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. 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 (CaCO₃) to generate shells or skeletons. Studies of potential effects of future levels of pCO₂ 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 pCO₂ 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 (ΩAᵣ = 0.71), the CaCO₃ 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.