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 photoplankton 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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