Enzymatic responses reveal different physiological strategies employed by eurytolerant fish during extreme hot and cold cycling acclimation temperatures

Abstract

Heat waves and cold snaps are projected to rise in magnitude, duration, interval, and harshness in the coming years. The current literature examining thermal impacts on the physiology of organisms rarely uses chronic, variable thermal acclimations despite the fact that climate change predictions project a more variable environment. If we are to determine species’ susceptibility to climate change, chronic and variable lab acclimations should be prioritized. Here, we acclimated the eurytolerant sheepshead minnow (Cyprinodon variegatus) to two extreme cycling thermal regimes: one warm [resting 27 °C with a spike to 33 °C for 8 h daily], one cold [resting 6.5 °C with a spike to 12 °C for 8 h daily], and three chronically stable conditions (10, 22, and 30 °C) for comparison. We measured enzymatic antioxidants (catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx)), total antioxidant capacity, lipid peroxidation (LPO) damage, and citrate synthase (CS) activity in white epaxial muscle. Of particular note, we found significant increases in log CAT activity and SOD concentration in the warm cycling temperatures, and significant increases in GPx activity in the cold cycling temperatures. We found no significant accumulation of LPO damage in any of our thermal acclimation treatments. Thus, sheepshead minnows demonstrate two particularly different mechanisms towards dealing with thermal variation in low and high temperatures. The enzymatic differences between low and high cycling temperatures may define pathways of eurytolerant organisms and how they may survive predicted variability in thermal regimes.