Variation in thermal tolerance: the interactive effects of thermal stress on cardiac mitochondrial parameters in the family Labridae

Abstract

Mitochondria are essential for almost all complex multicellular life, supplying ~90% cellular power as ATP. Just below upper habitat temperatures, mitochondria fail (mtTcrit), mediating heart failure and death in ectotherms. While work has examined mitochondrial bioenergetics with thermal stress, the exact mechanisms that induce mitochondrial disruption remain unclear, and limited work has examined the impact on mitochondrial ultrastructure. The primary aims of this thesis were to determine the interactive effects of temperature and oxygen (O2) on cardiac mitochondria as temperatures reach mtTcrit in wrasse species from different thermal environments, cold-temperate Notolabrus fucicola and tropical Scarus ghobban, and to explore whether mitochondrial mechanisms are involved in variations in thermal-tolerance. Using high-resolution-respirometers coupled with fluorometric measures, we explored mitochondrial bioenergetic interactions between mitochondrial O2 consumption efficiency, mitochondrial membrane potential (mtMP), mitochondrial work (Jmt) and ATP dynamics, and whether mitochondrial failure at mtTcrit results from disruption to mitochondrial ultrastructure. These measurements were performed from saturated O2 tensions to anoxia, in order to span from traditional hyperoxic experimental conditions to in situ heart mitochondrial environment. In translation to whole organism aerobic scope, we explored mitochondrial scope, which informs on the surplus proportion of mitochondrial respiration available above basal metabolism. Analysis of mitochondrial scope showed species were able to support mitochondrial respiration until species respective mtTcrit (N. fucicola 26oC; S. ghobban between 32oC-35oC). Importantly, mitochondrial scope was more severely impacted by temperature at subcellular O2 tensions, indicating mitochondrial oxidative phosphorylation is sensitive to small changes in O2, and can be limiting under physiological conditions. At mtTcrit, mitochondria in OXPHOS state from N. fucicola showed mtMP depolarisation, increased Jmt, and a mis-match in ATP production relative to hydrolysis. This in conjunction with changes in metabolite levels, may indicate progressive disruption of the electron transport system, directing metabolism to anaerobic pathways. In contrast, Scarus ghobban mitochondria sustained mtMP, Jmt and normal levels of ATP hydrolysis above mtTcrit, despite decreases in ATP production. Electron microscopy on intact cardiac tissue showed changes in mitochondrial cristae structure mediating progressive matrix swelling in response to warming. Importantly, changes in ultrastructure occurred at temperatures below mtTcrit. This thesis confirms that mitochondrial ultrastructure disruption mediates the loss of mitochondrial bioenergetics at mtTcrit, and loss of mitochondrial function fails the hot heart of N. fucicola and S. ghobban approaching their respective habitat thermal range.