Role of Settling Particles on Mercury Methylation in the Oxic Water Column of Freshwater Systems.
As the methylation of inorganic mercury to neurotoxic methylmercury has been attributed to the activity of anaerobic bacteria, the formation of methylmercury in the oxic water column of marine ecosystems has puzzled scientists over the past years. Here we show for the first time that methylmercury can be produced in particles sinking through oxygenated water column of lakes. Total mercury and methylmercury concentrations were measured in the settling particles and in surface sediments of the largest freshwater lake in Western Europe (Lake Geneva). While total mercury concentration differences between sediments and settling particles were not significant, methylmercury concentrations were about ten-fold greater in settling particles. Methylmercury demethylation rate constants (kd) were of similar magnitude in both compartments. In contrast, mercury methylation rate constants (km) were one order of magnitude greater in settling particles. The net potential for methylmercury formation, assessed by the ratio between the two rate constants (kmkd-1), was therefore up to ten fold greater in settling particles, denoting that in situ transformations likely contributed to the high methylmercury concentration found in settling particles. Mercury methylation was inhibited (∼80%) in settling particles amended with molybdate, demonstrating the prominent role of biological sulfate-reduction in the process.
Department F.-A. Forel for Environmental and Water Sciences, University of Geneva , Boulevard Carl-Vogt 66, 1211 Geneva 4, Switzerland.
Elena
E
Gascón Díez
Department F.-A. Forel for Environmental and Water Sciences, University of Geneva , Boulevard Carl-Vogt 66, 1211 Geneva 4, Switzerland.
Jean-Luc
JL
Loizeau
Department F.-A. Forel for Environmental and Water Sciences, University of Geneva , Boulevard Carl-Vogt 66, 1211 Geneva 4, Switzerland.
Claudia
C
Cosio
Laboratoire de Chimie Analytique Bio-Inorganique et Environnement, Institut des Sciences Analytiques et de Physico-Chimie pour l'Environnement et les Matériaux, UMR 5254 CNRS, Université de Pau et des Pays de l'Adour, Hélioparc , 64053 Pau, France.
Sylvain
S
Bouchet
Institute of Earth Sciences (ISTE), University of Lausanne , 1015 Lausanne, Switzerland.
Thierry
T
Adatte
Laboratoire de Chimie Analytique Bio-Inorganique et Environnement, Institut des Sciences Analytiques et de Physico-Chimie pour l'Environnement et les Matériaux, UMR 5254 CNRS, Université de Pau et des Pays de l'Adour, Hélioparc , 64053 Pau, France.
David
D
Amouroux
Limnology Department, Evolutionary Biology Centre, EBC, Norbyvägen 18D, 75236 Uppsala, Sweden.
Andrea G
AG
Bravo
D019015:Geologic Sediments; D008628:Mercury; D008745:Methylation; D008767:Methylmercury Compounds; D014867:Water; D014874:Water Pollutants, Chemical
D008767:Methylmercury Compounds; D014874:Water Pollutants, Chemical; D014867:Water; D008628:Mercury