Is the Acute Thermal Tolerance Limit of a Temperate Fish (Notolabrus fucicola) Determined by Changes to the Ultrastructure of its Heart Mitochondria?

Abstract

Anthropogenic climate change is increasing ocean surface temperatures globally, which is increasing the intensity, frequency, and variability of extreme weather events such as marine heatwaves. As such, marine organisms globally are coping with increasingly warm habitat conditions. Marine ectotherms, such as fish, and are unable to regulate their internal body temperatures physiologically and must regulate their body temperature through behavioural mechanisms. As a result, ocean warming has caused shifts in the distributions of many marine species as they move to track their preferred habitat temperatures. Tropical species are at even greater risk as many are already living “on a knife’s edge” near their thermal limits. As many people rely on the ocean as a primary food source and way of life, understanding how marine ectotherms respond to acute heat stress in a warming climate is of great importance. The ectotherm heart is often the first organs to fail with acute heat stress. This is because as temperature rises, oxygen demand increases, and the heart is forced to work harder. However, hearts can only increase their workload to a point before the onset of arrhythmia and their eventual failure. A leading hypothesis (oxygen- and capacity-limited thermal tolerance, OCLTT) proposes that an organism’s thermal tolerance limit is set by the capacity of its respiratory system to extract and utilise oxygen from the environment at high temperatures. However, recent studies have shown that organisms may not be oxygen-limited at high temperatures. Yet as they approach critical thermal limits their respiratory system fails, their heart stops, and the organism dies. This suggests that another mechanism is likely failing prior the organism becomes oxygen-limited. Our group has shown that just before an organism’s heart fails with acute heat stress (Tcrit) mitochondria fail (mtTcrit). Protons accumulate within folds of mitochondrial inner membranes (cristae), generating an electrochemical proton gradient that powers terminally-located ATP-synthases to make ATP. Mitochondrial ultrastructure is therefore essential to their function. This thesis tests the hypothesis that acute heat stress alters mitochondrial ultrastructure, in particular the organisation of cristae, and this is the mechanism that causes hearts to fail at high temperature. Techniques were used to determine changes to mitochondrial ultrastructure as acute temperature increases above the mtTcrit (26°C ) for our model species, N. fucicola. Temperatures chosen were 18°C, 24°C, 26°C, 28°C, and 30°C. It was found that at mtTcrit cristae lose their structural integrity (increased SA :V) and move apart at a rate of ~2.5 nm°C-1. A swelling experiment also showed that mitochondria appear to swell with increasing acute temperature by ~6-11% at mtTcrit, suggesting that even slight changes to mitochondrial volume is enough to cause their failure. These data suggest the mechanism of mitochondrial failure at high acute temperatures. The results of this thesis are discussed in the context of ocean warming and marine ecology.