In artificial media where the analytical concentrations of the components 
are precisely known, the exact composition of the system, including all soluble 
and insoluble species of the various metals and their activities, can be 
computed if a state of equilibrium or partial equilibrium is established (43). It 
should be underlined that such calculations of chemical speciation depend on 
correct identification of the principal species, and knowledge of the 
corresponding equilibrium constants. The calculations presented here were 
performed with the computer programs, REDEQL (25, 21) and MINEQL(49) 
which contain a list of possible species, and a selection of the necessary 
thermodynamic constants from a variety of sources (36, 38, 33). 
Results of chemical equilibrium computations of three media recipes, the 
freshwater medium WC (11) and the seawater media F/2 (12) and Aquil (26), 
are shown in Table 4-1. Possible adsorption processes are not considered in 
these calculations. Note that some heavy metals (Pb, Cd, Hg, Ni, Co, Cr) which 
are not part of the recipes, have been added in trace amounts (10‘^M) to 
illustrate how they would be speciated if they were present as contaminants in 
the media. Such low metal concentrations affect the rest of the chemical 
systems negligibly. Heavy metal speciation in all media is completely 
dominated by the chelation with ethylenediaminetatraacetate (EDTA) which is 
included in the recipes for the very purpose of chelating metals. An important 
exception is mercury, which, according to the computations, is present entirely 
as chloride complexes in F/2 and Aquil and half as hydroxide species in WC. 
For all metals, the free ion activities are several orders of magnitude smaller 
than their total concentrations. In all media, iron and manganese oxides and 
calcium phosphate (hydroxylapatite) are computed to precipitate at 
equilibrium. Calcium carbonate (calcite) is also shown to be saturated in the 
seawater media. Actual precipitation of these various solids is dependent on 
kinetic processes as will be discussed later. 
The trace metal chemistry of such culturing media can be grossly affected by 
the presence of algal cells due to metal uptake. For example, typical values for 
the uptake of copper by phytoplankton are in the range 10' 1(5 to 10'^ 
moles/cell (44, 35, 16). With the algal densities and the copper concentrations 
commonly used, a sizeable part of the total concentration of copper in the 
medium can thus be taken up by the cells. This underlines the necessity of 
“buffering” metal ion activities in toxicity experiments in order to render these 
activities relatively insensitive to total metal concentrations. The use of various 
chelating agents for this purpose will be discussed later. 
INORGANIC COMPLEXATION 
It has been observed in several instances that the toxicity of metals such as 
lead, cadmium, mercury or silver to a variety of organisms, from bacteria to 
40 
