THE CONTRIBUTION OF COMPLEXED COPPER TO THE METABOLIC INHIBITION OF ALGAE AND BACTERIA IN SYNTHETIC MEDIA AND RIVER WATER

The toxicity of copper is reduced due to complexation by ligands, e.g. humic acids or synthetic compounds like EDTA. Therefore, the concentr ation of free metal ions has been considered the main determinant of m etal toxicity. This early hypothesis was tested by adding different co ncentrations of copper to water from the River Rhine and to a syntheti c medium, supplemented with different concentrations of EDTA. Subseque ntly the following parameters were determined: (1) voltammetrically la bile copper (differential pulse anodic stripping voltammetry), (2) fra ction of copper retained on Chelex-100 columns, (3) photosynthetic rat e of the alga Selenastrum capricornutum (in synthetic medium) and (4) rate of [H-3]thymidine incorporation by multiplying bacteria (in river water). Addition of 5 muM Cu to the medium with 5 or 10 muM of EDTA i nhibited algal photosynthesis, although copper was not voltammetricall y detectable (<0.005 muM). Small spikes of copper (0.06-1.00 muM) adde d to river water inhibited the multiplication of bacteria; electrochem ical determination showed no detectable copper activity at the lower c oncentrations. Chelex-retained copper correlated well with the percent age inhibition of the two biological activities. According to calculat ions using the model TITRATOR copper-EDTA complexes and un-ionized sal ts (mainly CuCO3) were dominant copper species in synthetic solutions inhibiting photosynthesis. The GECHEQ model calculated that free coppe r as well as chelex-labile copper correlated well with the inhibition of bacterial growth rate. Therefore it seems that complexed copper is ''biologically available'' to a significant extent. Furthermore, inter mediate labile copper, such as chelex-retained copper, is an appropria te measure of copper toxicity.