Species-specific isotope tracking of mercury uptake and transformations by pico-nanoplankton in an eutrophic lake

Thibaut Cossart; Javier Garcia-Calleja; Isabelle A M Worms; Emmanuel Tessier; Killian Kavanagh; Zoyne Pedrero; David Amouroux; Vera I Slaveykova

doi:10.1016/j.envpol.2021.117771

Species-specific isotope tracking of mercury uptake and transformations by pico-nanoplankton in an eutrophic lake

Environ Pollut. 2021 Nov 1:288:117771. doi: 10.1016/j.envpol.2021.117771. Epub 2021 Jul 9.

Authors

Thibaut Cossart¹, Javier Garcia-Calleja², Isabelle A M Worms¹, Emmanuel Tessier², Killian Kavanagh¹, Zoyne Pedrero², David Amouroux², Vera I Slaveykova³

Affiliations

¹ Environmental Biogeochemistry and Ecotoxicology, Department F.-A. Forel for Environmental and Aquatic Sciences, Earth and Environmental Sciences, Faculty of Sciences, University of Geneva, Uni Carl Vogt, Bvd Carl-Vogt 66, CH-1211, Geneva 4, Switzerland.
² Université de Pau et des Pays de l'Adour, E2S UPPA, CNRS, IPREM, Institut des Sciences Analytiques et de Physico-chimie pour l'Environnement et les matériaux, Pau, France.
³ Environmental Biogeochemistry and Ecotoxicology, Department F.-A. Forel for Environmental and Aquatic Sciences, Earth and Environmental Sciences, Faculty of Sciences, University of Geneva, Uni Carl Vogt, Bvd Carl-Vogt 66, CH-1211, Geneva 4, Switzerland. Electronic address: vera.slaveykova@unige.ch.

PMID: 34271517
DOI: 10.1016/j.envpol.2021.117771

Abstract

The present study aims to explore the bioaccumulation and biotic transformations of inorganic (iHg) and monomethyl mercury (MMHg) by natural pico-nanoplankton community from eutrophic lake Soppen, Switzerland. Pico-nanoplankton encompass mainly bacterioplankton, mycoplankton and phytoplankton groups with size between 0.2 and 20 μm. Species-specific enriched isotope mixture of ¹⁹⁹iHg and ²⁰¹MMHg was used to explore the accumulation, the subcellular distribution and transformations occurring in natural pico-nanoplankton sampled at 2 different depths (6.6 m and 8.3 m). Cyanobacteria, diatoms, cryptophyta, green algae and heterotrophic microorganisms were identified as the major groups of pico-nanoplankton with diatoms prevailing at deeper samples. Results showed that pico-nanoplankton accumulated both iHg and MMHg preferentially in the cell membrane/organelles, despite observed losses. The ratios between the iHg and MMHg concentrations measured in the membrane/organelles and cytosol were comparable for iHg and MMHg. Pico-nanoplankton demethylate added ²⁰¹MMHg (~4 and 12% per day depending on cellular compartment), although the involved pathways are to further explore. Comparison of the concentrations of ²⁰¹iHg formed from ²⁰¹MMHg demethylation in whole system, medium and whole cells showed that 82% of the demethylation was biologically mediated by pico-nanoplankton. No significant methylation of iHg by pico-nanoplankton was observed. The accumulation of iHg and MMHg and the percentage of demethylated MMHg correlated positively with the relative abundance of diatoms and heterotrophic microorganisms in the pico-nanoplankton, the concentrations of TN, Mg²⁺, NO₃^-, NO₂^-, NH₄⁺ and negatively with the concentrations of DOC, K⁺, Na⁺, Ca²⁺, SO₄^2-. Taken together the results of the present field study confirm the role of pico-nanoplankton in Hg bioaccumulation and demethylation, however further research is needed to better understand the underlying mechanisms and interconnection between heterotrophic and autotrophic microorganisms.

Keywords: Biotic transformations; Enriched stable isotopes; Eutrophic lake; Inorganic mercury; Methylmercury; Natural pico-nanoplankton.

MeSH terms

Isotopes
Lakes
Mercury* / analysis
Methylmercury Compounds*
Water Pollutants, Chemical* / analysis

Substances

Isotopes
Methylmercury Compounds
Water Pollutants, Chemical
Mercury