The relationship between cupric ion activity and the toxicity of copper to phytoplankton

Abstract

The purpose of this investigation is to quantify the relationship between cupric ion activity and the toxicity of copper to phytoplankton and further to study the effect on copper toxicity of naturally occurring organic ligands. Culture experiments with an estuarine diatom Thalassiosira pseudonana (clone 3H) in highly chelated seawater media demonstrated that copper induced growth rate inhibition and 3 to 4 day cellular uptake of copper are both related to the calculated free cupric ion activity and are independent of the total copper concentration. Cupric ion activity and total copper concentration were independently altered through various combinations of chelator (trishydroxymethyl amino methane) concentration, total copper concentration, and pH. Cellular copper content, in moles per cell, followed a hyperbolic relationship Cu/cell = 4.8 x 1-016 aCu/aCU + 10-9.2 where aCu is the free cupric ion activity. The above relationship suggests a reversible binding of copper to a single set of cellular ligand sites having a total binding capacity of 4.8 x 10-16 moles per cell and an association constant for reaction with copper of 109.2. For T. pseudonana (clone 3H) copper was inhibitory at pCu values below 10.7 (i.e. cupric ion activities above 10-10.7) with total growth inhibition occurring at pCu values below 8.3. The relationship between growth rate inhibition and cupric ion activity was not a simple hyperbolic relationship as was observed in the case of copper uptake. For an estuarine green alga Nannochloris atomus (clone GSB Nanno) and an open ocean strain of T. pseudonana (clone 13-1) partial growth rate inhibition occurred in the pCu ranges 10.3 to 8.4 and approximately 10 to 8, respectively. Comparison of these growth inhibitory pCu levels with a calculated estimate of the pCu of seawater of pH 8.2 containing a typica1 total copper concentration of 0.012 μM and having no significant copper chelation, indicates that natural cupric ion activity levels in seawater may be inhibitory to these three clones. Evidence was found for the complexation of copper by extracellular products of the alga T. pseudonana (clone 3H). Cupric ion selective electrode measurements of copper complexation in unused low salinity culture media and in identical media in which algae had been grown and from which they were subsequently filtered showed a higher degree of copper complexation in the used media. Parallel studies of copper toxicity and cellular copper uptake in an unused medium and in a culture filtrate demonstrated a lower copper toxicity and a decreased cellular copper uptake in the used medium. Cupric ion-selective electrode measurements and bioassay experiments support the hypothesis that copper is complexed by organic ligands in at least some natural waters. Copper added to filtered untreated river water is more highly complexed than that added to river water that has been uv irradiated to remove some portion of the dissolved organic matter. Copper toxicity to N. atomus is significantly increased in seawater from Vineyard Sound and in salt marsh water subjected to prior ultraviolet irradiation.