Copper-binding organic ligands and phytoplankton growth in Japanese coastal waters

Abstract

Although copper (Cu) is necessary for phytoplankton growth, it becomes toxic at high concentrations. The bioavailability and toxicity of Cu are governed by its chemical speciation, with free inorganic Cu²⁺ being the most toxic species. In this study, we estimated free Cu²⁺ concentrations in Japanese coastal waters to evaluate the effects of Cu toxicity on phytoplankton growth. Incubation experiments were conducted using natural phytoplankton assemblages in seawater collected from Otsuchi Bay, northeastern Japan, and Goto-nada in the East China Sea. The cells were incubated for 14 days with Cu additions of up to 100 nM. Copper speciation parameters were determined using the reverse titration methods with competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV). Chlorophyll-a concentrations increased with the addition of <10 nM Cu but decreased at 100 nM Cu due to Cu toxicity. We found that phytoplankton growth caused negligible increases in Cu-binding organic ligand concentrations. Free Cu²⁺ concentrations were simulated using the initial Cu speciation parameters obtained from the reverse titration method under varying Cu concentrations. When assuming the presence of only one class of organic ligands, the estimated Cu²⁺ concentrations were inconsistent with the incubation results. In contrast, by considering two classes of ligands, the increase in the concentration of Cu²⁺ closely matched the incubation data. This suggests that the resolution of the weaker class of organic ligands is crucial for evaluating Cu toxicity in seawater.