## 4 Results and Discussion
### Biochemical Analysis
Saturated fatty acids (SFAs) vary from 39% of algae grown under 20 uE s\({}^{-1}\)m\({}^{-2}\) to 50% of those grown inside the annular reactor; Monounsaturated fatty acids (MUFAs) vary between 28% to 41% of algae grown under 20 and 210 uE s\({}^{-1}\)m\({}^{-2}\), respectively: Polyunsaturated fatty acids (PUFAs) vary from 8-9% in algae grown under 140, 210, 300 uE s\({}^{-1}\)m\({}^{-2}\) and in a 150 mm reactor to 32% of algae grown under 20 uE s\({}^{-1}\)m\({}^{-2}\). Looking at Figure 9, the most interesting aspect is that the PUFA content is highest when the cells were cultivated under low photon flux densities (20 uE s\({}^{-1}\)m\({}^{-2}\)), then gradually decreases until 140 uE s\({}^{-1}\)m\({}^{-2}\) and is stable under the highest tested photon flux densities. The same trend is shown when the thickness of the reactor decreases, meaning that the PUFA content is inversely related to the average photon density. An opposite trend may be observed for the SFA, while the MUFAs do not vary significantly. Table 3 reports in detail the composition in n-3 and in n-6 fatty acids, the ratio between n-3 and n-6 and the percentage composition of the eicosapentaenoic acid (EPA), a fatty acid of interest from an industrial point of view. The fatty acids of the n-3 series decrease with an increase in the average photon flux density, in an opposite way to those of the n-6 series. The content of EPA is again highest under the lowest average light intensities employed. In general, the fatty acid profile is coherent with that found by other authors [27], as well as the change in composition with light intensity [30]. The increase in PUFA and in n-3 fatty acids in cultures exposed to low-light conditions is related to a low-light adaption, because cells increase their photosynthetic active membranes quantities [31; 32]. It is worth highlighting, in any case, that the total lipid content is lower in cultures grown under low photon flux densities. It may be possible that the increase of lipids in algae grown under higher photon flux densities is due to an accumulation of storage lipids as SFAs and MUFAs, and that actually the PUFAs concentration is stable. Figure 10 reports an analysis of the carotenoids content of the biomass obtained at the end of the batch cultivations grown under different photon flux densities and in reactors with different configurations. The content of chlorophyll a (Figure 10a), total carotenoids (Figure 10b), beta-carotene (Figure 10c) and lutein (Figure 10d) varies from a minimum of 1.7, 1.7, 0.05 and 0.08 ug mg\({}^{-1}\) of dry weight, respectively, of the biomass grown under 450 uE s\({}^{-1}\)m\({}^{-2}\) to a maximum of 29.4, 8.71, 1.55, and 0.60 ug mg\({}^{-1}\) of dry weight, respectively, of the biomass grown under 20 uE s\({}^{-1}\)m\({}^{-2}\). The contents are in the same range observed earlier [27]. The content of all the analysed compounds shows the same trend: it is higher in conditions where the average light intensity is lower (20 uE s\({}^{-1}\)m\({}^{-2}\) and annular reactor) and coherently decreases along with increases in average light intensity. This response is caused by a photo-acclimatization mechanism, already known, through which the cells grown under low photon flux densities increase the number of photoactive systems, and consequently need higher quantities of carotenoids and chlorophyll [33; 34].