Oxygen supply capacity breathes new life into critical oxygen partial pressure (Pcrit)

The critical oxygen partial pressure (P-crit), typically defined as the P-O2 below which an animal's metabolic rate (MR) is unsustainable, is widely interpreted as a measure of hypoxia tolerance. Here, P-crit is defined as the P-O2 at which physiological oxygen supply (alpha(0)) reaches its maximum capacity (alpha; mu mol O-2 g(-1 )h(-1) kPa(-1)). alpha is a species- and temperature-specific constant describing the oxygen dependency of the maximum metabolic rate (MMR=P(O2)x alpha) or, equivalently, the MR dependence of P-crit (P-crit MR/alpha). We describe the a-method, in which t ere- the MR is monitored as oxygen declines and, for each measurement period, is divided by the corresponding P-O2 to provide the concurrent oxygen supply (alpha(0) =MR/P-O2). The highest alpha(0) value (or, more conservatively, the mean of the three highest values) is designated as alpha. The same value of alpha is reached at P-crit for any MR regardless of previous or subsequent metabolic activity. The MR need not be constant (regulated), standardized or exhibit a clear breakpoint at P-crit for accurate determination of alpha. The alpha-method has several advantages over P-crit determination and non-linear analyses, including: (1) less ambiguity and greater accuracy, (2) fewer constraints in respirometry methodology and analysis, and (3) greater predictive power and ecological and physiological insight. Across the species evaluated here, a values are correlated with MR, but not P-crit. Rather than an index of hypoxia tolerance, P-crit is a reflection of alpha, which evolves to support maximum energy demands and aerobic scope at the prevailing temperature and oxygen level.

Automated summary

P-crit is the critical oxygen partial pressure below which an animal's metabolic rate is unsustainable, while alpha reflects the oxygen supply capacity. The relationship between alpha values and metabolic rate is stronger than that between alpha and P-crit, indicating that P-crit is more of a reflection of oxygen supply capacity than hypoxia tolerance.