## 6 Lipid Characterization  
Carbon fixed by microalgae can be allocated into diverse metabolites. This distribution is dependent on microalgae metabolic and cellular organization, which in turn varies across distinct phylogenetic classes [140]. Thus, to improve microalgal biomass/product accumulation, several questions must be solved, namely how flux to desired product accumulation is controlled, and which interactions occur between and within metabolic pathways [141].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Phylum** & **Carotenoids** & **Xantophylls** \\ \hline Cyanobacteria & \(\beta\)-carotene & Myoxanthin, zeaxanthin \\ Prochlorophyta & \(\beta\)-carotene & Zeaxanthin \\ Glaucophyta & \(\delta\)-carotene & Zeaxanthin \\ Rhodophyta & \(\alpha\)- and \(\beta\)-carotene & Lutein \\ Crypophyta & \(\alpha\)-, \(\beta\)-, and \(\epsilon\)-carotene & Alloxanthin \\ Ocrophyta & \(\alpha\)-, \(\beta\)-, and \(\epsilon\)-carotene & Fucoxanthin, violaxanthin \\ Haptopophyta & \(\alpha\)- and \(\beta\)-carotene & Fucoxanthin \\ Dinophyta & \(\beta\)-carotene & Peridinin, fucoxanthin, diadinoxanthin, \\ Euglenophyta & \(\beta\)- and \(\boldsymbol{Y}\)-carotene & Diadinoxanthin \\ Chlorarachniophyta & absent & Lutein, neoxanthin, violaxanthin \\ Chlorophyta & absent & Lutein, prasinoxanthin \\ \hline \hline \end{tabular}
\end{table}
Table 5: Main carotenoids and xanthophylls of the algal phyla [136].  
Metabolic profiling is a promising approach for identifying and quantifying the intracellular metabolic fluxes of microalgae, under different cultivation conditions [129].  
Studies monitoring chemical diversity often screen a specific biological activity, a targeted compound class or individual molecule [142]. This constrains the development of microalgae-based industries once it overshadows the diversity of compounds produced by microalgae [143]. Several methods have been developed to allow a quantitative and simultaneous analysis of many groups of metabolites on complex mixtures by capillary electrophoresis mass spectroscopy (CE-MS), gas chromatography-mass spectroscopy (GC-MS), liquid chromatography-mass spectroscopy (LC-MS), nuclear magnetic resonance spectroscopy (NMR), and Fourier transform ion cyclotron resonance-mass spectroscopy (FTICR-MS) [144; 145]. These strategies allow to save time, with respect to laborious isolation and quantification procedures, and give prompt information for complex mixtures, constituting a major advantage for microalgal-based industries [145].  
Lipidomic comprises the detailed identification and quantification of lipid classes [146]. GC-MS has been widely used for the determination of the fatty acid compositions of microalgal lipids, which, in turn, are the basis of microalgae lipidomic studies [146]. However, the challenge with microalgal lipidomic studies lies in addressing its vast complexity and chemical heterogeneity [147]. GC-MS methods are recognized by their high detection sensitivity, accuracy, and excellent reproducibility; nevertheless, sample pretreatment (e.g., hydrolysis, derivatization) needs to be performed for samples [148]. A trimethylsilyl 2,2,2-trifluoro-N-trimethylsilylethanolamine (BSTFA) derivatization with GC-MS has been used as a simple, fast, low-cost, and powerful tool to gain in-depth knowledge on unknown but relevant lipids [149]. Through this non-target approach, it is possible to simultaneously analyze fatty acids, sterols, monoglycer, aliphatic alcohols, glycosyl sterols, and other lipid-soluble molecules, such as \(\alpha\)-tocopherol, without prior knowledge of the sample composition [150].