AUTHOR=Hu Marian Y. , Michael Katharina , Kreiss Cornelia M. , Stumpp Meike , Dupont Sam , Tseng Yung-Che , Lucassen Magnus 

TITLE=Temperature Modulates the Effects of Ocean Acidification on Intestinal Ion Transport in Atlantic Cod, Gadus morhua

JOURNAL=Frontiers in Physiology

VOLUME=7

YEAR=2016

URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2016.00198

DOI=10.3389/fphys.2016.00198

ISSN=1664-042X

ABSTRACT=<p>CO<sub>2</sub>-driven seawater acidification has been demonstrated to enhance intestinal bicarbonate secretion rates in teleosts, leading to an increased release of CaCO<sub>3</sub> under simulated ocean acidification scenarios. In this study, we investigated if increasing CO<sub>2</sub> levels stimulate the intestinal acid–base regulatory machinery of Atlantic cod (<italic>Gadus morhua</italic>) and whether temperatures at the upper limit of thermal tolerance stimulate or counteract ion regulatory capacities. Juvenile <italic>G. morhua</italic> were acclimated for 4 weeks to three CO<sub>2</sub> levels (550, 1200, and 2200 μatm) covering present and near-future natural variability, at optimum (10°C) and summer maximum temperature (18°C), respectively. Immunohistochemical analyses revealed the subcellular localization of ion transporters, including Na<sup>+</sup>/K<sup>+</sup>-ATPase (NKA), Na<sup>+</sup>/H<sup>+</sup>-exchanger 3 (NHE3), Na<sup>+</sup>/<inline-formula><mml:math id="M1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>HCO</mml:mtext></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> cotransporter (NBC1), pendrin-like Cl<sup>−</sup>/<inline-formula><mml:math id="M2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>HCO</mml:mtext></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> exchanger (SLC26a6), V-type H<sup>+</sup>-ATPase subunit a (VHA), and Cl<sup>−</sup> channel 3 (CLC3) in epithelial cells of the anterior intestine. At 10°C, proteins and mRNA were generally up-regulated for most transporters in the intestinal epithelium after acclimation to higher CO<sub>2</sub> levels. This supports recent findings demonstrating increased intestinal <inline-formula><mml:math id="M3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>HCO</mml:mtext></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> secretion rates in response to CO<sub>2</sub> induced seawater acidification. At 18°C, mRNA expression and protein concentrations of most ion transporters remained unchanged or were even decreased, suggesting thermal compensation. This response may be energetically favorable to retain blood <inline-formula><mml:math id="M4" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>HCO</mml:mtext></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> levels to stabilize pH<sub>e,</sub> but may negatively affect intestinal salt and water resorption of marine teleosts in future oceans.</p>