## 3 CO\({}_{2}\) Capture and Utilization (CCU)  
Renewable and green energy sources, such as solar, wind, hydrodynamics, and biomass, are the most effective in minimizing CO\({}_{2}\) emissions by reducing their carbon footprint. In particular, biological methods are considered to be dynamic and promising for efficient CO\({}_{2}\) capture and storage. Microalgae absorbing CO\({}_{2}\) from power plant exhausts can be used to lower greenhouse emissions into the air. In relation to CO\({}_{2}\) emitted, using microalgae for the production of biofuels could, through the photosynthetic process, balance the production of CO\({}_{2}\) by power plants, resulting in the net release of CO\({}_{2}\) tending to zero [21]. In particular, microalgae are more effective through photosynthetic mechanisms against terrestrial plants. Microalgae have faster growth rates and efficiency in capturing CO\({}_{2}\) (10-50 times more) than terrestrial plants and can use higher concentrations of CO\({}_{2}\), as emitted by the exhaust gases of power plants and other sources. Microalgae can absorb CO\({}_{2}\) from the atmosphere, power plants, and soluble carbonates such as Na\({}_{2}\)CO\({}_{3}\) and NaHCO\({}_{3}\), and cope with extreme conditions. In relation to the exhaust gases of power plants, the use of microalgae must be preceded by cleaning from SO\({}_{x}\) and NOx, as these have a negative effect on biomass productivity due to the reduction in the pH of the crop, leading to reduced carbon sequestration and reduced growth. An additional limiting factor is the high temperature of the exhaust gases, which makes the use of heat-resistant microalgae strains mandatory [27]. For example, the marine microalgae _Chlorococcum littorale_ can grow at CO\({}_{2}\) concentrations of up to 40%, while the same resistance (40%) has been shown by strains of the genus _Chlorella_ from hot springs, and even at a temperature of 42 \({}^{\circ}\)C [21]. CO\({}_{2}\) is one of the controlling factors for microalgae photosynthesis. Microalgae capture CO\({}_{2}\) under alkaline conditions and intensify their biomass. Increasing CO\({}_{2}\) levels improves photosynthetic efficiency, which leads to higher biomass yield. Below pH 5, the capture of CO\({}_{2}\), which is converted into biomass with a simultaneous release of oxygen, is not facilitated. Additionally, the permeability of cells and the forms of inorganic salts, and consequently the reactions between, them may be impacted by pH. Carbon binding takes place in chloroplasts, and, specifically, the process of carbon binding CO\({}_{2}\) from the microalgae takes place during the Calvin cycle and is called the dark phase of photosynthesis. Photosynthesis involves two stages: the light phase, where reactions occur only when cells receive light, and the dark phase, during which carbon capture reactions take place, both in the presence and absence of light. The enzyme carboxylase/oxygenase of ribulose 1,5-diphosphate (Rubisco) produces 3-phosphoglycerate, which is the first product in the series of carbon capture reactions. Taking into account the average chemical composition of microalgae biomass, approximately 1.8 kg CO\({}_{2}\) is required to produce 1 kg of biomass. Most microalgae species develop well in CO\({}_{2}\) concentrations from 0.038 to 10%; however, some species demonstrate high resistance to higher concentrations. Some microalgae grow well even at higher CO\({}_{2}\) concentrations, up to 100%, while biomass productivity and carbon sequestration are usually higher at low concentrations. The reason for the decrease in growth in large CO\({}_{2}\) concentrations for most species is that the pH from the production of carbonate ions decreases, resulting in the extension of the latency phase, and consequently lowering growth rates in the exponential phase.