Effects of temperature acclimation on the upper thermal tolerance of two Arctic fishes

Abstract

The thermally dynamic nearshore Beaufort Sea, Alaska, is experiencing climate change-driven temperature increases. Measuring thermal tolerance of broad whitefish (Coregonus nasus) and saffron cod (Eleginus gracilis), both important species in the Arctic ecosystem, will enhance understanding of species-specific thermal tolerances. The objectives of this study were to determine the extent that acclimating broad whitefish and saffron cod to 5°C and 15°C changed their critical thermal maximum (CT_max) and HSP70 protein and mRNA expression in brain, muscle and liver tissues. After acclimation to 5°C and 15°C, the species were exposed to a thermal ramping rate of 3.4°C · h^-1 before quantifying the CT_max and HSP70 protein and transcript concentrations. Broad whitefish and saffron cod acclimated to 15°C had a significantly higher mean CT_max (27.3°C and 25.9°C, respectively) than 5°C-acclimated fish (23.7°C and 23.2°C, respectively), which is consistent with trends in CT_max between higher and lower acclimation temperatures. There were species-specific differences in thermal tolerance with 15°C-acclimated broad whitefish having higher CT_max and HSP70 protein concentrations in liver and muscle tissues than saffron cod at both acclimation temperatures. Tissue-specific differences were quantified, with brain and muscle tissues having the highest and lowest HSP70 protein concentrations, respectively, for both species and acclimation temperatures. The differences in broad whitefish CT_max between the two acclimation temperatures could be explained with brain and liver tissues from 15°C acclimation having higher HSP70a-201 and HSP70b-201 transcript concentrations than control fish that remained in lab-acclimation conditions of 8°C. The shift in CT_max and HSP70 protein and paralogous transcripts demonstrate the physiological plasticity that both species possess in responding to two different acclimation temperatures. This response is imperative to understand as aquatic temperatures continue to elevate.

Keywords: Arctic teleosts; HSP70 expression; critical thermal maximum; thermal plasticity.

Figure 1

The CT_max (°C) for broad whitefish and saffron cod at the 5°C and 15°C acclimation temperatures. The median (line) and mean (dot) values are reported. Significant differences are denoted by ^*P < 0.05, ^**P < 0.01, ^***P < 0.001 and ^****P < 1 × 10⁻⁴ from a Wilcoxon rank-sum test.

Figure 2

a. Comparisons of 70-kDa heat shock protein (HSP70) concentrations (OD unit) between acclimation temperatures 5°C and 15°C and between broad whitefish and saffron cod. The median (line) and mean (dot) values are reported. Significant differences are denoted by ^*P < 0.05, ^**P < 0.01, ^***P < 0.001 and ^****P < 1 × 10⁻⁴ from a Wilcoxon rank-sum test. b. Comparisons of HSP70 protein concentrations (OD unit) between brain, liver and muscle samples in broad whitefish and saffron cod and between acclimation temperatures 5°C and 15°C. The median (line) and mean (dot) values are reported. Significant differences are denoted by ^*P < 0.05, ^**P < 0.01, ^***P < 0.001 and ^****P < 1 × 10⁻⁴ from a Kruskal-Wallis test followed by a Dunn’s post hoc test.

Figure 3

The mRNA HSP70 TPM between broad whitefish liver and muscle tissue samples in addition to between the acclimation temperatures 5°C and 15°C and the control group. The control group were broad whitefish samples that were left in lab-acclimation conditions at 8°C. The image on the left is a result from transcript A, and on the right is transcript B. The median (line) and mean (dot) values are reported. Significant differences are denoted by ^*P < 0.05, ^**P < 0.01, ^***P < 0.001 and ^****P < 1 × 10⁻⁴ from a Kruskal-Wallis test followed by a Dunn’s post hoc test.