Physiological responses of European sea bass (Dicentrarchus labrax) exposed to increased carbon dioxide and reduced seawater salinities

The iono- and osmoregulatory capacities of marine teleosts, such as European sea bass (Dicentrarchus labrax) are expected to be challenged by high carbon dioxide exposure, and the adverse effects of elevated CO₂ could be amplified when such fish migrate into less buffered hypo-osmotic estuarine environments. Therefore, the effects of increased CO₂ on the physiological responses of European sea bass (Dicentrarchus labrax) acclimated to 32 ppt, 10 ppt and 2.5 ppt were investigated.

Following acclimation to different salinities for two weeks, fish were exposed to present-day (400 µatm) and future (1000 µatm) atmospheric CO₂ for 1, 3, 7 and 21 days. Blood pH, plasma ions (Na+, K+, Cl-), branchial mRNA expression of ion transporters such as Na⁺/K⁺–ATPase (NKA), Na⁺/K⁺/2Cl^– co-transporters (NKCC) and ammonia transporters (e.g. Rhesus glycoproteins Rhbg, Rhcg1 and Rhcg2) were examined to understand the iono- and osmoregulatory consequences of elevated CO₂.

A transient but significant increase in the blood pH of exposed fish acclimated at 10 ppt (day 1) and 2.5 ppt (day 21) was observed possibly due to an overshoot of the blood HCO₃⁻ accumulation while a significant reduction of blood pH was observed after 21 days at 2.5ppt. However, no change was seen at 32 ppt. Generally, Na + concentration of control fish was relatively higher at 10 ppt and lower at 2.5 ppt compared to 32 ppt control group at all sampling periods. Additionally, NKA was upregulated in gill of juvenile sea bass when acclimated to lower salinities compared to 32 ppt control group. CO₂ exposure generally downregulated NKA mRNA expression at 32ppt (day 1), 10 ppt (days 3, 7 and 21) and 2.5ppt (days 1 and 7) and also a significant reduction of NKCC mRNA level of the exposed fish acclimated at 32 ppt (1–3 days) and 10 ppt (7–21 days) was observed. Furthermore, Rhesus glycoproteins were generally upregulated in the fish acclimated at lower salinities indicating a higher dependance on gill ammonia excretion. Increased CO₂ led to a reduced expression of Rhbg and may therefore reduce ammonia excretion rate.

Juvenile sea bass were relatively successful in keeping acid base balance under an ocean acidification scenario. However, this came at a cost for ionoregulation with reduced NKA, NKCC and Rhbg expression rates as a consequence.