The transcriptomic responses of Atlantic salmon (Salmo salar) to high temperature stress alone, and in combination with moderate hypoxia

Background: Increases in ocean temperatures and in the frequency and severity of hypoxic events are expected with climate change, and may become a challenge for cultured Atlantic salmon and negatively affect their growth, immunology and welfare. Thus, we examined how an incremental temperature increase alone (Warm & Normoxic-WN: 12 -> 20 degrees C; 1 degrees Cweek(-1)), and in combination with moderate hypoxia (Warm & Hypoxic-WH: similar to 70% air saturation), impacted the salmon's hepatic transcriptome expr\ession compared to control fish (CT: 12 degrees C, normoxic) using 44K microarrays and qPCR. Results: Overall, we identified 2894 differentially expressed probes (DEPs, FDR<5%), that included 1111 shared DEPs, while 789 and 994 DEPs were specific to WN and WH fish, respectively. Pathway analysis indicated that the cellular mechanisms affected by the two experimental conditions were quite similar, with up-regulated genes functionally associated with the heat shock response, ER-stress, apoptosis and immune defence, while genes connected with general metabolic processes, proteolysis and oxidation-reduction were largely suppressed. The qPCR assessment of 41 microarray-identified genes validated that the heat shock response (hsp90aa1, serpinh1), apoptosis (casp8, jund, jak2) and immune responses (apod, c1ql2, epx) were up-regulated in WN and WH fish, while oxidative stress and hypoxia sensitive genes were down-regulated (cirbp, cyp1a1, egln2, gstt1, hif1 alpha, prdx6, rraga, ucp2). However, the additional challenge of hypoxia resulted in more pronounced effects on heat shock and immune-related processes, including a stronger influence on the expression of 14 immune-related genes. Finally, robust correlations between the transcription of 19 genes and several phenotypic traits in WH fish suggest that changes in gene expression were related to impaired physiological and growth performance. Conclusion: Increasing temperature to 20 degrees C alone, and in combination with hypoxia, resulted in the differential expression of genes involved in similar pathways in Atlantic salmon. However, the expression responses of heat shock and immune-relevant genes in fish exposed to 20 degrees C and hypoxia were more affected, and strongly related to phenotypic characteristics (e.g., growth). This study provides valuable information on how these two environmental challenges affect the expression of stress-, metabolic- and immune-related genes and pathways, and identifies potential biomarker genes for improving our understanding of fish health and welfare.