Cadmium partition coefficients of cultured benthic foraminifera Ammonia beccarii

Abstract

Cadmium partition coefficients (D_Cd) between calcitic tests of the benthic foraminifera Ammonia beccarii (Linné) 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 18°C and 2.3, 5.3 nmol l^-1 at 13°C 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.