Humic and fulvic acids from lakes in Ontario, Canada, have been isolated and been examined for their ability to bind methylmercury(II). Equilibrium dialysis has been applied to differentiate between bound and unbound fractions of MeHg(II). Losses of methylmercury(II) from solutions containing 0.5-100 ng/L MeHg(II) and 10 mg/L humic substances (HS) are negligible over a 24-h time interval when using acid-cleaned glass beakers, membrane closures, and cellulose ester membranes having a molecular weight cutoff of 500. Dialysis equilibrium for MeHg(II) is reached within this period of time. For each humic material, two classes of binding sites could be detected. Log K values of conditional stability constants for the strong and the weaker binding sites were in the range of 13.02-14.48 and 12.15-13.07, respectively. Binding capacities have been calculated as 0.2-13 of ng MeHg(II)/mg of HS for the strong sites and 1.2-72 ng of MeHg(II)/mg of HS for the weaker sites. Calculation of equilibrium distribution shows that methylmercury(II)-HS complexes predominate in oxidized freshwaters with low sulfide concentrations. In anoxic waters, CH 3 HgSH should prevail. The CH 3 HgCl complex is too weak to compete with either of the latter complexes.

Environmental Toxicology is a comprehensive introductory textbook dealing with most aspects of the subject, from the molecular to the ecosystem level. Early chapters deal with basic and advanced concepts, methods and approaches. The next tier discusses the environmental toxicology of individual or groups of substances. The third part addresses complex issues, in which many of the concepts, approaches and substances covered in earlier tiers are incorporated. The fourth part includes chapters on risk assessment, rehabilitation and regulatory toxicology. The book concludes with a summary of present and future areas of emphasis. Each chapter contains a comprehensive list of references and further reading, case studies from different jurisdictions, and student exercises. Environmental Toxicology is primarily a textbook for undergraduate and graduate students in environmental toxicology, environmental chemistry, ecotoxicology, applied ecology, environmental management, and risk assessment. It will also be valuable for specialists in ecology, environmental science, and chemistry.

The cell walls of plants, including those of algae, have the capacity to bind metal ions in negatively charged sites. The authors had already shown that the wild type (walled) strain of the unicellular green alga Chlamydomonas reinhardtii Dangeard was more tolerant to Cd, Co, Cu, and Ni than a wall-less mutant of the same species. The objective of the present study was to determine if the tolerance to metals was associated with an increased adsorption of the same metals to the cell wall. Adsorbed metal was defined as that fraction that could be removed with a solution containing Na(2)EDTA and CaCl(2). The fraction that remained after the EDTA/CaCl(2) wash was considered to be strongly bound in the cell. When exposed to metals, singly, in solution for 24 h, cells of both strains accumulated the metals. The original hypothesis was supported by the results for Cd, Co, and Ni insofar as significantly higher concentrations of these metals were in the loosely bound fraction of the walled strain in comparison with the wall-less strain. However, there are three reasons why the potentially protective effect of the cell wall did not explain differential tolerance of the two strains. After 24 h of exposure (1) less Cd was accumulated internally by the wall-less strain than by the walled strain, (2) very little of the accumulated Cu was in the loosely bound fraction of the walled strain, and (3) the two strains accumulated comparable and relatively high amounts of internal Cu. Unexpectedly, significant amounts of Cd and Cu were also removable from the surface of the wall-less cells. One possible explanation for these apparently externally bound metals in the wall-less strain is that the cells exuded metal-chelating molecules that decreased the ability of metal ions to penetrate the plasma membrane. It was concluded that metal tolerance in this alga must involve a complex of mechanisms involving both internal and external detoxification of metal ions.

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