The effect of organic ligands exuded by intertidal seaweeds on copper complexation

Abstract

Copper complexation in marine systems is mainly controlled by organic matter, partially produced by micro- and macroalgae that release exudates with the capacity to bind metals. This feature is important as it influences bioavailability, bioaccumulation, toxicity, and transport of copper through biological membranes. The release of Cu-complexing ligands by seaweeds cultured under copper excess was studied in the laboratory. Five macroalgae belonging to different functional groups were used, including the filamentous Chaetomorpha firma (Chlorophyta), the foliose Ulva lactuca (Chlorophyta) and Porphyra columbina (Rhodophyta), the corticated Gelidium lingulatum (Rhodophyta), and the leathery Lessonia nigrescens (Phaeophyceae). The concentration of ligands and their copper-binding strength (log K′) of exudates released by each species was determined by anodic stripping voltammetry (ASV). The selected algae released exudates in a wide range of concentration (42–117 nM) after 48 h of culture, and addition of 157 nM copper increased the production of ligands up to 8 times. A relationship between structural complexity or thallus thickness and the amount of ligands released was not observed. The binding strength (log K′) varied among species from 7.6 to 8.9, a response that was not modified by exposure to sub-lethal copper excess. The kelp L. nigrescens showed a fast response to copper excess, releasing ligands that reduced toxicity of the metal in hours. Results suggest that intertidal and shallow subtidal macroalgae might have been overlooked regarding their role as producers of organic ligands and, therefore, as modulators of metal complexing capacity in coastal waters.