Rising CO₂ enhances hypoxia tolerance in a marine fish

Global environmental change is increasing hypoxia in aquatic ecosystems. During hypoxic events, bacterial respiration causes an increase in carbon dioxide (CO₂) while oxygen (O₂) declines. This is rarely accounted for when assessing hypoxia tolerances of aquatic organisms. We investigated the impact of environmentally realistic increases in CO₂ on responses to hypoxia in European sea bass (Dicentrarchus labrax). We conducted a critical oxygen (O_2crit) test, a common measure of hypoxia tolerance, using two treatments in which O₂ levels were reduced with constant ambient CO₂ levels (~530 µatm), or with reciprocal increases in CO₂ (rising to ~2,500 µatm). We also assessed blood acid-base chemistry and haemoglobin-O₂ binding affinity of sea bass in hypoxic conditions with ambient (~650 μatm) or raised CO₂ (~1770 μatm) levels. Sea bass exhibited greater hypoxia tolerance (~20% reduced O_2crit), associated with increased haemoglobin-O₂ affinity (~32% fall in P₅₀) of red blood cells, when exposed to reciprocal changes in O₂ and CO₂. This indicates that rising CO₂ which accompanies environmental hypoxia facilitates increased O₂ uptake by the blood in low O₂ conditions, enhancing hypoxia tolerance. We recommend that when impacts of hypoxia on aquatic organisms are assessed, due consideration is given to associated environmental increases in CO₂.