Abstract. Coral calcification is strongly dependent on both the pH and the dissolved inorganic carbon (DIC) of the calcifying fluid. Skeletal oxygen and carbon isotope fractionation of high-Mg calcite skeletons of \textit{Octocorallia} corals directly record the biological manipulation on sources of DIC in response to environmental changes. The coral skeletons were enriched in light isotopes (¹⁶O and ¹²C) relative to the expected values based on habitat environmental parameters and Mg/Ca of the skeletons. The differences between the expected and observed values ranged from −4.66 to −1.53 for δ¹⁸O and from −7.34 to −1.75 for δ¹³C. The large variability cannot be explained by the ambient environment, the contribution of metabolic carbon, or the precipitation rate of the skeleton. Therefore, the most plausible explanation for the observed O and C isotope differences in high-Mg calcite coral skeletons is the existence of two carbon sources, aqueous carbon dioxide in the calcifying fluid and dissolved inorganic carbon in seawater. Positive correlations of B/Ca with δ¹⁸O and δ¹³C suggest that skeletal isotopic compositions are enriched in light isotopes when conditions are less alkaline. Therefore, the relative contribution of isotopically heavy DIC from seawater through the skeleton and pericellular channels decreases under the reduced pH of the extracytoplasmic calcifying fluid. Our data suggest an even stronger biological effect under lower pH. Skeletal δ¹⁸O and δ¹³C values record the response of the sources of DIC in the coral calcifying fluids to ambient seawater pH. These changes give insight into how ocean acidification impacts the physiological mechanisms as well as the pH offset between calcifying fluid and seawater in response to ocean acidification.