# Integrated Carbonate-Based CO\({}_{2}\) Capture--Biofixation through Cyanobacteria
## 1 Introduction
Specifically, the microalgae strain must thri in highly alkaline environments, corresponding to the complete shift in pH from bicarbonate (pH = 8) to carbonate (pH = 12). It is worth noting that the pH buffering ability of the HCO\({}_{3}\)\({}^{-}\)/CO\({}_{3}\)\({}^{2-}\) couple could aid in adapting the culture. In addition, the algae strain must tolerate a high ionic strength [15] as this will define the maximum tolerated bicarbonate concentration, and consequently, the maximum carbon density (minimum volume) of the carrier. The  
Figure 1: Schematic representation of the integrated carbonate-based CCS biofixation system, where the blue line refers to the capture of CO\({}_{2}\) from the flue gas, the black lines refer to the carbonate-bicarbonate cycle, and the green line refers to microalgae growth and harvesting. solubility of NaHCO\({}_{3}\) in water (96 g/L at 20 \({}^{\circ}\)C) exceeds the saline tolerance limit of known algae cultures. * Biofixation rate and efficiency. For an optimal operation, both the CO\({}_{2}\) absorption rate (R) and CO\({}_{2}\) utilization efficiency (U) are essential. As the rate-determining step, this defines the overall processing speed of the integrated system (Figure 1). The operativity of the carbon capture step must be modulated based on the biofxation speed to avoid the need for storing large volumes of LCC+ (awaiting biofxation). Factors such as temperature, pH, light intensity, and nutrient availability need to be carefully controlled and optimized. Cultivation is often harvested near the end of the exponential growth phase, where the increased turbidity of the cell suspension begins to limit the amount of diffused light, affecting the photosynthetic process [16]. * Harvesting easiness. The system requires an efficient harvesting method to separate the microalgae biomass from the cultivation medium, which can be challenging and costly [17]. * Economic viability. The system involves important CapEx, which includes the purchase and set-up of equipment for algae cultivation and for the carbon capture stage. The system also requires a significant amount of energy, water, and materials (e.g., nutrients) [11]. The higher the carrier concentration tolerated by the culture, the lower the volume, together with lower capital and operational expenses. The value of the produced biomass can improve the overall economic balance. * Carrier recyclability. Aqueous carbonates and bicarbonatees are not degraded during the cycling, but the accumulation of certain metabolites may require periodic purification. * Scaling up issues. Maintaining system efficiency and productivity on a larger scale requires significant engineering and logistical considerations, especially regarding light irradiation efficiency. Moreover, for consistent performance at scale, it is necessary to develop strategies to prevent and control microalgae contamination. Alkaline pH should limit some kind of contamination. * Closed loop vs. open loop plant design. Each biofixed CO\({}_{2}\) molecule is accompanied by the production of one molecule of oxygen (Equation (3)). It is known that an increase in dissolved oxygen can lead to microalgae growth inhibition due to photorespiration [18], a condition where O\({}_{2}\) competes with CO\({}_{2}\) in the interaction with the rubisco enzyme [19]. Increased oxygen levels in closed-loop systems can also pose plant safety issues. Open-loop systems appear to be a simpler solution, but the outgassing of some CO\({}_{2}\), especially during the initial phases of operation when the pH is lower, must be evaluated. From the above considerations, it can be deduced that the development of an efficient microalgae-based ICCU system strongly relies on identifying an alkaline-tolerant, saline-tolerant strain capable of high growth rates. However, isolating industrially useful alkaliophilic green algae can be a complex undertaking.