| Summary: | 1. Ocean acidification by anthropogenic carbon dioxide emissions is projected to depress metabolic and physiological activity in marine calcifiers. To evaluate the sensitivity of marine organisms against ocean acidification, the assimilation of nutrients into carbonate shells and soft tissues must be examined. 2. We designed a novel experimental protocol, reverse radioisotope labelling, to trace partitioning of nutrients within a single bivalve species under ocean acidification conditions. Injecting CO 2 gas, free from radiocarbon, can provide a large contrast between carbon dissolved in the water and the one assimilated from atmosphere. By culturing modern aquifer organisms in acidified seawater, we were able to determine differences in the relative contributions of the end members, dissolved inorganic carbon (DIC) in seawater and metabolic CO 2 , to shell carbonate and soft tissues. 3. Under all pCO 2 conditions (463, 653, 872, 1137, and 1337 μatm), radiocarbon (Δ 14 C) values of the bivalve ( Scapharca broughtonii ) shell were significantly correlated with seawater DIC values; therefore, shell carbonate was derived principally from seawater DIC. The Δ 14 C results together with stable carbon isotope (δ 13 C) data suggest that in S. broughtonii shell δ 13 C may reflect the kinetics of isotopic equilibration as well as end-member contributions; thus, care must be taken when analyzing end-member contributions by a previous method using δ 13 C. The insensitivity of S. broughtonii to perturbations in pCO 2 up to at least 1337 µatm indicates that this species can withstand ocean acidification. 4. Usage of radioisotope to dope for tracer experiments requires strict rules to conduct any operations. Yet, reverse radioisotope labeling proposing in this study has a large advantage and is a powerful tool to understanding physiology of aquifer organisms that can be applicable to various organisms and culture experiments, such as temperature, salinity, and acidification experiments, to improve understanding of the ...
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