Charlotte Nelson, Samantha L. Barlow, and Michael Berenbrink

Atlantic cod is a species affected by climate change with a major polymorphic hemoglobin component (HbI) whose two polymorphs show an inverse change in frequency along a latitudinal temperature cline in the North East Atlantic, and that have been associated with differences in performance and behavioural traits. An earlier study at the northern distribution limit of the species reported differential temperature sensitivities of red blood cell oxygen (O2) affinity between the northern cold-water HbI-2 polymorph and its southern, warm-water HbI-1 counter-part, which has since widely been held as adaptive for the species across its distributional range. The present study critically re-examined this hypothesis by comparing the thermal sensitivity of O2 binding in both purified HbI polymorphs from the southern, high temperature distribution limit of the species under controlled conditions of allosteric modifiers of Hb function. Contrary to the prevailing view the O2-affinity of the major HbI polymorphs did not differ from each other under any of the tested conditions. Depending on pH and ATP concentration, the temperature-sensitive and the temperature-insensitive Hb-O2 affinity phenotypes -previously exclusively ascribed to the HbI-1 and HbI-2, respectively- could be induced in both HbI polymorphs. These results are the first to establish a molecular mechanism behind a reversed temperature-dependence of red blood cell O2 affinity in an non-endotherm fish and lay the basis for future studies on alternative mechanisms behind the differences in distribution, performance, and behavioural traits associated with the different HbI polymorphs of Atlantic cod.

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