Life on the edge: O2 binding in Atlantic cod red blood cells near their southern distribution limit is not sensitive to temperature or haemoglobin genotype.

Atlantic cod are a commercially important species believed to be threatened by warming seas near their southern, equatorward upper thermal edge of distribution. Limitations to circulatory O transport, in particular cardiac output, and the geographic distribution of functionally different haemoglobin (Hb) genotypes have separately been suggested to play a role in setting thermal tolerance in this species. The present study assessed the thermal sensitivity of O binding in Atlantic cod red blood cells with different Hb genotypes near their upper thermal distribution limit and modelled its consequences for the arterio-venous O saturation difference, Sa-v , another major determinant of circulatory O supply rate. The results showed statistically indistinguishable red blood cell O binding between the three HbI genotypes in wild-caught Atlantic cod from the Irish Sea (53° N). Red blood cells had an unusually low O affinity, with reduced or even reversed thermal sensitivity between pH 7.4 and 7.9, and 5.0 and 20.0°C. This was paired with strongly pH-dependent affinity and cooperativity of red blood cell O binding (Bohr and Root effects). Modelling of Sa-v  at physiological pH, temperature and O partial pressures revealed a substantial capacity for increases in Sa-v  to meet rising tissue O demands at 5.0 and 12.5°C, but not at 20°C. Furthermore, there was no evidence for an increase of maximal Sa-v  with temperature. It is suggested that Atlantic cod at such high temperatures may solely depend on increases in cardiac output and blood O capacity, or thermal acclimatisation of metabolic rate, for matching circulatory O supply to tissue demand.