Quantification of the increase in Pb bioavailability to marine organisms caused by different types of DOM from terrestrial and river origin.

Dissolved organic matter (DOM) has metal binding properties and decreases the concentration of free metal ions in the aquatic environments. However, humic acids (HA) have been shown to increase Pb bioavailability for marine invertebrates, including Pb toxicity to sea urchin embryos. The low solubility and other properties of commercially available HA, which may not be the most representative fraction of DOM in natural waters, limit the environmental relevance of these findings. The present study tested the effect of more soluble DOM, fulvic acids (FA) and DOM extracted from the Suwannee River (SRDOM), on the acute toxicity of Pb to Paracentrotus lividus embryos. It was confirmed that FA and SRDOM, despite their Pb binding properties, also increased Pb toxicity for sea urchin embryos, even though this increase was not as high as that previously observed for HA. The enhancing effect of DOM on Pb bioavailavility was quantified using multiple regression equations for the present data, as well as for previously published data on Pb uptake by the gills of the marine mussel Mytilus sp. and Pb internalization by the marine microalgae Isochrysis galbana and Thalassiosira weissflogii. These results confirm that dissolved Pb bioavailability in seawater in the presence of DOM is higher than predicted using current bioavailability models based on chemical equilibrium chemistry. The experimental evidence suggests that the mechanisms by which DOM enhances Pb uptake and toxicity implies direct contact of the organic compounds with the plasma membrane.