Bioaccumulation of waterborne Ni in Dreissena polymorpha: a stable isotope experiment to assess the effect of zinc, calcium, and dissolved organic matter.

The effect of Ca, Zn, and dissolved natural organic matter (NOM) on waterborne Ni accumulation was investigated in a freshwater mussel. An enriched stable metal isotope tracer was required to measure the Ni uptake rate accurately. Zebra mussels were exposed to environmentally relevant concentrations of (62) Ni (from 0.5 to 8 µg/L) for 48 h in media spiked with Ca, Zn, or dissolved NOM. The (62)Ni uptake was inhibited by Ca (from 0.138 ± 0.021 to 0.061 ± 0.010 L/g/d for Ca concentrations ranging from 43 to 133 mg/L) and enhanced by Zn (from 0.051 ± 0.006 to 0.109 ± 0.007 L/g/d for Zn concentrations ranging from 6.6 to 38.3 µg/L). The mechanisms behind the synergistic effect of Zn remain unclear, yet it can be hypothesized that Ni uptake is facilitated by Zn-dependent transport sites. To formalize the effects of Ca and Zn, a model was proposed to express the Ni uptake rate as a function of the mussels' filtration rate and of Ca and Zn concentrations. The (62)Ni uptake increased at low NOM concentrations and decreased at higher concentrations. This could be explained by the influence of NOM on both the speciation of Ni and the filtration activity of mussels. At high NOM concentrations, a modification of the membrane's permeability might also have favored Ni uptake, although this was not clearly established in this study. Therefore, the effect of water composition on Ni bioavailability to zebra mussels cannot be predicted by competition and complexation models alone, because it also influences the animal's physiology.
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