Characterizing the effects of diel temperature fluctuations on thermal and hypoxia tolerance in adult zebrafish

Abstract

Aquatic ecosystems naturally experience temperature fluctuations, which are often accompanied by shifts in dissolved oxygen. However, climate change has exacerbated the prevalence and magnitude of these fluctuations, often leaving organisms exposed to sub-optimal conditions. Due to their inherent relationship, these abiotic factors are thought to share common signalling pathways (cross-talk) and protective mechanisms (cross-tolerance) that confer cross-protection. Much of what we understand about the effect of temperature on fish originates from static exposures, despite physiological performance differing in fluctuating environments. Therefore, in this study, I aimed to determine whether diel temperature fluctuations influence thermal and hypoxia tolerance, while simultaneously characterizing the underpinning molecular and biochemical adjustments. Three experimental series were conducted, during which adult zebrafish were acclimated to either static control conditions (27ºC) or a thermal flux (23-33ºC) for approximately two weeks. Following the acclimation, I quantified whole-animal tolerance (critical thermal maximum and time to loss of equilibrium), heat-shock response genes (heat-shock proteins; hsp70 and hsp90αα), hypoxia response genes (hypoxia-inducible factor 1 alpha: hif-1αb; insulin-like growth factor binding protein: igfbp1), microRNAs (let-7d-5p, miR-301c-5p, miR-29a, miR-22b-3p), heat-shock proteins (Hsp70 and Hsp90α), and enzymatic activity (lactate dehydrogenase: LDH; citrate synthase: CS; pyruvate kinase: PK) in the brain and liver tissue. Together, the data suggest that fish acclimated to diel thermal variability exhibit distinct molecular and biochemical responses that may be involved in the observed increase in thermal and hypoxia tolerance