# Integrated Carbonate-Based CO\({}_{2}\) Capture--Biofixation through Cyanobacteria
## 1 Introduction
This integration is enabled by microalgae's capacity to directly uptake bicarbonate, facilitated by the \"CO\({}_{2}\) concentrating mechanism\", which involves membrane-bound and intracellular carbonic anhydances [10; 11]. Figure 1 illustrates the concept of the system, using sodium salts as an example. Initially, the CO\({}_{2}\)-rich flue gas is chemically fixed in an aqueous carbonate solution (pH\(-\)12, Liquid Carbon Carrier in its \"discharged\" form, LCC\(-\)), resulting in the formation of a bicarbonate solution (Equation (1)) with double the concentration (pH\(-\)8.0, Liquid Carbon Carrier in its \"charged\" form, LCC\(+\)). Bicarbonate can enter metabolic pathways of microalgae, where it is converted into CO\({}_{2}\) for fixation by rubisco inside the pyrenoid [12; 13]. Hydroxyl ions are also co-produced (Equation (2)). The released CO\({}_{2}\) can then be fixed as reduced carbon compounds, leading to biomass growth (Equation (3)), which exemplifies the case of bioreduction to glucose). However, the interaction of hydroxyl ions with bicarbonate causes carbonate to progressively become the dominant inorganic carbon species in solution (Equation (4)), a form that microalgae cannot metabolize. Consequently, the solution remaining after biomass harvesting can be directly recycled as LCC\(-\) and can further be used to chemically adsorb more CO\({}_{2}\) (Equation (1)). \[\text{HCO}_{3}^{-}\rightarrow\text{CO}_{2}+\text{OH}^{-} \tag{2}\]  
\[6\text{ CO}_{2}+6\text{ H}_{2}\text{O}\rightarrow\text{C}_{6}\text{ H}_{12}\text{O}_{6}+6\text{ O}_{2} \tag{3}\]  
\[\text{ HCO}_{3}^{-}+\text{OH}^{-}\rightarrow\text{CO}_{3}^{2-}+\text{ H}_{2}\text{O} \tag{4}\]A number of advantages are offered by this biotechnological process, such as:  
* Energy efficiency. The integrated system eliminates the need for thermal desorption of CO\({}_{2}\), compression, and distribution, thereby reducing the CO\({}_{2}\) management costs. Biofixation occurs at near room temperature, making it more energy efficient than conventional chemical processing at higher temperatures. * Process intensification. The process features fewer components, resulting in a reduction in capital expenditures (CapEx), labor required for operation, and maintenance costs. * Resource recycling. Operating as a closed-loop system, it allows for the reuse of the liquid carbonate carrier in multiple carbon capture cycles, reducing the need for continuous input of alkaline species and water. Additionally, a portion of the required nutrients can often be recycled. * Flue gas purification. Many attempts to directly use flue gas as an economical source of CO\({}_{2}\) gas come with several critical issues, including low biofixation efficiency due to low CO\({}_{2}\) water solubility, and a potential hindrance to algal growth caused by toxic substances such as SO\({}_{\mathsf{X}}\), NO\({}_{\mathsf{X}}\), and heavy metals. Using a carbonate carrier enables the selective fixation of CO\({}_{2}\), resulting in purification from harmful species present in flue gas, such as SO\({}_{\mathsf{X}}\), NO\({}_{\mathsf{X}}\), and heavy metals. * Limitation of CO\({}_{2}\) outgassing. At pH values lower than around 8.0, part of the CO\({}_{2}\) is present as dissolved gas [14], which has very low solubility in water (approximately \(1.25\times 10^{-5}\) mol/L in standard conditions), resulting in rapid CO\({}_{2}\) loss to the atmosphere. Working at pH values higher than 8.0 converts CO\({}_{2}\) into stable dissolved inorganic carbon (bicarbonate and carbonate). * Biomass production. The produced microalgae biomass has various potential applications, depending on the specific strain, including biofuels, food production, biochemicals, and biomaterials. However, some limitations and challenges also exist:
* Algae suitability. The effective operation of the system depends on the identification of suitable microalgae strains capable of ensuring consistent performance.