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

Abstract

CO2-driven seawater acidification has been demonstrated to enhance intestinal bicarbonate secretion rates in teleosts, leading to an increased release of CaCO3 under simulated ocean acidification scenarios. In this study, we investigated if increasing CO2 levels stimulate the intestinal acid–base regulatory machinery of Atlantic cod (Gadus morhua) and whether temperatures at the upper limit of thermal tolerance stimulate or counteract ion regulatory capacities. Juvenile G. morhua were acclimated for 4 weeks to three CO2 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+/K+-ATPase (NKA), Na+/H+-exchanger 3 (NHE3), Na+/HCO3− cotransporter (NBC1), pendrin-like Cl−/HCO3− exchanger (SLC26a6), V-type H+-ATPase subunit a (VHA), and Cl− 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 CO2 levels. This supports recent findings demonstrating increased intestinal HCO3− secretion rates in response to CO2 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 HCO3− levels to stabilize pHe, but may negatively affect intestinal salt and water resorption of marine teleosts in future oceans.

Elevated CO2 increases energetic cost and ion movement in the marine fish intestine

“…The demand to secrete HCO 3 − in the intestine to maintain water balance is well-conserved across marine teleosts 30 . Increased intestinal HCO 3 − secretion with CO 2 exposure as reported here has also been demonstrated at higher CO 2 levels in the plainfin midshipmen (~50,000 μatm CO 2 ) 46 , in the toadfish (5000–20,000 μatm CO 2 ) 23 , and suggested from gene expression and/or protein assays in the Japanese ricefish (7000 μatm CO 2 ) 34 and the Atlantic cod (1,200 and 2,200 μatm CO 2 ) 41 . These studies suggest that increased intestinal HCO 3 − secretion and metabolic demand during CO 2 exposure could be a ubiquitous response to elevated CO 2 throughout marine bony fishes.…”

“…Altered mitochondrial capacity 5 , shifts in energetic budgets 34 38 39 , increased expression/activity of gill and intestinal ionoregulatory genes and proteins 15 , increased ventilation 40 , and increased protein turnover 38 have all been noted during ocean acidification-relevant CO 2 exposures with little impact to whole-animal measurements. The importance of integrating multiple techniques is apparent in a recent study on CO 2 exposure (1,200 and 2,200 μatm CO 2 ) in Atlantic cod exhibiting increased intestinal NKA mRNA and protein concentration, while exhibiting no change in NKA protein activity 41 . While the present study of isolated intestinal tissue precluded normal hormonal cascades or feedback mechanisms, it offered the advantage of careful control of blood-side (serosal) saline conditions and made it possible to identify mechanistic differences in tissue function that were impossible to observe in previous in vivo work 22 .…”

“…Calculations using estimates of toadfish standard metabolic rate 43 suggest that the energetic cost of CO 2 acclimation in the intestine would account ~0.5% of whole animal O 2 consumption 21 . Albeit small, any factor that promotes an energy reallocation or increases standard metabolic rate could exacerbate already narrow metabolic constraints 44 or possibly interact with projected temperature elevations to increase overall impact 36 39 41 45 .…”

Ocean acidification promotes broad transcriptomic responses in marine metazoans: a literature survey

“…Follow-up studies suggest that rather than a decreased production of ATP in response to OA, S. purpuatus larvae reallocate the total ATP produced; more ATP goes toward protein synthesis and ion transport when exposed to high pCO 2 , leaving less energy to maintain other cellular functions [83]. However, larvae from Strongylocentrotus purpuratus Embryos/Larvae [78][79][80][81][82][83][84][85][86] Adults [87] Chordata Fish Acanthochromis polyacanthus Juveniles [88,89] Dicentrarchus labrax Larvae [90] Gadus morhua Adults [91,92] Oryzias latipes Embryos/Hatchlings/Adults [93] Pagothenia borchgrevinki Adults [94] Sciaenops ocellatus Adults [95] Sebastes caurinus Juveniles [96] Sebastes mystinus Juveniles [96] Trematomus bernacchii Adults [97] Fig. 2 Exposure times in studies examining gene expression responses to ocean acidification across life-history stages.…”

“…Marine fishes are thought to be more robust to increases in pCO 2 than invertebrates due to their high capacity to maintain acid-base homeostasis, although there is variability in sensitivities across fish taxa [131]. Atlantic cod, Gadus morhua, exposed to 3 different pCO 2 levels (550 μatm, 1200 μatm, 2200 μatm) across 2 temperatures (10°C, 18°C) for 4 weeks showed temperaturedependent responses in multiple ion transport proteins, often with dose-dependent responses across pCO 2 levels in gill tissue [91], while temperature had a stronger impact on expression of these same ion transport proteins in intestines [92]. These studies are further substantiated by protein levels of the same transport proteins.…”