Respiratory adaptations to a combination of oxygen deprivation and extreme carbon dioxide concentration in nematodes

To examine physiological adaptations to the two combined stressors O₂ deprivation and extreme CO₂ concentrations, we compared respiratory responses of two nematode species occurring in natural CO₂ springs. The minimum O₂ concentration allowing maintenance of respiration in both species was 0.0176μmol O₂ml^-1 (corresponds to 1.4% O₂ in air). After exposure to anoxia, individuals resumed respiration immediately when O₂ was added, but on a lower level compared to control and without showing a respiratory overshoot. A species-specific response was found in respiration rate during 20% CO₂: the more tolerant species maintained respiration rates, whereas the sensitive species showed a decreased respiration rate as low as after anoxia. The results indicate that during 20% CO₂ the sensitive species undergo a survival state. We conclude, that the ability to maintain respiration even under low oxygen and high CO₂ concentrations may allow the better adapted species to occupy an ecological niche in the field, where others cannot exist.