Intestinal response to ocean acidification in the European sea bass (Dicentrarchus labrax)

The intestine of marine fishes contributes to the ocean carbon cycle producing carbonate aggregates as part of the osmoregulatory process. Therefore, this study aimed to evaluate physiological adjustments of European sea bass (Dicentrarchus labrax) intestine to a higher pCO(2) environment likely in the near future (similar to 1700 mu atm). At the whole-body level, hypercapnia for 5 weeks resulted in fish having a significantly diminished specific growth rate, condition factor and hepatosomatic index. An increase in plasma osmolality and HCO3- concentration was detected, paralleled by decreased metabolites concentrations. In the intestine, high seawater pCO(2) was without effect on ouabain-sensitive ATPase activities, while Bafilomycin A1-sensitive ATPase activity significantly decreased in the anterior intestine. Anterior and mid intestine were mounted in Ussing chambers in order to measure bioelectrical parameters and bicarbonate secretion by pH-Stat ex-vivo. Hypercapnia induced a 2.3 and 2.8-fold increase in bicarbonate secretion rates in the anterior and mid intestine, respectively. In the intestinal fluid, HCO3- concentration increased 2.2-fold, and carbonate precipitates showed a 4.4-fold increase in response to hypercapnia, paralleled by a > 3-fold increase of drinking and a > 2-fold increase of intestinal volume at any given time. At the molecular level, hypercapnia elicited higher intestinal mRNA expression levels for atp6v1b (V-ATPase B subunit), slc4a4, slc26a3, and slc26a6, both in the anterior and mid intestine. As a whole, our results show that the intestine of sea bass responds to high seawater pCO(2), a response that comes at a cost at the whole-body level with an impact in the fish specific growth rate, condition factor, and hepatosomatic index.