Antagonistic interactions of Pb and Cd on Cu uptake, growth inhibition and chelator release in the marine algae Emiliania huxleyi

Some trace metals are known to act synergistically or antagonistically to influence phytoplankton growth limitation or toxicity. However, little is known about the effects of Pb and/or Cd on Cu uptake and liberation of exudates by marine micro-algae. The work presented here was undertaken using cultures of Emiliania huxleyi in coastal seawater enriched only with nitrate and phosphate, and in which the background total dissolved metal concentrations and Cu-, Pb- and Cd-complexing ligands had been previously determined. The concentrations of Cu, Pb and Cd, and the concentrations of released organic compounds and their ligand concentrations, in the cultures at different Pb and Cd levels, were monitored by cathodic and anodic stripping voltammetry (CSV and ASV). Compounds that behaved in CSV like cysteine and glutathione were also determined by an independent method. Addition of 10 nM Pb (64 pM ionic Pb ([Pb2+])) reduced the growth rate but did not promote the liberation of organic ligands. The cellular levels of Cu decreased or did not change, depending on the age of the cultures, suggesting that Pb slightly antagonised Cu uptake ([Cu2+]=0.32 pM, [Cu]d=29 nM). Similar effects were observed for 310 pM initial [Pb2+] (25 nM [Pb]d); 120 pM Initial [Cd2+] (10 nM [Cd]d) as well as 400 pM initial [Cd2+] (25 nM [Cd]d) reduced the rate of algal growth for the first 7 days, although the rate of cell division subsequently increased. It was also noted that 120 pM initial [Cd2+] but not 400 pM initial [Cd2+] slightly antagonised Cu uptake, whilst Cd strongly promoted the liberation of exudates, particularly those identified by CSV as glutathione. The effects of simultaneous additions of Pb and Cd (10 nM Pb and 10 nM Cd) could not be interpreted as simple additive effects.