Ocean acidification at high latitudes: potential effects on functioning of the Antarctic bivalve Laternula elliptica

Abstract

Ocean acidification is a well recognised threat to marine ecosystems. High latitude regions are predicted to be particularly affected due to cold waters and naturally low carbonate saturation levels. This is of concern for organisms utilising calcium carbonate (CaCO(3)) to generate shells or skeletons. Studies of potential effects of future levels of pCO(2) on high latitude calcifiers are at present limited, and there is little understanding of their potential to acclimate to these changes. We describe a laboratory experiment to compare physiological and metabolic responses of a key benthic bivalve, Laternula elliptica, at pCO(2) levels of their natural environment (430 µatm, pH 7.99; based on field measurements) with those predicted for 2100 (735 µatm, pH 7.78) and glacial levels (187 µatm, pH 8.32). Adult L. elliptica basal metabolism (oxygen consumption rates) and heat shock protein HSP70 gene expression levels increased in response both to lowering and elevation of pH. Expression of chitin synthase (CHS), a key enzyme involved in synthesis of bivalve shells, was significantly up-regulated in individuals at pH 7.78, indicating L. elliptica were working harder to calcify in seawater undersaturated in aragonite (Ω(Ar) = 0.71), the CaCO(3) polymorph of which their shells are comprised. The different response variables were influenced by pH in differing ways, highlighting the importance of assessing a variety of factors to determine the likely impact of pH change. In combination, the results indicate a negative effect of ocean acidification on whole-organism functioning of L. elliptica over relatively short terms (weeks-months) that may be energetically difficult to maintain over longer time periods. Importantly, however, the observed changes in L. elliptica CHS gene expression provides evidence for biological control over the shell formation process, which may enable some degree of adaptation or acclimation to future ocean acidification scenarios.

Figure 1. mRNA expression of HSP70 in Laternula elliptica mantle tissue after 21 days at experimental pH.

Levels shown are means (+ SE) relative to β-actin (expressed as a ratio, fold induction).

Figure 2. mRNA expression of CHS in Laternula elliptica mantle tissue after 21 days at experimental pH.

Levels shown are means (+ SE) relative to β-actin (expressed as a ratio, fold induction).

Figure 3. Laternula elliptica adductor tissue Total RNA_adj after A. 21 and B. 120 days at experimental pH.

Figure 4. O₂ consumption (µmol O₂ g⁻¹ AFDW h⁻¹) of Laternula elliptica after 120 days at experimental pH.

Figure 5. Physiological condition of Laternula elliptica in each experimental treatment on Day 120.

Condition of individuals on Day 0 is also shown.

Figure 6. The change in each physiological condition index between Days 0 and 120.

The % change that this represents is also given.