## 4 Sterols as an Underexploited Lipid Resource from Microalgae  
When linking microalgae with food and human health, the predominant terms are fatty acids and carotenoids (Figure 4). Nevertheless, some microalgae are high-level producers of phytosterols, which have been playing a key role in the functional food market [79; 80]. In addition, mixtures of phytosterols can function as skin conditioning in cosmetic products (creams and lipstick), and in pharmaceutics, they are gaining interest for the production of therapeutic steroids [53; 81]. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**MicroaIga** & **Strain** & **Factors Used** & **Biomass** & **Lipid** & **PUFA Content** & **Notes** & **Ref. ** \\ \hline \multirow{6}{*}{_Chlamydomonas reinhardtii_} & \multirow{2}{*}{CC124} & Phosphorus & & & PUFA: 17.15–45.23 \(\mu\)g & & \\  &  
Plant-derived phytosterols have been added to food products for their ability to reduce serum cholesterol levels and prevent coronary heart diseases [79; 82]. Cyanophyta _Nostoc commune_ var. _sphaeroides_ lipid extracts have been found to inhibit the expression of key regulatory genes involved in cholesterol and fatty acid biosynthetic pathways; this property could contribute to lower serum cholesterol as well as triglyceride concentrations [83]. Phytosterols occur in four common forms: as free sterols (FS), as fatty acid esters (sterol is esterified to fatty acid; SE), as steryl glycosides (bound to sugar with a glycosidic bond; SG), and as acylated steryl glycosides (sugar moiety is acylated with a fatty acid; ASG) [84]. Due to the poor solubility of free phytosterols, major phytosterol/phytostanol products being marketed are in their conjugated forms (SE, SG, and ASG) as it is easier to add them into food products [84]. Table 2 summarizes information with respect to some brands that commercialize phytosterol fortified food products (yoghurts, spreads, soft cheese, and drinks), and other phytosterol-based products (supplements and paste). Although the raw materials used for phytosterols isolation for the food industry are tall oil (fat-soluble by-product obtained from trees) and vegetable oil, the scarcity of land resources has pulled the attention of scientific and industrial communities towards the search for new sustainable natural sources of phytosterols [85; 86]. The concentration of phytosterols in different vegetable oils was estimated by [PERSON] et al. [87] and ranged between 142.64 (camellia oil) and 1891.82 (rice bran oil) mg 100 g\({}^{-1}\). The potential of Chlorophytes _Dunaliella tertioleta_ and _Dunaliella salina_ as sources of phytosterols was studied by [PERSON] et al. [94]; in its study, the highest yields of total sterols were 1.3% and 0.89% dw for _D. tertioleta_ and _D. salina_, respectively. Another promising microalgal strain for phytosterol production, investigated by [PERSON] et al. [79], was _Pavlova lutheri_, with the phytosterol content reaching up to 5.1% dw. Comparing these values with the ones previously mentioned for vegetable oils, it is possible to recognize that microalgae have the potential to become a useful alternative source of phytosterols to use as functional ingredients. Microalgal-derived phytosterols are mainly distributed among four groups: 4-desmethyl-\(\Lambda\)5-sterols; 4-desmethyl-\(\Lambda\)7-sterols; 4-methylsterols; and di-hydroxylated sterols [80], with 4-desmethyl-\(\Lambda\)5-sterols being the predominant phytosterol in microalgae [80; 95]. According to [PERSON] et al. [84], 4-desmethyl sterols and stanols have been shown to inhibit the uptake of cholesterol from the intestine, resulting in a decrease of serum cholesterol levels. This once again strengthens the potential of using microalgae-phytosterols as a novel industrial application.