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Abstract Title: Exploring Mercury Uptake and Transformations by Pico-Nanoplankton in Eutrophic Lake by using Isotopic Dilution
Presenter Name: Thibaut Cossart
Company/Institution: University of Geneva
Session: Mercury in Freshwater Ecosystems
Co-Authors: Thibaut Cossart,Javier Garcia-Calleja,Isabelle A. M. Worms,Emmanuel Tessier,Killian Kavanagh,Zoyne Pedrero,David Amouroux,Vera Slaveykova

Abstract Information :

Aquatic microorganisms are recognized to play a key role in the Hg transformations and bioaccumulation. However, the role of photoautotrophs in these processes is still to confirm. The present study explores the bioaccumulation and transformations of inorganic (iHg) and monomethyl mercury (MMHg) by pico-nanoplankton communities of an eutrophic lake (Soppen Lake, Switzerland). Special attention was given to the subcellular localization of the accumulated species, given their importance for the Hg trophic transfer. Pico-nanoplankton communities sampled at 2 depths, mainly composed of bacterioplankton, mycoplankton and phytoplankton, were exposed to an enriched isotope mixture of 1 nM 199iHg and 0.1 nM MM201Hg for 24h. Cyanobacteria, diatoms, cryptophyta and heterotrophic microorganisms were identified as the major groups of pico-nanoplankton with diatoms prevailing at the deepest depth. The results showed that both Hg species were accumulated by the nano-phytoplankton communities. The ratios between Hg accumulated in the membrane and cytosolic fractions were comparable for both Hg compounds. The formation of newly Hg species (201iHg and MM199Hg) through biotic transformations revealed that pico-nanoplankton communities demethylated MM201Hg. Comparison of the newly formed 201iHg from MM201Hg revealed that about 82 % of the demethylation was biologically mediated by pico-nanoplankton. No significant methylation of 199iHg by pico-nanoplankton was observed with our experimental design. Statistical analysis has shown that Hg biouptake and MMHg demethylation correlated positively with the relative abundance of diatoms, the nitrogen content, and the concentration of Mg2+, NO3-, NO2-, NH4+ and, negatively correlated with the carbon content and the concentration in K+, Na+, Ca2+, SO42-. The results of this field study revealed the involvement of pico-nanoplankton communities in Hg biouptake and transformations processes through demethylation. Further research is required to better assess the role of these primary producers, to understand the prevailing mechanisms and to understand the possible interconnections between heterotrophic and autotrophic microorganisms.

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