Aquatic organisms produce organic ligands via the active release of photosynthates, degradation of senescent cells, rupture of cells due to grazing, and cell lysis by pathogens. The release of organic ligands has important implications for metal toxicity in seawater, as such ligands are able to bind metals, altering their toxicity.
While it is widely accepted that these ligands binds free metal ions, reducing their availability, some studies have concluded that they increase metal bioavailability. It is postulated that organic ligands bind the algal cell wall resulting in increased
membrane permeability of metal complexes and hydrophilic substances. The same
binding also increases the negative charge at the algal surface. This may result in the
formation of a ternary complex between dissolved organic ligand, metal, and algae.
Hence, it is at present unknown to what extent algal exudates may affect metal
bioavailability. This work describes species-specific ligand profiles, which were
observed for five macroalgae (Laminaria hyperborea, Fucus spiralis, Fucus
vesiculosus, Ascophyllum nodosum, and Pelvetia canaliculata) as a function of
exposure to multiple metals (Cd, Cu, Pb, Zn) and a range of environmental
conditions. Phenolic and CDOM exudation were estimated by measurement of the
absorbance of water at 274 nm and 440 nm, respectively. The determination of total
dissolved metal, complexing ligands, glutathione and cysteine concentrations of the
culture solutions by cathodic stripping voltammetry (CSV) are discussed. This