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 (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. 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.
Highlights
The ocean and the land are ‘sinks’ for the excess atmospheric CO2 produced by burning of fossil fuels, deforestation and land use changes
To convert O2 consumption values to per unit ash free dry weight (AFDW; methods provided below), we derived the following relationship between L. elliptica AFDW and wet tissue weight: AFDW = 0.37014 wet weight0.55134 ; R2 = 0.63 As there were no differences in this relationship between animals from different treatments we considered this to be a good estimator of AFDW for the O2 consumption animals
After just 21 days we found that lowering pH resulted in a significant increase in expression of the chitin synthase (CHS) gene, which codes for a key enzyme involved in synthesis of bivalve shells [28,67], indicating the animals were working harder to calcify
Summary
The ocean and the land are ‘sinks’ for the excess atmospheric CO2 produced by burning of fossil fuels, deforestation and land use changes. Twenty five to thirty percent of the total anthropogenic CO2 emissions produced since the industrial revolution have been absorbed by the oceans [1] This excess CO2 dissolves in the surface ocean, causing increased hydrogen ion (H+) concentrations and decreased carbonate ion (CO322) concentrations in seawater [2]. The taxonomic groups considered most susceptible to ocean acidification are calcifying organisms with CaCO3 skeletons and shells, such as corals, coralline algae, coccolithophores and molluscs. This is due to the predicted reduced availability of the CO322 ions they require for precipitation of CaCO3 [5,7]. Too, not all species, nor all life stages of a particular species, have responded negatively to pCO2 conditions predicted for the future (e.g., [16,17])
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