## 4 Discussion
This fatty acid intake has significant improvements in terms of human health, improving the vascular and neural health [50; 51]. Moreover, considering that a large fraction of the developed countries population presents a very low intake of EPA and that the largest source of this _PUFA_ is fish intake, another aspect that is highly reduced in western populations, this EPA content acquires a reinforced role as a possible alternative source of these key fatty acids [50; 51]. Biosynthesis of EPA is thought to occur in the endoplasmic reticulum via two complementary pathways [52]. In the \(\omega\)-3 pathway \(\alpha\)-Linolenic acid (18:3, ALA) is desaturated to 18:4 which suffers subsequent elongation to 20:4, while in the \(\omega\)-6 pathway 18:2 is desaturated to GLA and subsequently elongated to eicosatrienoic acid (20:3, ETA) and desaturated to 20:4. A \(\Delta\)5-desaturase is reported to act in both pathways, converting 20:3 to 20:4 in the \(\omega\)-6 pathway, and 20:4 to EPA in the \(\omega\)-3 pathway [52; 53]. Overall, cultures grown under red light revealed increased abundance of 16:3 and EPA, a trend which was also reflected in the TAG composition. It is possible that red light induced recycling of plastid membrane fatty acids through phospholipase A activity given the increased abundance of 16:3 in TAG [54]. The overall increase in C16 fatty acids, namely 16:2 and 16:3, could suggest variations in lipid classes such as monogalactosyldiacylglyceroll (MGDG) and digalactosyldiacylglyceroll (DGDG), responsible for fucoxanthin chlorophyll proteins (FCP) stabilization and electron transport [55]. The decreased abundance of 18:4 in cultures grown under red illumination could also suggest a prominence of \(\Delta\)6-elongase activity or gene expression in the \(\omega\)-3 pathway converting 18:4 to 20:4, rather than GLA to ETA in the \(\omega\)-6 pathway, and increased \(\Delta\)5-desaturase activity, resulting in the observed EPA increase. Blue light induced an increase of 16:1 and decrease in EPA content overall; however, the major changes in TAG composition were observed in C18 fatty acids. In sum, cultures grown under red LED illumination presented a high concentration of chloroplast fatty acids and long chain PUFAs, while blue light-growth induced the accumulation of EPA precursors in TAG (Figure 11). These results could imply an interaction between light quality, photoreceptors and regulation of genes involved in the lipid and fatty acid metabolism. It is interesting to notice that stearidonic acid, a precursor of EPA with known significant health promoting effects [56], also had its concentration increased in the cultures grown under blue light illumination. In sum, cultures grown under red LED illumination presented a high concentration of long-chain fatty acids, while blue light-growth induced the accumulation of its precursors (Figure 11). Another group of interesting value-added products produced by marine diatoms are the carotenoids with relevant applications for biotechnology and human health as nutraceuticals [57; 58]. From the wide array of pigments produced by diatoms some like \(\beta\) -carotene and Fx have been widely investigated and targeted in studies aiming to enhance its production in microalgae cultures [2; 57; 58]. Generally speaking, carotenoid pigment production is related to local environmental conditions (such as light conditions and nutrient availability). Fucoxanthin, chlorophyll \(a\) and \(c\) are the main light-harvesting pigments in diatoms, whereas \(\beta\)-carotene, DD and DT are mainly involved in photoproduction mech  
Figure 11: Schematic model of the metabolic changes induced by different wavelengths reaching the different depth layers of the ocean water column (blue and red arrows represent the penetration capacity of the red and blue wavelengths, respectively). anisms [45]. Regarding the potential nutritional aspects of carotenoids produced these also highlight the potential application of red and blue illumination for improving _P. tri-corutum_ nutraceuticalular.