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 tlie 

 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 (lO'^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 

 system 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" 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 



