Hypercarbia enhances metabolic thermotolerance of the cold temperate nototheniod Notothenia angustata

Chair: Philip Munday

Felix C Mark (1)*, Anneli Strobel (1,2), Daniel W Baker (3,4), Michael Oellermann (1), Fathima I. Iftikar (3), Hans O. Pörtner (1), Anthony J.R. Hickey (3)

1 Alfred Wegener Institute, Bremerhaven, D-27570, Germany

2 Universität Basel, Basel, CH-4003, Switzerland

3 University of Auckland, Auckland, New Zealand

4 Vancouver Island University, Nanaimo, BC V9R 5S5, Canada

Background
Many Antarctic notothenioid fish are considered losers of global change, due to their low thermal tolerance and lack of regulative mechanisms that enhance physiological plasticity. The Austral nototheniid congener Notothenia angustata provides an alternative model to explore the effects of ocean acidification and warming, as it inhabits cold temperate to subpolar waters. It is a eurythermal species, with greater capacities for thermal acclimation relative to Antarctic congeners, and therefore presents a useful model against which Antarctic notothenioids can be contrasted.
Methods
We investigated the long-term effects of hypercarbic acclimation on whole animal and cardiac mitochondrial function for the Austral nototheniid Notothenia angustata. Fish were acclimated under hypercarbic (0.2 kPa CO2, 15 days, n=6) and normocarbic conditions (control 0.04 kPa CO2, n=10). Routine metabolic rates (RMR) were determined with acute increases in temperature (3°C/d) under normocarbic and hypercarbic conditions. Mitochondrial function was then tested within permeabilised cardiac muscle fibres, and assays conducted in normocarbic (0.04 kPa CO2) and hypercarbic (3.0 kPa CO2) media at 9, 15 and 21°C. Metabolic profiles were determined in red skeletal muscle.
Findings
Whole animal critical temperature thresholds occurred below 19°C for normocarbic exposed fish, while acutely hypercarbic exposed fish maintained routine metabolic rates up to 21oC. Overall mitochondria mirrored the responses of acutely exposed whole animals, with an increased mitochondrial performance in fish acclimated to chronic hypercarbia. Chronically hypercarbic exposed animals also exhibited altered metabolomes of red muscle, but not liver with apparent increases in metabolites consistent with enhanced anaerobic metabolism and elevated contents of histidine and tryptophan that may contribute to acid-base buffering.
Conclusions
Overall enhanced cardiac mitochondrial capacities coincide with increasing hypercarbic and elevated temperature tolerance. This response suggests sufficient metabolic plasticity for Austral nototheniids to acclimate to a warming and acidifying ocean, which has not been observed to that extent in Antarctic notothenioids.