Rising CO(2) 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 (CO(2)) while oxygen (O(2)) declines. This is rarely accounted for when assessing hypoxia tolerances of aquatic organisms. We investigated the impact of environmentally realistic increases in CO(2) 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 O(2) levels were reduced with constant ambient CO(2) levels (~530 µatm), or with reciprocal increases in CO(2) (rising to ~2,500 µatm). We also assessed blood acid-base chemistry and haemoglobin-O(2) binding affinity of sea bass in hypoxic conditions with ambient (~650 μatm) or raised CO(2) (~1770 μatm) levels. Sea bass exhibited greater hypoxia tolerance (~20% reduced O(2crit)), associated with increased haemoglobin-O(2) affinity (~32% fall in P(50)) of red blood cells, when exposed to reciprocal changes in O(2) and CO(2). This indicates that rising CO(2) which accompanies environmental hypoxia facilitates increased O(2) uptake by the blood in low O(2) 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 CO(2).