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Abstract Title: Comparison of Hg species Intracellular Handling in two Freshwater Phytoplankton sp. : Cyanobacterium vs Diatom.
Presenter Name: Javier Garcia-Calleja
Company/Institution: University of Pau
Session: Mercury in Freshwater Ecosystems
Co-Authors: Javier Garcia-Calleja,Zoyne Pedrero,Thibaut Cossart,Joao Santos,Vera Slaveykova,Emmanuel Tessier,Laurent Ouerdane,David Amouroux

Abstract Information :

To address the importance of phytoplankton intracellular handling on the fate of Hg species (inorganic mercury ? Hg(II), methylmercury ? MeHg) in freshwater ecosystems, we have investigated Hg-phytoplankton interactions in two model organisms, diatom Cyclotella meneghiniana and cyanobacterium Synechocystis sp. PCC 6803. Laboratory experiments were conducted using isotopically-labelled Hg species at low Hg concentration (3 nM / 600 ng L-1 199Hg (II) & 0.3 nM / 60 ng L-1 Me201Hg) combined with hyphenated techniques based on gas or liquid chromatography coupled to elemental and molecular mass spectrometry (GC-ICPMS, SEC-ICPMS, HILIC-ICP-MS/MS-MS). The time-course (t=5min to 96hrs) intracellular bioaccumulation of Hg species, (sub-) cellular distribution and formation of Hg intracellular complexes in both species were performed. The intracellular behaviors of Hg species were investigated by determining the newly-formed 199Hg (II) and 201MeHg binding biomolecules in the cytosolic fraction. For Hg species bioaccumulation in Cyclotella meneghiniana, the concentration of both 199Hg (II) and 201MeHg in the intracellular fraction decreased ten times after 96 hours of exposure (from 0.19 to 0.02 amol cell-1 for 199Hg (II) and from 0.02 to 0.003 amol cell-1) being found in the extracellular fraction. In Synechocystis sp, the intracellular content of both Hg (II) and MeHg remained constant over 96 hours (0.04 amol cell-1). The screening of the intracellular Hg species ?biocomplexes reveals both Hg species-specific and changing bioligands affinities over time from high to low molecular weight (LMW) bioligands. The identification of LMW bioligands binding Hg (II) and MeHg was achieved and clearly demonstrates the role glutathione in Hg intracellular handling. Overall, we have observed different intracellular Hg species behavior (sequestration / release) over time in two different freshwater model microorganisms that influence the Hg fate and therefore, the potential trophic transfer in freshwater ecosystems.

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