The beneficial role of dietary Nannochloropsis oculata (N. oculata) on health parameters of Nile tilapia (Oreochromis niloticus ) under air exposure stress were investigated. Fish were fed for 4 weeks in three groups: the control group, N5 (5% of N. oculata ), and N10 (10% of N. oculata). The fish in the supplemented groups were then exposed to the air stress experiment for 1 hour (h) and designed as N5+AE and N10+AE, respectively. The results revealed that serum cortisol and glucose were elevated only in the N10+AE group, whereas C-reactive protein (CRP) and Immunoglobulin-M (IgM) decreased and increased; respectively, in the supplemented-only groups compared to the control (Fig.1) .
Intricate changes in stress and cytokine gene expression patterns were evident. In the N10 group, the highest transcription levels of hepatic and intestinal heat shock protein 70/ HSP70 were noted, whereas lower and higher transcription levels of hepatic and intestinal HSP70 were evident in N10+AE and N5+AE groups, respectively.
The hepatic glutathione peroxidase/ GPx, intestinal glutathione S-transferase/ GST, and GPx were induced in the N5 group but declined significantly upon air exposure in the same group. The intestinal transforming growth factor β1/ TGFβ1 and the IL-10 expression level were significantly induced in the N10 group compared to the N5 group. Interestingly, the N5+AE group exhibited a significant upregulation of TGF-β1, and IL-10 expression levels compared to N5 and N10+AE groups. Dietary supplementation of N. oculata maintained the liver and intestinal histomorphology and mitigated the air-exposure impacts on the induced tissue histopathological changes. Overall, both levels of N. oculata dietary supplementation showed advantages in different scenarios regarding mitigation of air-exposure stress; nonetheless, these findings support the vital role of N. oculata as feed additives on fish health and in mitigating physical stressors, like air exposure stress.