TRACE METAL SPECIATION AND TOXICITY 

 IN PHYTOPLANKTON CULTURES 



F.M.M. Morel, N.M.L. Morel, D.M. Anderson, 



D.M. McKnight and J.G. Rueter, Jr. 



Division of Water Resources 



and Environmental Engineering 



Civil Engineering Department 



Massachusetts Institute of Technology 



Cambridge, Massachusetts 02139 



ABSTRACT 



The toxicity of trace metals to phytoplankton has been demonstrated to 

 depend on metal ion activities. The various chemical processes that control 

 metal speciation, and thus activities in aquatic systems, are inorganic 

 complexation, chelation, precipitation and adsorption. For example, the 

 activity of metals such as mercury, cadmium or lead are controlled in saline 

 waters of low organic content by the formation of chloride and bromide 

 inorganic complexes. For mercury, this is also the case in typical 

 phytoplankton culturing media. Artificial chelating agents permit convenient 

 manipulation of metal ion activities in algal toxicity experiments. However, 

 kinetic phenomena can result in transient peaks in metal ion activities and lead 

 to large overestimations of toxicity. The release of metal complexing agents by 

 algae is not expected, in general, to affect markedly the chemistry of metals in 

 highly chelated artificial media except in cases of high specific affinity. The 

 greatest complication in interpretation of phytoplankton toxicity 

 experiments arises from the presence of solids in the culture medium. These 

 can precipitate during the preparation of the medium, or as a result of the 

 pH increase due to photosynthetic carbon uptake. The kinetics of 

 precipitation of these solids, their aging and the adsorption of trace metals 

 on their surface, lead to variations in metal activities that are difficult to 

 quantify, and do not permit proper assessment of the toxic effects. 

 Understanding the global aquatic chemistry of trace metals in algal culture 

 media, is a sine qua non prerequisite to proper design and interpretation of 

 toxicity experiments. 



INTRODUCTION 



Using copper as the principal example, this study aims at establishing a 

 chemical framework for the study of laboratory and natural processes involving 

 trace metal toxicity to phytoplankton. If it is a reasonable assumption that the 



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