# Integrated Carbonate-Based CO\({}_{2}\) Capture--Biofixation through Cyanobacteria
## 1 Introduction  
Since mankind transitioned from wood to fossil fuels as their primary energy source, global energy demand has continued to rise exponentially. This shift has resulted in a heavy reliance on the combustion of gas, coal, or oil, accounting for approximately 79% of energy production in 2019 [1]. However, the use of fossil fuels has been linked to detrimental effects on the environment due to the significant amounts of pollutants and greenhouse gases released into the atmosphere. For instance, in 2019 alone, anthropogenic activities generated a staggering 40 gigatons of CO\({}_{2}\), according to the International Energy Agency [2]. The accumulation of greenhouse gases in the atmosphere acts as a thick blanket, effectively trapping infrared radiation and preventing it from escaping into outer space. This phenomenon triggers a steady rise in temperatures of the Earth's atmosphere and surface, commonly referred to as global warming, along with several related consequences, such as an enhanced frequency and intensity of extreme weather events, prolonged droughts, and heatwaves [3, 4]. In order to partly compensate for anthropogenic CO\({}_{2}\) emissions, one promising solution is the implementation of carbon capture and sequestration (CCS) systems. These systems typically involve capturing CO\({}_{2}\) from low-concentration gaseous sources (including the atmosphere itself, industrial flue gases, or output from anaerobic digesters) using solid or liquid alkaline absorbents, such as alkali (hydr)oxides or organic amines. These absorbents can be regenerated through thermal treatment, allowing for the subsequent utilization of the released CO\({}_{2}\)[5]. Carbonates have been widely used as CO\({}_{2}\) absorbents, rapidly forming bicarbonate as indicated by Equation (1). Upon heating above around 80 \({}^{\circ}\)C (depending on the counter ion), these bicarbonate can regenerate carbonates (reverse arrow). The use of a liquid CO\({}_{2}\) carrier offers several advantages in various applications, primarily due to its ease of handling, such as pumping, in comparison to solid carriers. Therefore, it is often preferable to use an aqueous solution of a soluble carbonate (often sodium, potassium, or ammonium), prepared either before or after transporting the carrier. \[\text{CO}_{3}^{2-}+\text{CO}_{2}+\text{H}_{2}\text{O}\rightleftarrows 2\text{ HCO}_{3}^{-} \tag{1}\]  
Several avenues exist for the application of captured CO\({}_{2}\). A prevalent option is the chemical reduction of CO\({}_{2}\) into simple organic molecules, such as CO or methane, which can serve as a valuable fuel source [6]. Despite its clear concept, CCS methods present significant challenges due to the overall expenses of CO\({}_{2}\) management, including the energy requirements for absorbent thermal regeneration, CO\({}_{2}\) compression, and distribution. For example, [PERSON] et al. estimated that the capital and operating costs for compressing, drying, and transporting CO\({}_{2}\) through a pipeline over a distance of 100 km would amount to approximately EUR 11.3 per metric ton [7]. These costs can be even higher in the absence of existing infrastructure. An integrated carbon capture and utilization (ICCU) system, which combines CO\({}_{2}\) capture with its direct utilization, offers several advantages in comparison to systems featuring separate stages [8]. This results not only in reduced CO\({}_{2}\) management costs but also leads to process intensification [9], wherein multiple unit operations can be incorporated in simpler machinery. This can lead to synergism simplification. The integration of a carbonate-based CCS system with microalgal cultivation, referred to as \"Bicarbonate-based Integrated Carbon Capture and Algae Production System\" (BICCAPS) or \"CO\({}_{2}\) Absorption and Microalgae Conversion\" (CAMC) [9], serves as an attractive example.