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Abstract Title: Methylmercury partitioning in marine phytoplankton and their response to increasing water temperature
Presenter Name: Yuya Tada
Company/Institution: National Institute for Minamata Disease
Session: Mercury in Marine Ecosystems
Co-Authors: Yuya Tada

Abstract Information :

Evaluation of methylmercury (MeHg) distribution in the natural marine phytoplankton community is important for understanding bioaccumulation process of MeHg in the marine ecosystem. In addition, the temperature effect on MeHg distribution in phytoplankton cell is useful information for predicting the change of global mercury cycling in response to the climate change. In this study, the MeHg concentration of the phytoplankton and ambient seawater were estimated at 5 stations (north-south transect) in the Oyashio region in the western North Pacific, which is known as highly productive area with the spring phytoplankton (mainly diatom) bloom in Japan. During the bloom season (May 2021), we collected the phytoplankton samples with the different size using distinct mesh size (180 ?m, 20 ?m, 3 ?m, and 0.2 ?m). The MeHg concentration in phytoplankton ranged from 0.11 to 0.55 pmol/g wet (n = 18), and MeHg/total Hg were varied 2.2-19% (n = 5) in the Oyashio region. Remarkable differences of MeHg concentration among size fractions were not observed. Bioconcentration factor (BCF) of MeHg ranged from 0.4 x 10^4 to 2.1 x 10^4 [L/kg]. Interestingly, relatively high BCF value were observed in the southernmost station with relatively high temperature and low chlorophyll concentration. To evaluate the temperature response of MeHg distribution in phytoplankton cell, we conducted the incubation experiment with distinct temperature (Temp.: 2~15§C, Time: 0~8 h, MeHg-Cysteine conc.: 1 ng/L) using a diatom culture, Thalassiosira nordenskioeldii, which known as one of the dominant species in the Oyashio region. These results showed that the MeHg uptake rate and BCF value of diatom increased with temperature (e.g. BCF: 5.2 x 10^3 at 2 §C, 7.0 x 10^3 at 10 §C), suggesting that methylmercury concentration in the phytoplankton community in the Oyashio region can be affected by temperature increase associated with climate change.

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