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

Ocean acidification is a well recognised threat to marine ecosystems. Highlatitude regions are predicted to be particularly affected due to cold watersand naturally low carbonate saturation levels. This is of concern for organismsutilising calcium carbonate (CaCO₃) to generate shells or skeletons.Studies of potential effects of future levels of pCO₂ on high latitudecalcifiers are at present limited, and there is little understanding of theirpotential to acclimate to these changes. We describe a laboratory experimentto compare physiological and metabolic responses of a key benthic bivalve, Laternulaelliptica, at pCO₂ levels of their natural environment(430 µatm, pH 7.99; based on field measurements) with those predictedfor 2100 (735 µatm, pH 7.78) and glacial levels (187 µatm, pH8.32). Adult L. elliptica basal metabolism (oxygen consumptionrates) and heat shock protein HSP70 gene expression levelsincreased in response both to lowering and elevation of pH. Expression ofchitin synthase (CHS), a key enzyme involved in synthesisof bivalve shells, was significantly up-regulated in individuals at pH 7.78,indicating L. elliptica were working harder to calcify inseawater undersaturated in aragonite (Ω_Ar = 0.71),the CaCO₃ polymorph of which their shells are comprised. The differentresponse variables were influenced by pH in differing ways, highlighting theimportance of assessing a variety of factors to determine the likely impactof pH change. In combination, the results indicate a negative effect of oceanacidification on whole-organism functioning of L. ellipticaover relatively short terms (weeks-months) that may be energetically difficultto maintain over longer time periods. Importantly, however, the observed changesin L. elliptica CHS gene expression provides evidence forbiological control over the shell formation process, which may enable somedegree of adaptation or acclimation to future ocean acidification scenarios.