| Popis: |
The critical oxygen partial pressure (Pcrit) 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 Pcritremain topics of active debate. The debate remains unresolved in part because Pcrit, as defined above, is a purely descriptive metric that lacks a clear mechanistic basis. Here we redefine Pcritas the PO2at which physiological oxygen supply is maximized and refer to these values, thus determined, as Pcrit-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 PO2. Thisαis easily determined using respirometry and provides a precise and robust estimate of the minimum oxygen pressure required to sustain any metabolic rate. To determineα, 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 PO2. We show that Pcrit-αcan 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 Pcrittrials 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 Pcrit-αmethod are strongly correlated with Pcritvalues reported in the literature. Advantages of Pcrit-αinclude: 1) Pcrit-αis directly measured without the need for complex statistics that hinder measurement and interpretation; 2) it makes clear that Pcritis 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 PO2, 5) Pcrit-αsheds light on the temperature- and size-dependence of oxygen supply and metabolic rate and 6) Pcrit-αcan be determined with greater precision than traditional Pcrit. |
