Identical Hg Isotope Mass Dependent Fractionation Signature during Methylation by Sulfate-Reducing Bacteria in Sulfate and Sulfate-Free Environment Vincent Perrot,* ,†,§ Romain Bridou, †,‡ Zoyne Pedrero, † Remy Guyoneaud, ‡ Mathilde Monperrus, † and David Amouroux* ,† † Laboratoire de Chimie Analytique Bio-Inorganique et Environnement, Institut des Sciences Analytiques et de Physico-Chimie pour l′Environnement et les Mate ́ riaux, CNRS-UPPA-UMR-5254, Hé lioparc, 2 Avenue du Pre ́ sident Pierre Angot, Pau, 64053, France ‡ Equipe Environnement et Microbiologie, Institut des Sciences Analytiques et de Physico-Chimie pour l′Environnement et les Mate ́ riaux, CNRS-UPPA-UMR-5254, IBEAS, Avenue de l′Universite ́ , Pau, 64013, France * S Supporting Information ABSTRACT: Inorganic mercury (iHg) methylation in aquatic environments is the first step leading to monomethylmercury (MMHg) bioaccumulation in food webs and might play a role in the Hg isotopic composition measured in sediments and organisms. Methylation by sulfate reducing bacteria (SRB) under sulfate-reducing conditions is probably one of the most important sources of MMHg in natural aquatic environments, but its influence on natural Hg isotopic composition remains to be ascertained. In this context, the methylating SRB Desulfovibrio dechloracetivorans (strain BerOc1) was incubated under sulfate reducing and fumarate respiration conditions (SR and FR, respectively) to determine Hg species specific (MMHg and IHg) isotopic composition associated with methylation and demethylation kinetics. Our results clearly establish Hg isotope mass-dependent fractionation (MDF) during biotic methylation (−1.20 to +0.58‰ for δ 202 Hg), but insignificant mass-independent fractionation (MIF) (−0.12 to +0.15‰ for Δ 201 Hg). During the 24h of the time- course experiments Hg isotopic composition in the produced MMHg becomes significantly lighter than the residual IHg after 1.5h and shows similar δ 202 Hg values under both FR and SR conditions at the end of the experiments. This suggests a unique pathway responsible for the MDF of Hg isotopes during methylation by this strain regardless the metabolism of the cells. After 9 h of experiment, significant simultaneous demethylation is occurring in the culture and demethylates preferentially the lighter Hg isotopes of MMHg. Therefore, depending on their methylation/demethylation capacities, SRB communities in natural sulfate reducing conditions likely have a significant and specific influence on the Hg isotope composition of MMHg (MDF) in sediments and aquatic organisms. 1. INTRODUCTION Mercury (Hg) is a global pollutant that has a complex biogeochemical cycle within earth compartments. 1 Methylation of inorganic mercury (IHg) in aquatic ecosystems is a critical step, since it leads to the formation of methylmercury (MMHg) easily accumulated and biomagnified in food webs. 2 Microbial activities play a crucial role in the transformations of Hg species in the environment such as methylation/demethylation and reduction/oxidation reactions. 3 Among sulfate-reducing bac- teria (SRB), which have been identified as the principal IHg methylators in anoxic sediments, 4 representatives of the genus Desulfovibrio have been extensively studied for their methyl- ation/demethylation capacity. 5−8 Recently Desulfovibrio desul- f uricans ND132 was selected as a model organism, 9,10 leading to the identification of two specific genes hgcA and hgcB determined to be mandatory for Hg methylation in all the bacterium. 11 In-situ and pure culture experiments show that MMHg formation rate depends on several parameters such as environmental/physiological conditions, 12,13 bacterial strain, 6,14,15 growth phase 16,17 and Hg bioavailability. 8 Inves- tigations of bacterial methylation are hampered by difficulties in constraining/monitoring the physiological changes that affect the biochemical pathways of Hg methylation. 6,15,18 Thus, relating bacterial communities and activities to the MMHg budget in aquatic ecosystems is still an important challenge. The natural Hg stable isotope variations are a powerful tool to trace Hg sources 19,20 and its species transformations. 21−23 Previous studies reported that aquatic organisms were enriched in heavier Hg isotopes relative to the sediment which is assumed to be the source of the Hg they accumulate. 24,25 This probably results from the combined effects of of (1) IHg adsorption−desorption to particles 26,27 and dark/biotic reduc- tion, 23,28 followed by (2) the methylation of bioavailable IHg, 29 Received: July 9, 2014 Revised: December 17, 2014 Accepted: January 7, 2015 Article pubs.acs.org/est © XXXX American Chemical Society A DOI: 10.1021/es5033376 Environ. Sci. Technol. XXXX, XXX, XXX−XXX