We studied the relationship between macro-algae Chara (Stoneworts) calcite (CaCO3) encrustation and the speciation of cadmium (Cd) accumulated by the plant. Results showed that 17% of the total Cd (0.3 mg kg(-1)) accumulated by Chara fibrosa exposed to 1 mu g Cd L-1 was carbonate-bound. The percentage of carbonate-bound Cd in the plant exposed to 10 mu g Cd L-1 increased from 48% in young thalli (total Ca < 50 mg g(-1), total Cd: 125 mg kg(-1)) to 63% in calcified mature thalli (total Ca: 190 mg g(-1); total Cd: 134 mg kg(-1)). Based on mineral saturation calculation and reliability analysis of the sequential fractionation procedure, precipitation of otavite (CdCO3) and co-precipitation of Cd with calcite, occurring in the alkaline regions of Chara cell wall, are probably the mechanisms of carbonate-bound Cd formation. Thick marl sediment frequently found beneath charophyte meadows suggests a long-term storage of Ca as well as the precipitated or co-precipitated Cd in the sediment after the plant senescence and decomposition. (C) 2009 Elsevier B.V. All rights reserved.