## 3 CO\({}_{2}\) Capture and Utilization (CCU)
The most important thing about microalgae application for the CO\({}_{2}\) capture process is that biomass is produced at the same time, which has great potential to be changed to numerous bioproducts, such as biofuels, colors, cosmetics, food, feed, and other products. Therefore, microalgae are a satisfactory and sustainable choice for the scientific community as an important CO\({}_{2}\) capture agent at global level. A critical prerequisite is the development of an effective microalgae cultivation system, since changes in physicochemical parameters play an important role in enhancing biomass productivity and CO\({}_{2}\) capture [28]. Photobioreactors utilize CO\({}_{2}\) far more efficiently when compared to open lakes, where the majority of the CO\({}_{2}\) is released into the atmosphere. In large-scale applications, design and operation are very important, so as to achieve good stirring and aeration in order to facilitate light absorption and CO\({}_{2}\) capture [13]. Before being fed into photobioreactors(PBRs), CO\({}_{2}\) gas from point emission sources is compressed, cooled, and filtered for efficient CO\({}_{2}\) capture. Mass transfer is a very intricate process. The system consists of three phases: mass transfer from gas to liquid, gas to solid, and liquid to solid. In this system, CO\({}_{2}\) is the gas, the culture medium is the liquid, and the microalgae cells are the solid. In order for the bubbles to rise and disintegrate, CO\({}_{2}\) is blasted through the aeration device to the PBR's bottom. When CO\({}_{2}\) gas first reacts with water, carbonic acid (H\({}_{2}\)CO\({}_{3}\)) is created. This acid quickly decays to produce bicarbonate (HCO\({}_{3}\)) and the carbonate radical CO\({}^{3-}\). As the following reactions demonstrate, cells primarily take up carbon in these two forms rather than directly absorbing CO\({}_{2}\), which happens at a much slower rate:  
\[\text{CO}_{2(\text{g})}+\text{H}_{2}\text{O}\rightarrow\text{H}_{2}\text{CO}_ {3}\leftrightarrow\text{HCO}_{3}^{-}+\text{H}^{+}\]  
\[\text{HCO}_{3}^{-}\leftrightarrow\text{CO}_{3}^{2-}+\text{H}^{+}\]  
The conventional aeration system technique for adding CO\({}_{2}\) results in uneven bubbles and does not ensure uniform mass transfer. This disadvantage is removed by the use of hollow fiber membranes. Thus, significantly less CO\({}_{2}\) is used and the crop growth rate is optimized. Aeration is improved by the technique of using diffuse fine bubbles to supply the CO\({}_{2}\) current, in microalgae culture, as it increases mass transfer to the crop, avoids CO\({}_{2}\) loss, and is considered a lower-energy-cost technique. While in conventional ventilation systems the size of bubbles ranges from 1 to 3 mm, in microbubble systems the range is 200-800 \(\upmu\)m. Due to the large surface area to volume, they have the possibility of long residence time in the suspension of the culture, as they move slowly towards the top of the bioreactor, and thus the transfer of mass between gas and liquid takes place at higher rates. By applying such a system to an airlift bioreactor, an increase of 20-40% in the specific growth rate of microalgae culture and 6-8 times higher chlorophyll content were recorded. By optimizing physicochemical parameters such as light, temperature, and pH, microalgae cultures achieve greater CO\({}_{2}\) sequestration, but increase the total cost of cultivation. In this context, it is necessary to improve the resilience of microalgae through genetic engineering. Using microalgae models such as _Chlamydomonas reinhardtii_, genetic manipulations are performed to enhance CO\({}_{2}\) fixation, targeting carbonic anhydrase and Rubisco genes, or by interfering with metabolic pathways that lead to enhanced lipid and carbohydrate production [27].