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.
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