Toxicity of copper complexes to the marine diatom Nitzschia closterium

Abstract

Copper ions initially bind to Nitzschia with a stability constant, β 1, in the range log β 1 < 13 > 10, a value which precludes sulfur bonding and suggests involvement of a membrane protein. Although the algal growth rate in sea water was halved by 20 μg Cu·1 −1, photosynthesis was not affected until the copper concentration was above 100 μg Cu·1 −1. This decoupling of growth and photosynthesis did not occur to the same extent with other algae species tested, and it is proposed that it results from an intracellular reaction between Cu 2+ and GSH, leading to a lowering of the GSH/GSSG ratio and suppression of mitosis. Water-soluble ligands generally decreased the toxicity of copper, whereas lipid-soluble copper complexes were highly toxic. Oxine, 2,9-dimethyl-1,10-phenanthroline, and pyridyl- and thiazolyl-hydroxyazo compounds formed exceptionally toxic complexes. The toxicity of many redox-active compounds and their copper complexes appears to be due, at least partly, to the intracellular generation of hydrogen peroxide during oxidation of these compounds, an effect exacerbated by the ability of copper to inhibit catalase. Hydrogen peroxide is highly toxic towards marine algae, and may be a natural growth-inhibiting factor. Hydroxyl radicals (OH·), generated intracellularly from a mixture of hydrogen peroxide and the lipid-soluble thiol, cysteine, were also extremely toxic. However, when OH· or superoxide radicals O 2 sce; were generated extracellularly, no effect on growht rate was observed, even though singlet oxygen ( 1O 2) did seriously reduce growth.