Salinity change evokes stress and immune responses in Atlantic salmon with microalgae showing limited potential for dietary mitigation

Abstract

Smoltification was found to impact both immune and stress responses of farmed Atlantic salmon (Salmo salar), but little is known about how salinity change affects salmon months after completed smoltification. Here, we examined (1) the effect of salinity change from brackish water to seawater on the stress and immune responses in Atlantic salmon and (2) evaluated if functional diets enriched with microalgae can mitigate stress- and immune-related changes. Groups of Atlantic salmon were fed for 8 weeks with different microalgae-enriched diets in brackish water and were then transferred into seawater. Samples of the head kidney, gill, liver and plasma were taken before seawater transfer (SWT), 20 h after SWT, and 2 weeks after SWT for gene-expression analysis, plasma biochemistry and protein quantification. The salmon showed full osmoregulatory ability upon transfer to seawater reflected by high nkaα1b levels in the gill and tight plasma ion regulation. In the gill, one-third of 44 investigated genes were reduced at either 20 h or 2 weeks in seawater, including genes involved in cytokine signaling (il1b) and antiviral defense (isg15, rsad2, ifit5). In contrast, an acute response after 20 h in SW was apparent in the head kidney reflected by increased plasma stress indicators and induced expression of genes involved in acute-phase response (drtp1), antimicrobial defense (camp) and stress response (hspa5). However, after 2 weeks in seawater, the expression of antiviral genes (isg15, rsad2, znfx1) was reduced in the head kidney. Few genes (camp, clra, c1ql2) in the gill were downregulated by a diet with 8% inclusion of Athrospira platensis. The results of the present study indicate that salinity change months after smoltification evokes molecular stress- and immune responses in Atlantic salmon. However, microalgae-enriched functional diets seem to have only limited potential to mitigate the related changes.

Keywords: functional feed; immunity; microalgae; salinity; salmon; smoltification.

FIGURE 1

Boxplot with median and 1.5 x interquartile range whiskers showing sodium-ion (A) and chloride-ion (B) concentration in plasma of Atlantic salmon fed the control diet (CD) after 8 weeks experimental period in brackish water (T1), 20 h in seawater (T2) and 2 weeks in seawater (T3), as well as log-10- transformed copy numbers of nkaα1a/b transcripts for the three timepoints (C); n = 6–8. Note that the y-axis in panel (A), (B) and (C) does not start at zero.

FIGURE 2

Principal component analysis (A) of 44 stress-and immune-regulated genes in the head kidney of Atlantic salmon after 8 weeks experimental period in brackish water (BW, T1), 20 h in seawater (T2) and 2 weeks in seawater (T3). Factor map (B) explaining the contribution of individual genes to the overall separation of the PCA. Length and color of the arrows indicate percent contribution of each gene. Heatmap (C) of fold changes for the genes investigated in relation to timepoint T1 (BW) based on relative gene expression values. Statistical significance compared to timepoint T1 was assessed using multiple contrast tests (*, p < 0.05). The rows represent different genes categorized into functional groups as illustrated in the legend on the right. The columns display the time points 20 h in seawater (T2) and 2 weeks in seawater (T3). Each cell is colorized based on the fold change of that gene, as visualized in the legend on the right. For each figure panel (A–C) n = 35–50 per timepoint.

FIGURE 3

Principal component analysis (A) of 44 stress-and immune-regulated genes in the gill of Atlantic salmon after 8 weeks experimental period in brackish water (BW, T1), 20 h in seawater (T2) and 2 weeks in seawater (T3). Factor map (B) explaining the contribution of individual genes to the overall separation of the PCA. Length and color of the arrows indicate percent contribution of each gene. Heatmap (C) of fold changes for the genes investigated in relation to timepoint T1 (BW) based on relative gene expression values. Statistical significance compared to timepoint T1 was assessed using multiple contrast tests (*, p < 0.05). The rows represent different genes categorized into functional groups as illustrated in the legend on the right. The columns display the time points 20 h in seawater (T2) and 2 weeks in seawater (T3). Each cell is colorized based on the fold change of that gene, as visualized in the legend on the right. For each figure panel (A–C) n = 36–50 per timepoint.

FIGURE 4

Boxplot with median and 1.5 x interquartile range whiskers showing sodium-ion (A) and chloride-ion (B) concentration in plasma of Atlantic salmon 20 h in seawater (T2) fed with the microalgae enriched diets C. vulgaris intact (CVI), C. vulgaris broken (CVB), T. chuii (TC), A. platensis (AP) or S. limacinum (SL) at 8% inclusion or a control diet (CD); n = 9. Note that the y-axis in panel (A) and (B) does not start at zero.

FIGURE 5

Boxplot with median and 1.5 x interquartile range whiskers showing plasma cortisol (A) and glucose (B) of Atlantic salmon 20 h in seawater (T2) and 2 weeks in seawater (T3) fed with the microalgae enriched diets C. vulgaris intact (CVI), C. vulgaris broken (CVB), T. chuii (TC), A. platensis (AP) or S. limacinum (SL) at 8% inclusion or a control diet (CD); n = 9.

FIGURE 6

Boxplot with median and 1.5 x interquartile range whiskers showing protein abundance of myeloperoxidase (A) and superoxide dismutase 1 (B) in liver tissue of Atlantic salmon 20 h in seawater (T2) and 2 weeks in seawater (T3) fed with the microalgae enriched diets C. vulgaris intact (CVI), C. vulgaris broken (CVB), T. chuii (TC), A. platensis (AP) or S. limacinum (SL) at 8% inclusion or a control diet (CD); n = 3 pool of 3 fish per tank.

FIGURE 7

Boxplot with median and 1.5 x interquartile range whiskers showing gene expression of clra (A), c1ql2 (B) and camp (C) in the gill of Atlantic salmon 20 h in seawater (SW) and 2 weeks in seawater fed with the microalgae enriched diets C. vulgaris intact (CVI), C. vulgaris broken (CVB), T. chuii (TC), A. platensis (AP) or S. limacinum (SL) at 8% inclusion or a control diet (CD), n = 6-9.