency. Utilizing by-products from fermentation processes, such as volatile fatty acids, can enhance the economic feasibility by providing additional revenue streams (Ghimire et al., 2015). Moreover, integrated systems that combine CO2 fixation with other fermentation processes, such as ethanol production, can reduce costs by utilizing CO2 produced during ethanol fermentation (Wu et al., 2012; Zhang et al., 2017). Converting CO2 into organic acids offers significant environmental benefits by reducing greenhouse gas emissions and producing valuable bio-based chemicals. For instance, the production of succinic acid from CO2 can replace petroleum-based production methods, contributing to sustainability and reducing the carbon footprint (Ferone et al., 2019). Additionally, integrated fermentation processes that utilize CO2 from ethanol production can further enhance environmental benefits by minimizing CO2 emissions. The use of non-photosynthetic microorganisms for CO2 fixation has also been shown to be more efficient than microalgae-based biofuels, providing a more effective pathway for carbon capture and utilization (Zhang et al., 2017). 5 Case Studies 5.1 Case study 1: engineering E. coli for enhanced succinic acid production from CO2 The engineering of Escherichia coli for enhanced succinic acid production from CO2 involves several genetic modifications and process optimizations. One approach includes the heterologous expression of genes from the reductive tricarboxylic acid (rTCA) cycle. Specifically, ten genes encoding key rTCA cycle enzymes such as α-ketoglutarate:ferredoxin oxidoreductase, ATP-dependent citrate lyase, and fumarate reductase/succinate dehydrogenase were cloned into E. coli. This transgenic strain demonstrated enhanced growth and the ability to assimilate external inorganic carbon with a gaseous CO2 supply (Lo et al., 2021). Additionally, metabolic engineering strategies have been employed to improve CO2 utilization efficiency, including the use of micro-nano bubbles and CO2 adsorption materials (Chen et al., 2023). Directed evolution of enzymes like propionyl-CoA carboxylase has also been applied to enhance the catalytic efficiency of CO2 fixation pathways (Liu et al., 2020). The genetically modified E. coli strains showed significant improvements in CO2 assimilation and succinic acid production. For instance, the transgenic strain with rTCA cycle genes exhibited CO2-enhanced growth and upregulation of genes involved in chemotaxis, flagellar assembly, and acid-resistance under anaerobic conditions (Lo et al., 2021) (Figure 2). Computational analyses have indicated that with optimized parameters, microbial succinate production processes could reach economically viable levels, making them promising alternatives to traditional sugar-based fermentations (Liebal et al., 2018). These advancements suggest that engineered E. coli could play a crucial role in sustainable industrial applications, potentially reducing greenhouse gas emissions and providing a cost-effective method for producing bio-based chemicals. 5.2 Case study 2: cyanobacteria as a platform for lactic acid production Cyanobacteria have been explored as a platform for lactic acid production due to their ability to fix CO2 and convert it into valuable chemicals. Strategies to enhance lactic acid yield include metabolic engineering to increase the flux through the reductive TCA branch and the use of nutritional supplements like corn-steep liquor (CSL) to boost production. For example, the recombinant strain PCCK of Synechocystis sp. PCC6803, which expresses foreign ATP-forming phosphoenolpyruvate carboxykinase (PEPck) along with overexpressed intrinsic phosphoenolpyruvate carboxylase (Ppc), showed increased production of C4 dicarboxylic acids, which can be further converted into lactic acid (Hidese et al., 2022). The economic viability and scalability of using cyanobacteria for lactic acid production depend on several factors, including the efficiency of CO2 fixation and the overall production costs. High-density cultivation and the use of non-sterile CSL have been shown to significantly enhance the production of malate, fumarate, and succinate, which are precursors for lactic acid production (Hidese et al., 2022). These findings suggest that with further optimization, cyanobacteria could provide a scalable and economically viable platform for lactic acid production, contributing to a sustainable bioeconomy.
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