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 (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 .