| Abstract
| - To better understand the relationship between leadspeciation and bioavailability in natural freshwaters, theinteraction of lead with the freshwater alga Chlorella kessleriiwas studied in the presence of the Suwannee Riverfulvic acid (SRFA). Special attention was paid to directinteractions of the fulvic acid on the algae, as well aspotential physiological (membrane permeability and algalmetabolism) influences. Lead-free ion concentrationmeasurements were carried out using a novel ion-selectiveelectrode. Pb uptake decreased in the presence of SRFAwith respect to noncomplexed Pb, but uptake fluxes, cellularPb, Pb bound to the transport sites, and total adsorbedPb were all higher than predicted from Pb2+ activities, inaccordance with the free ion activity model (FIAM).The discrepancies between the observed values andthose predicted by the FIAM in the presence and absenceof synthetic ligands increased with increasing concentrationof SRFA. Several hypotheses were examined to explainthe observed differences. No contributions of labile and/or hydrophobic Pb−SRFA complexes were found.Furthermore, direct biological effects, including variationsin membrane permeability or algal metabolism, couldnot account for the observations. On the other hand, changesin the algal surface charge due to SRFA adsorptionseemed to account, at least partially, for the observedincrease in lead uptake in the presence of SRFA as comparedto that corresponding to the same Pb2+ concentration inthe presence of synthetic ligands.
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