Breathing new life into the critical oxygen partial pressure (Pcrit): a new definition, interpretation and method of determination

The critical oxygen partial pressure (P crit ) is most commonly defined as the oxygen partial pressure below which an animal’s standard metabolic rate can no longer be maintained. It is widely interpreted as measure of hypoxia tolerance, which influences a species’ aerobic scope and, thus, constrains biogeography. However, both the physiology underlying that interpretation and the methodology used to determine P crit remain topics of active debate. The debate remains unresolved in part because P crit , as defined above, is a purely descriptive metric that lacks a clear mechanistic basis. Here we redefine P crit as the PO 2 at which physiological oxygen supply is maximized and refer to these values, thus determined, as P crit- α . The oxygen supply capacity ( α ) is a species- and temperature-specific coefficient that describes the slope of the relationship between the maximum achievable metabolic rate and PO 2 . This α is easily determined using respirometry and provides a precise and robust estimate of the minimum oxygen pressure required to sustain any metabolic rate. To determine α , it is not necessary for an individual animal to maintain a consistent metabolic rate throughout a trial (i.e. regulation) nor for the metabolic rate to show a clear break-point at low PO 2 . We show that P crit- α can be determined at any metabolic rate as long as the organisms’ oxygen supply machinery reaches its maximum capacity at some point during the trial. We reanalyze published representative P crit trials for 40 species across five phyla, as well as complete datasets from six additional species, five of which have not previously been published. Values determined using the P crit- α method are strongly correlated with P crit values reported in the literature. Advantages of P crit- α include: 1) P crit- α is directly measured without the need for complex statistics that hinder measurement and interpretation; 2) it makes clear that P crit is a measure of oxygen supply, which does not necessarily reflect hypoxia tolerance; 3) it alleviates many of the methodological constraints inherent in existing methods; 4) it provides a means of predicting the maximum metabolic rate achievable at any PO 2 , 5) P crit- α sheds light on the temperature- and size-dependence of oxygen supply and metabolic rate and 6) P crit- α can be determined with greater precision than traditional P crit .