Uranium accumulation and toxicity in the green alga Chlamydomonas reinhardtii is modulated by pH.

The effects of pH on metal uptake and toxicity in aquatic organisms are currently poorly understood and remain an evolving topic in studies about the biotic ligand model (BLM). In the present study, the authors investigated how pH may influence long-term (4 d) uranium (U) accumulation and chronic toxicity in batch cultures of the freshwater green alga Chlamydomonas reinhardtii. The toxicity expressed as a function of the free uranyl ion was much greater at pH 7 (effective concentration, 50% [EC50] = 1.8 × 10(-9)  M UO2 (2+) ) than at pH 5 (EC50 = 1.2 × 10(-7)  M UO2 (2+) ). The net accumulation rate of U in algal cells was much higher at pH 7 than at pH 5 for the same free [UO2 (2+) ], but the cells exposed at pH 5 were also more sensitive to intracellular U than the cells at pH 7 with EC50s of 4.0 × 10(-15) and 7.1 × 10(-13)  mol of internalized U cell(-1) , respectively. The higher cellular sensitivity to U at pH 5 than at pH 7 could be explained partly by the increase in cytosolic U binding to algal soluble proteins or enzymes at pH 5 as observed by subcellular fractionation. To predict U accumulation and toxicity in algae accurately, the important modulating effects of pH on U accumulation and U cellular sensitivity should be considered in the BLM.