## 4 Discussion
A possible explanation for the observed similar cell densities at the end of the trials under different LED regimes was also reported in the centric diatom _Coscinodiscus granii_, pointing towards illumination conditions supporting maximum diatom cell growth [45]. In sum, in photochemical terms, cells are not substantially affected by the different illumination conditions at the end of the experiment, under the given nutrient conditions and PPFD availabilities. However, it is important to point out that, according to our data, a culture medium may replenish with nutrients at the end of 48 h, as indicated by higher exponential growth under red illumination in the beginning of the trial. In terms of fatty acid and TAG accumulation, cell cultures under Blue LED light exhibited high total fatty acid and TAG concentrations. This high accumulation of lipids was already reported before, in the diatom _Skeletonema marinoi_[46], pointing out an effective energy and carbon storage strategy [47]. The increase in storage lipids such as TAG, under high frequency blue light has also been attributed to their role in preventing photo-oxidative damages [48]. In fact, and as abovementioned, the cultures subjected to Blue LED illumination showed higher dissipated energy fluxes, also indicative of the need to dissipate excessive energy, another strategy to prevent photo-inhibition. In terms of bioenergy production, the results also have a relevant role. High amounts of SFAs provide a superior oxidative stability, while PUFAs provide better cold-flow properties at the cost of oxidative stability [49]. Examining the saturation degree of the different cultures, two important characteristics were evident. Both in terms of whole fatty acid and TAG contents, the cells grown under red LED illumination presented a high content in _PUFA_, i.e., higher cold-flow properties. These cultures also showed another important feature in terms of bioenergy production. The fatty acid saturation degree was linearly correlated to the cetane number (CN), a fuel quality parameter which is related to ignition delay and combustion quality of the fuels [49]. At this level also the cells grown under red light illumination showed a substantially higher UFAs percentage, reinforcing the promising characteristics of the fatty acids produced under these experimental conditions. Cultures subjected to blue light illumination presented a higher fatty acid content and in terms of quality for bioenergy applications presented higher saturation degree and thus better characteristics for biodiesel production. Some nutritional characteristics can also be highlighted for the fatty acid profiles produced under the different light qualities. As abovementioned two indices can be used to evaluate the potential role of certain fatty acid profiles for human health: the _IA_ value is related to the plaque formation and to the decrease in the levels of esterified fatty acid, cholesterol, and phospholipids, thereby preventing the appearance of micro- and macro-coronary diseases [33]; the _IT_ value is related to the tendency to form clots in the blood vessels [33]. Since saturated FAs are considered to be proatherogenic and UFAs antiatherogenic, a low _IA_ ratio is recommended [33]. Regarding _IT_, this index indicates the propensity to form masses in the blood vessels and is defined as the relationship between the prothrombogenic (saturated fatty acids) and the antithrombotic _MLIFA_, n-3 and n-6 _PUFA_). Therefore, a low _IT_ value is also desirable [33]. Tuna, a fish considered to have a high nutritional value in terms of its fatty acid profile, has an _IA_ and _IT_ of approximately 0.7 and 0.3, respectively [33], and is therefore used as a reference. At this level, the cultures grown under red wavelength illumination present a better nutritional quality, with very low _IA_ and _IT_ values. Moreover, red illumination promoted the accumulation of PUFAs, namely EPA.