135-2 Sunday, Jan. 8 13:45 - 14:00 How Does Mitochondrial Functioning Constrain Energy Efficiency? SALIN, K*; AUER, SK; ANDERSON, G; VILLASEVIL, EM; SELMAN, C; METCALFE, NB; SALIN, Karine; University of Glasgow, UK email@example.com http://www.gla.ac.uk/researchinstitutes/bahcm/staff/karinesalin/
Although a great number of ecophysiological studies have focused on factors affecting energy acquisition and allocation, surprisingly few have considered energy processes at the mitochondrial level. Energy derived from the diet becomes usable only after being oxidized and converted into adenosine triphosphate (ATP) by the mitochondria. Here we illustrate the role of intraspecific variation in mitochondrial functioning in constraining animal energetics using evidence from an ectotherm, the brown trout Salma trutta. We show that conspecifics living in the same environment displayed up to a 3-fold variation in the rate of energy dissipation through mitochondrial proton leak respiration. Those that had a greater mitochondrial leak respiration may be to partially offset this leak, as revealed by a higher whole-organism metabolic rate. These individuals also had the poorest performance at high ambient high temperatures. However, it is important to note that mitochondrial properties are not fixed but change according to conditions: fasting caused disproportionate changes in mitochondrial capacities of the liver, such that substrate oxidation increased far more than did the ATP synthesis. As a result, the ATP/O ratio (the amount of ATP produced per unit of oxygen consumed, i.e. the efficiency of ATP production) decreased in response to fasting. These illustrations, combined with examples from the literature, suggest that mitochondria can be a significant constraint in the use of energy resources and their allocation into ATP. Among-individual variation in mitochondrial functioning is therefore likely to contribute to the proximate causes of differences in animal performance.