## 4 Sterols as an Underexploited Lipid Resource from Microalgae
### Strategies for Sterol Enhancement  
To boost phytosterol accumulation in microalgae, it is crucial to understand its trigger mechanisms. Table 3 summarizes some strategies already employed for inducing microalgae phytosterol accumulation. For Haptophyta _P. lutheri_, the effects of nutrient-induced changes, salinity, ultraviolet-C (UV-C) radiation, and sampling days have been assessed aiming phytosterol production [79; 82]. From these variables, the most effective were UV-C radiation and sampling days [79; 82]. Although the UV-C radiation equipment is simple and easy for operation and maintenance, this physical stressor has a great disadvantage for large-scale production, which is the significant cell damage connected with UV-C radiation mutagenic factor (as it attacks an organism's deoxyribonucleic acid (DNA)) [107; 108; 109]. The mutagenic factor also poses a concern since little is known about the stability of modified algal strains or whether they can potentially take any environmental risks [110]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Miconalga** & **Variables Studied** & **Total Sterols** & **Major Sterols** & **Observations** & **Ref.** \\ \hline \hline \multirow{4}{*}{_Diacomenum hatteri (synt._} & UV-C radiation & 9.9-20.3 mg g\({}^{-1}\) dw & \multirow{4}{*}{\(\uparrow\)TS was found at 100 m) cm\({}^{-2}\)} & \multirow{4}{*}{[82]} \\  & (50-250 m) m\({}^{-2}\) & & 19.5–30.9 mg g\({}^{-1}\) & & \\  & Hydrogen peroxide (H\({}_{2}\)O\({}_{2}\): & & 19.5–30.9 mg g\({}^{-1}\) & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\ \end{tabular}
\end{table} 
Table 3: Some strategies evaluated for microalgae sterol enhancement. According to [PERSON] et al. [79] and [PERSON] and [PERSON] [82], _P. Iutheri_ did not increased its sterol content when subjected to peroxide hydrogen input, variations in nitrogen and phosphorus concentrations, and changes in salinity. Still, previous studies have described salinity-induced changes as an effective tool to induce phytosterol accumulation in microalgae. [PERSON] et al. [94] studied the effect of different salinity concentrations for two Chlorophytas (_D. tertiolecta_ and _D. salina_) using a different approach from the study performed to Haptophyta _P. [PERSON]_, and good yields of total sterols were observed at a lower salt concentration [94]. Thus, the different observations made in both studies might be derived from (i) species-specific differences and/or (ii) the different approaches used to impose salinity-induced changes. Besides the growth conditions, the specific growth phase at which microalgal biomass is harvested can influence lipid yields and composition for specific purposes [70,114]. The effect of growth phase on the sterol content of dinoflagellate species (_Prorocentrum dongluiense_, _Prorocentrum minimum_, _Karenia mikimotoi_) in batch cultures was assessed by [PERSON] et al. [112]. In this study, the sterol content of two microalgae was susceptible to changes in the growth phase (from exponential to stationary phase), with the greatest increase being verified for dinosterol (168%) in _P. minimum_, and for Brassicosterol (423%) in _K. mikimotoi_. When assessing product accumulation over the growth phase, it is also important to take into consideration that there are some growth phases in which the production of stress-associated molecules (e.g., carotenoids, lipids) is still balanced with good amounts of growth-associated ones (e.g., proteins) [115]. This point is especially important for co-exploitation of high-value microalgae products. Interactive effects of nutrient and abiotic factors have been reported to enhance the production of phytosterol in some freshwater microalgal species [111,116]. For instance, varying light intensities in both high- and low-phosphorus environments was shown to affect sterol accumulation for Chlorophyta _Scenedesmus quadrica_ and _Chlamydomonas globosa_, and Bacillariophyta _Cyclotella meneghiniana_, with sterol contents increasing with light intensity under high phosphorus [111].