## 3 Polyunsaturated Fatty Acids (PUFA) Exploitation from Autotrophic Microalgae
### Microalgae--PUFA Enhancement Strategies  
Although the largest share of the EPA/DHA oil market comes from wild fish, the declining fish stocks and susceptibility to contamination by pollutants (such as mercury) have turned the attention of PUFA exploitation to microalgae [66]. In this sense, the oxidative stability, sustainability, suitability for vegetarians, and the absence of fishy taste/smell are some of the advantages that make microalgae a feasible source for PUFA commercialization [66]. However, the immature production process is one of the weaknesses that must be surpassed for making PUFA exploitation from microalgae a feasible process [66]. Figure 6: Lipid mediators of inflammatory process derived from arachidonic acid (AA), eicosanentaenoic acid (EPA), and docosahexaenoic acid (DHA) [63; 64]. EETs—epoxyeicosatrienoic acids; HETEs—hydroxyeicosatetraenoic acids; PG—prostaglandin. New competitors of PUFA exploitation from microalgae are DHA producers belonging to thraustochytrids (_Traustochytrium_ sp. and _Aurantiochytrium_) [66]. These unicellular heterotrophic organisms have already been authorized for food/feed/nutraceuticalals and are marketed by DSM/Evonik and Source-Omega companies [66]. However, some disadvantages from the exploitation of these organisms for \(\omega\)3-production (mostly produce DHA; the production chain increases pressure on arable land--since heterotrophic organisms need a sugar input to grow, and produces CO\({}_{2}\)) are opportunities for the production of \(\omega\)3-fatty acids from microalgae [66]. Since the microalgae composition is known to vary with growth conditions, the study of strategies for microalgae PUFA enhancement is crucial to overcome the challenge of the undeveloped production process of phototrophic PUFA exploitation from microalgae. Although several strategies have been proposed to enhance microalgae lipids production, most of these have been projected for biofuel production. However, nowadays, more attention has been given towards PUFA production. Conventional approaches for enhancing microalgae lipid accumulation include nutrient stress (e.g., alterations in nitrogen, phosphorus, and carbon supply) or changes in cultivation conditions (e.g., light, and temperature) [67]. The main advantage of nutrient stress is its easy applicability at both lab and large-scale cultivation, while cultivation conditions such as light have high operational costs and are not easy to control within open cultivation systems [67]. Within nutrient regime alterations, nitrate limitation is a commonly employed strategy to enhance microalgal lipids quantity [67]. As essential constituent of proteins, nucleotides, vitamins, and coenzymes, any changes in the nitrogen source and concentrations can trigger growth changes and biochemical remodeling in microalgae species [68; 69; 70]. Although the study of [PERSON] et al. [71] had the purpose of improving lipids properties of the microalgal strains _Tetraselmis subcoriformis_ SHOU-S05, _Nannochloropsis oculata_ SHOU-S14, and _Pavlova viridis_ SHOU-S16, the insights on the fatty acid composition at different nitrogen concentrations enables to see some trends with respect to PUFA accumulation. Therefore, towards high nitrogen supplementation, the lipid content decreased, whereas the PUFA proportion increased for _N. oculata_ and _P. viridis_. For _T. subcoriformis_, the highest PUFA percentage was registered at lower nitrogen supplementation as with the highest lipid content. As with [PERSON] [72], this seems to suggest that the growth conditions for the maximization of PUFA production are similar to the conditions required to maximize biomass production for _N. oculata_ and _P. viridis_. Other cultivation conditions affecting microalgae growth and chemical diversity are the salinity, light intensity, and photoperiod. [PERSON] et al. [73] studied the effects of these factors in _Nannochloropsis gaditana_ CCNM1032 strain, and concluded that the most positive factor for fatty acid enhancement was the photoperiod.