Abstract:
Mitochondria are essential for sustaining complex life. They dominate fuel conversion, redox transfer, electron tunnelling, proton pumping and reduction of O2 to ultimately store significant energy in the form of ATP, and this complex machinery is believed to have occurred through endosymbiosis with an ancestral nucleated cell approximately 1.45 billion years ago. With the coupling of O2 consumption and ATP production via oxidative phosphorylation (OxPhos), sufficient O2 is required to match cellular energy demands. However, when O2 becomes limited, a succession of physiological changes may alter cell functions, and ultimately the organism’s life. Most vertebrates are sensitive to hypoxic insults and oxidative damage caused by rapid reoxygenation. However, some species have evolved within environments frequently exposed to dramatic O2 fluctuations, and this includes some intertidal species that inhabit rockpools exposed to tidal hypoxic and hyperoxic cycles. This thesis aimed to resolve whether mitochondrial adaptations play a role in the hypoxia-tolerance of intertidal New Zealand triplefin fishes and anoxia-tolerant sharks. The first study assessed the hypoxia-tolerance of intertidal and subtidal triplefins and the overall mitochondrial O2 consumption capacity in brain tissue, as the organ is extremely sensitive to O2 deprivation. Hypoxia-tolerance was found and verified and was accompanied with greater O2 consumption and OxPhos capacities in the rock pool species relative to the subtidal ones. This suggests that intertidal fish have the capacity to better utilise O2 for ATP production while presumably better conserving carbohydrate stores, even in both hypoxia and hyperoxia. As OxPhos flux diminishes with hypoxia, glycolytic flux likely accelerates and this mediates an increase in intracellular lactate level, and associates with an overall intracellular acidosis. The second study hence assessed the effect of acidosis mediated by graded lactic acid titrations, on the mitochondrial function in situ. Not only do intertidal species display greater mitochondrial pH buffering capacities, they appear to utilise acidosis to maintain function and maintain mitochondrial membrane potentials and sustain ATP production. In effect this also increases the efficiency of oxygen use given that OxPhos is depressed and this will benefit survival with decreasing oxygen. While hypoxia is relatively well tolerated by these species, rapid reoxygenation presents another challenge to overcome. The rapid oxidation of succinate that has accumulated in the hypoxic brain mediates excess reactive oxygen species (ROS) that can subsequently cause oxidative damage. In the third study, the combined effect of anoxia-reoxygenation and graded succinate in vitro on respiration and ROS production was measured. Intertidal species produced less ROS and electrons from succinate oxidation, which were better directed to respiration rather than ROS production. Hypoxia-tolerant species have acquired adaptations in different groups, and it is unknown whether similar or different mechanisms have evolved. The last chapter aimed to explore whether traits of hypoxia-tolerance were also apparent by more ancient species, i.e. elasmobranchs. The epaulette shark and grey carpet shark have evolved for ~150 M years and may retain traits that permitted survival on Earth, which had half the atmospheric O2 of today. Both species are hypoxia tolerant yet display different physiological and behavioural strategies when hypoxic. In the last chapter, shark brain mitochondria were exposed to elevated succinate and exposed to anoxia-reoxygenation. Mitochondria from these two anoxia tolerant shark species displayed contrasting responses, which likely mirror their respective physiological behaviour on anoxia exposure. Overall, this thesis reveals some traits of the mitochondrial function that likely confer hypoxia-tolerance in intertidal fish.