Stable oxygen isotopes of Thoracosphaera heimii (Dinophyceae) in relationship to temperature; a culture experiment

To establish a relationship between temperature and the stable oxygen isotopic composition (δ18O) of vegetative cysts of the photosynthetic calcareous dinoflagellate cyst Thoracosphaera heimii, two unicellular cultures of T. heimii have been cultured under different temperatures by using a temperature gradient box. is a clear relationship between temperature variance and the isotopic composition of T. heimii cysts according to the relationship: T (°C) = − 6.827 (δ18Oc–δ18Ow)–3.906 (R = 0.921), with c = calcite and w = water. this paper we are the first to discuss the possible vital effects that might cause an offset between the temperature–isotope relationship found for T. heimii calcite and that of equilibrium inorganic calcite precipitation. No indication for strong kinetic effects as result of fast calcite precipitation can be found. We observed a positive relationship between δ18Oc–δ18Ow and ambient mediumwater pH. We speculate that this might be the result of the presence of external carbonate anhydrase, which is common in photosynthetic dinoflagellates. The efficiency of this enzyme increases rapidly between pH 7.5 to 9, which could result in an increase in CO2 uptake relative to HCO3− with increasing pH. We furthermore discuss the possibility of T. heimii using respirative carbon at least as part of its carbon source for calcite precipitation, which can be based on the light values of δ18Oc–δ18Ow and δ13Cc–δ13CDIC (DIC = dissolved inorganic carbon) found in this and previous studies on the isotopic composition of calcareous dinoflagellates. sults of this study as well as the broad geographic distribution of T. heimii, its stable position within the water column, its presence in the geological record since the Late Cretaceous and its resistance against dissolution compared to other plankton groups underlines the potential for a wide usability of the oxygen isotope composition of T. heimii as palaeotemperature proxy for the deeper parts of the photic zone.