Biological uptake of Cd, Se(IV) and Zn by Chlamydomonas reinhardtii in response to different phosphate and nitrate additions

We investigated the influences of different nutrient regimes on the accumulation of Cd, Zn, and Se(IV) in the freshwater green alga Chlamydomonas reinhardtii over a 4 h exposure period. After the cells had been acclimated to different ambient P levels from 0.1 to 10.0 µM for 2 d in differ- ent media (buffer, basic and simplified uptake media), their intracellular metal concentrations increased by 3.6 to 14× for Cd and 1.6 to 4.0× for Zn. Se uptake was, however, decreased by 7.7 to 44× . Semi-continuous culture experiments further demonstrated that the alga's metal concentrations were enhanced 26× for Cd and 8.0× for Zn, whereas Se accumulation was inhibited 75× at the same medium P levels. The uptake rates in semi-continuous cultures increased by 269× for Cd and 11× for Zn, but Se uptake decreased 92× with increasing P concentration. Medium P additions also signifi- cantly increased intracellular partitioning for Cd and Zn, but there was no apparent effect for Se. For P-starved or P-repleted cells, the P-enriched medium dramatically increased the cellular accumula- tion of Cd and Zn compared with the P-depleted medium. A surge uptake of Cd and Zn occurred in P-starved cells. P starvation also resulted in a remarkable increase in Se accumulation in the P- deplete medium. N enrichment from 5 to 200 µM significantly increased the alga's uptake and intra- cellular partitioning for Cd and Zn, although the effects were not strictly concentration-dependent. Our study strongly suggests that ambient P enrichment greatly stimulates Cd and Zn accumulation and sequestration in C. reinhardtii, presumably due to rapid P uptake and subsequent formation of polyphosphate in the cells, and distinctly inhibits Se uptake possibly due to the competition of similar anionic forms for transport between Se and P.