Selenium enrichment in the marine microalga Nannochloropsis oceanica

Se-enriched ingredients have recently gained interest in the aquaculture industry as feed supplements due to their positive effects on fish health, growth, and potential effects on animal welfare. This study aims to assess which inorganic selenium (Se) species is suitable to produce Se-enriched Nannochloropsis oceanica (N. oceanica) biomass for aquafeed applications. The effective concentration for 50% growth inhibition (EC₅₀) and Se bioaccumulation of the two inorganic forms of Se, sodium selenite (Na₂SeO₃), and sodium selenate (Na₂SeO₄), were assessed at different concentrations after twelve days of cultivation. Toxicity results showed that selenate, EC₅₀ = 32.93 μM, had a greater negative effect on cell growth than selenite, EC₅₀ = 163.82 μM. Total intracellular Se was analysed by inductively coupled plasma - optical emission spectrometry (ICP-OES) and high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS), which revealed that selenite was better accumulated by N. oceanica. Further investigation at 30 μM of selenite in the growth medium resulted in Se bioaccumulation with a minor effect on cell growth and reached a Se intracellular content of 0.131 g_Se/kg_biomass after 12 days. Thus, 30 μM of selenite was selected for batch pilot-scale cultivation in a 1500 L tubular photobioreactor. Total Se accumulated in the biomass at pilot-scale was in the same order of magnitude compared with flasks (0.104–0.159 g_Se/kg_biomass). The results from this pilot-scale study are fundamental for a proof of concept from laboratory to pilot-scale production and they represent a critical bridging step for the potential use of Se-enriched N. oceanica in aquafeed.