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

Abstract

Abstract The effects of increased CO2 on the physiological responses of European sea bass (Dicentrarchus labrax) which were progressively 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 NKA, NKCC and ammonia transporters (e.g. Rhesus glycoproteins) were examined to understand the iono- and osmoregulatory consequences of the experimental conditions. A transient but significant increase in the blood pH of exposed fish acclimated at 10 ppt (day 1) and 2.5 ppt (day 21) possibly due to an overshoot of the blood HCO3- accumulation. However, no change was seen at 32 ppt. Additionally, plasma [Na+] of exposed fish reared at 10 ppt was significantly reduced at day 1 relative to control fish. 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. We also found that NKA was upregulated in gill of juvenile sea bass and NKA Mrna level of control fish was relatively higher when acclimated to lower salinities compared to 32 ppt control group. Elevated expression could be due to increased activity of acid-base transporters, which rely on the Na + gradient created by the NKA enzyme. Meanwhile, 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 which could be an adaptive response to foster ion retention during hypercapnia in sea bass acclimated at lower salinities. Furthermore, Rhesus glycoproteins were generally upregulated in the fish acclimated at lower salinities. It suggests that increased CO2 may enhance ammonia excretion rate.