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Polyunsaturated fatty acids (PUFAs) are essential in the animal diet, notably eicosapentaenoic and docosahexaenoic acids (EPA and DHA, respectively), and in recent years, microalgae like marine diatoms are being considered as an alternative source of PUFA. Therefore, this study evaluated the effect of three nitrogen/phosphorus (N/P) ratios (21/1, 14/1 and 7/1) and two growth phases (exponential and stationary) on both EPA concentration and gene transcript levels, four front-end desaturases and two elongases, involved in EPA biosynthesis of the microalgae Phaeodactylum tricornutum Bohlin. The experiment was carried out for 12 days, and the samples were assessed for growth and nutrient intake; in addition, qPCR was performed, as well as an analysis of fatty acids. Gene transcript levels were growth phase-dependent, and only two genes, PTD6 and PTD5A, were responsive to N/P ratios. EPA levels in P. tricornutum biomass were shown to be growth phasedependent, with higher concentrations in the exponential phase (28.64% of total fatty acids), irrespective of N/P treatments. In at least one N/P ratio (21/1), the transcription of one gene, PTD5A, could be associated with EPA levels. Data from gene transcript levels and EPA concentrations, as well as the lack of some EPA intermediates, might indicate some type of regulatory steps between gene transcription and actual EPA biosynthesis.

This study aims to evaluate a Pacific white shrimp and Nile tilapia integrated system using biofloc technology with or without the addition of the microalgae Scenedesmus obliquus and with or without fish feed supplementation in a two-factor 62-day experiment. The shrimp (2.16 ± 0.01 g) were reared under a density of 400 shrimp m−3 and the fish (1.53 ± 0.12 g) were reared under a density of 522 fish m−3. The microalgae was added to the culture water two times a week. Growth performance, sludge production, and water microbiology were evaluated. Fish feed and the microalgae addition improved fish final biomass in 58% and 14%, respectively (p < 0.05). Fish survival was significantly higher when microalgae was added (93.9 ± 1.8%) compared with the treatments without microalgae addition (86.2 ± 7.6%) (p < 0.05). The yield of the overall system was higher in the treatments with fish feed supplementation (4.2 ± 0.2 kg m−3) compared with no addition (3.9 ± 0.2 kg m−3) (p < 0.05). These results suggest that fish feed supplementation at the rate of 1% of the biomass and microalgae inoculation can improve fish growth performance and system yield, without affecting sludge production and water microbiology. This work is an expansion of a conference paper with the same title.