[M-ZZ45] Frontiers in geochemistry: discussing its appeal and future prospects

[MZZ45-P14] Copper-toxicity for marine phytoplankton in Japanese coastal waters

*Hajime Obata¹, Wiwit -¹, Kuo Hong Wong², Chia-Jung Lu¹, Hideki Fukuda³, Hiroshi Ogawa¹, Kazutaka Takahashi⁴, Asami Mashio², Shigenobu Takeda⁵ (1.Chemical Oceanography Division, Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Institute of Science and Engineering, Kanazawa University, 3.Center for International and Local Research Cooperation, Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Graduate School of Agriculture and Life Sciences, The University of Tokyo, 5.Graduate School of Fisheries and Environmental Sciences, Nagasaki University)

Keywords:ocean, Cu, organic ligand, reverse titration

Copper in seawater is essential for phytoplankton growth, but it is toxic at high concentrations. The bioavailability and toxicity of Cu depend on its chemical species. It is well known that free cupric ion is the most toxic for phytoplankton. In this study, we estimated the free cupric ion concentrations in Japanese coastal waters to evaluate the effect of Cu toxicity on phytoplankton growth. Incubation experiments were carried out using natural phytoplankton assemblages in seawaters collected at Otsuchi Bay on the Pacific Ocean side of the northern Japan and around Goto Nada in the East China Sea. They were incubated for 14 days with the addition of up to 100 nM Cu. Copper speciation parameters were obtained by using reverse titration methods with competitive ligand exchange adsorptive cathodic stripping voltammetry. During the incubation experiments, chlorophyll-a contents increased with the addition of less than 10 nM Cu but decreased with the addition of 100 nM Cu due to Cu toxicity. During the experiments, we did not detect increases of Cu-binding organic ligand accompanying with phytoplankton growth. We simulated the free cupric ion concentrations using initial Cu speciation parameters obtained using competitive ligand exchange adsorptive cathodic stripping voltammetry with varying Cu concentrations. Assuming one class of organic ligand in the sample, estimated free cupric ion concentrations were inconsistent with our incubation results. In contrast, by assuming two classes of ligands, we obtained reasonable free cupric ion concentrations in seawater which agreed well with our incubation results. This simulation results suggested that the accurate detection of weaker class of organic ligand is crucial for evaluating Cu toxicity in seawater.