# Extraction of Pigments from Microalgae and Cyanobacteria--A Review on Current Methodologies
## 7 Supercritical Fluid Extraction  
From scalable techniques, supercritical extraction stands out as an efficient method for obtaining pigments from microalgae and cyanobacteria, especially carotenoids and chlorophylls. Supercritical fluid extraction (SFE) is based on the use of a supercritical solvent, which can permeate the cell by acting like a gas while also solubilizing compounds by acting like a liquid solvent [57]. Because of its low polarity, supercritical CO2 is the most commonly used solvent in carotenoids and chlorophyll extraction, and due to its non-flammable and non-toxic properties, this method is associated with a more environmentally friendly approach [3; 58]. In general, SFE is recognized as a green methodology due to lower consumption of organic solvents and shorter extraction time, when compared to classic solvent extraction. However, the main limitation of SFE is still the cost of implementing and operating it [3]. Supercritical extraction may also open the possibility for processes operating under a \"biorefinery\" approach, once it allows further treatment and extraction of the remaining biomass [59]. The SFE consists of a high-pressure pump for CO2, a heating chamber, an extraction chamber, and a collecting chamber. The CO2 is pumped into the heating chamber where it will reach the supercritical conditions, then it is pumped into the extraction vessel, where it diffuses into the biomass. The extraction occurs as a result of the solvent permeating the biomass, which causes cell expansion and simultaneous solubilization of the target compounds. The extract then goes to the collecting chamber where CO2 is cooled, re-compressed and recycled, leaving an oil-like highly concentrated extract containing carotenoids and chlorophyll [57; 60]. Figure 6 shows a schematic representation of SFE and its effects on the cell. Supercritical CO2 extraction has been applied to the extraction of carotenoids in many microalgae and cyanobacteria. In _D. salina_, [PERSON] et al. [61] found a pressure of 443 bar and a temperature of 27.5 \({}^{\circ}\)C to be the optimal conditions for the extraction of \(\beta\)-carotene. Figure 6: Schematic representation of supercritical fluid extraction (SFE) in microalgae and cyanobacteria cells. In another work, [PERSON] et al. [62] showed that the optimal operating conditions for extraction of carotenoids from _D. salina_ were 400 bar and 55 \({}^{\circ}\)C, although the highest carotenoids/chlorophylls ratio was indeed found at a lower temperature (30 \({}^{\circ}\)C). Changes in pressure have been associated to the selective extraction of carotenoids. [PERSON] et al. [63] optimized supercritical CO\({}_{2}\) extraction in _Nannochloropsisulata_, and the system used ethanol addition (as co-solvent) to enhance the extraction yield. The optimal condition for lipids and carotenoids extraction was set at 350 bar, 50 \({}^{\circ}\)C, and 16.7% of ethanol addition. In the case of _Nannochloropsis gaditana_, [PERSON] et al. [64] used supercritical CO\({}_{2}\) extraction for carotenoids and chlorophyll extraction. The highest yield was obtained at a pressure of 400 bar and a temperature of 60 \({}^{\circ}\)C. The best carotenoids/chlorophylls ratio was also obtained by decreasing the pressure (200 bar) and maintaining the temperature at 60 \({}^{\circ}\)C. The pressure of 400 bar and a temperature of 60 \({}^{\circ}\)C was also the optimal condition for lutein and \(\beta\)-carotene extraction in _S. almeriensis_. Although the extraction yield of \(\beta\)-carotene is up to 50% of its total content, the percentage of extraction for lutein is very low [65]. For _Scenedesmus obliquus_, [PERSON] et al. [66] tested supercritical CO\({}_{2}\) extraction of carotenoids and chlorophylls for use in food processing. The highest carotenoid yield and ratio of total carotenoids to chlorophyll was attained at 250 bar and 60 \({}^{\circ}\)C, using ethanol (7.7%, \(v/v\)) as co-solvent. Regarding astaxanthin from _H. pluvialis_, [PERSON] et al.