Cadmium partition coefficients of cultured benthic foraminifera Ammonia beccarii

Cadmium partition coefficients (D-Cd) between calcitic tests of the benthic foraminifera Ammonia beccarii (Linne) and brackish water were measured from sediment free microcosms. Dissolved cadmium concentrations were prepared at 4.3, 6.4 and 9.0 nmol l(-1) at 18degreesC and 2.3, 5.3 nmol l(-1) at 13degreesC and we obtained D-Cd of 1.8 +/- 0.2 (n = 7), 1.0 +/- 0.1 (n = 8), 1.2 +/- 0.2 (n = 19), 2.8 +/- 0.6 (n = 6) and 1.0 +/- 0.2 (n = 10), respectively. The values of D-Cd close to 1.0 show that foraminiferal carbonates do not lead to a cadmium segregation with the surrounding water and in the light of these data, a published biomineralization model is discussed. The hypothesis of a diffusion limited uptake of ions during the mineralization process is developed to explain D-Cd = 1.0. The hypothesis is tested by calculations in the case of sea water (warm surface, S = 35 psu) and culture experiments. These calculations revealed that carbonate diffusion time are consistent with the chamber formation time usually observed in culture experiments for this species. It shows that CO32+ is the first limiting ion but does not allow us to conclude that Ca2+ uptake is also diffusion limited. Equations showed that ion diffusion coefficients could also control D-Cd. From cultures 2.3 and 4.3 nmol l(-1), D-Cd was 2.8 +/- 0.6 and 1.8 +/- 0.2, respectively. The assumption of remaining living Dunaliella cells in the food of these cultures could lead to an unexpected absorption of Cd2+. The hypothesis of solution mixture during experiment stoping a diffusion limited uptake, could also explain these partitioning values.