Electronic supplemental materials from Aerobic scope falls to nil at P_crit and anaerobic ATP production increases below P_crit in the tidepool sculpin, Oligocottus maculosus

The critical oxygen tension of whole-animal oxygen uptake rate, or P_crit, has historically been defined as the oxygen partial pressure (Po₂) at which aerobic scope falls to zero and further declines in Po₂ require substrate-level phosphorylation to meet shortfalls in aerobic ATP production, thereby time-limiting survival. Despite the inclusion of aerobic scope and anaerobic ATP production in the definition, little effort has been made to verify that P_crit measurements, the vast majority of which are obtained using respirometry in resting animals, actually reflect the predictions of zero aerobic scope and a transition to increasing reliance on anaerobic ATP production. To test these predictions, we compared aerobic scope and levels of whole-body lactate at oxygen partial pressures (Po₂s) bracketing P_crit obtained in resting fish during progressive hypoxia in the tidepool sculpin, Oligocottus maculosus. We found that aerobic scope falls to zero at P_crit and, in resting fish exposed to Po₂s < P_crit, whole-body lactate accumulated pointing to an increased reliance on anaerobic ATP production. These results support the interpretation of P_crit as a key oxygen threshold at which aerobic scope falls to nil and, below P_crit, survival is time-limited based on anaerobic metabolic capacity.