Evaluation of mercury methylation and methylmercury demethylation rates in vegetated and non-vegetated saltmarsh sediments from two Portuguese estuaries * Rute Ces  ario a, b , Holger Hintelmann c, 1 , Ricardo Mendes b , Kevin Eckey d , Brian Dimock c , Beatriz Araújo e , Ana Maria Mota a , Jo ~ ao Can  ario a, *, 1 a Centro de Química Estrutural, Instituto Superior T ecnico, Universidade de Lisboa, Av. Rovisco Pais, Lisboa 1,1049-001 Lisboa, Portugal b IPMA-Instituto Portugu^ es do Mar e Atmosfera, Av. Brasília, 1449-006 Lisboa, Portugal c Water Quality Centre, Trent University,1600 West Bank Drive, Peterborough, ON K9J 0G2, Canada d Institute of Inorganic and Analytical Chemistry, University of Muenster, Schlossplatz 2, 48149 Munster, Germany e Centro de Bioci^ encias e Biotecnologia, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000, Horto, Campus dos Goytacazes, Rio de Janeiro, Brazil article info Article history: Received 19 October 2016 Received in revised form 23 February 2017 Accepted 31 March 2017 Available online xxx Keywords: Mercury methylation Methylmercury demethylation Salt-marsh sediments Estuaries abstract Neurotoxic methylmercury (MMHg) is formed from inorganic divalent mercury (Hg 2þ ). However, it is poorly understood to what extent different mercury (Hg) pools contribute to existent MMHg levels. In this study, ambient concentrations of total Hg (THg) and MMHg as well as rates of methylation and demethylation were measured simultaneously in sediments with and without salt-marsh plant vege- tation, which were collected in Guadiana and Tagus estuaries, Portugal. Concurrent processes of Hg methylation and MMHg demethylation were directly monitored and compared by spiking sediments cores with stable isotope tracers of 199 Hg 2þ and CH 3 201 Hg þ followed by gas chromatographic separation and isotope-specific detection using inductively coupled plasma mass spectrometry. Compared to the Guadiana estuary, where concentrations were comparatively low, THg and MMHg levels varied between vegetated and non-vegetated sediments collected at the Ros ario site (ROS) of the Tagus estuary. Methylation (K M ) and demethylation rates (K D ) were also different between estuaries being dependent on the presence of vegetation. In addition, the type of macrophyte species influenced K M and K D values. In fact, the highest K M value was found in Sarcocornia fruticosa vegetated sediments at the Castro Marim site in Guadiana (CM, 0.160 day 1 ) and the lowest K M was observed in non-vegetated sediments at the Alcochete site in Tagus (ALC, 0.009 day 1 ). K D varied by a factor of three among sites with highest rates of demethylation observed in non-vegetated sediments in Guadiana (12 ± 1.3 day 1 , corresponding to a half-life of 1.4 ± 0.2 h). This study clearly shows that the presence of vegetation in sediments favors the formation of MMHg. Moreover, this effect might be site specific and further studies are needed to confirm the findings reported here. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Mercury (Hg) is known as a global pollutant due to its long- range transport and persistence in various compartments of the environment (Fitzgerald et al., 1998; Fitzgerald and Lamborg, 2007; Sonke et al., 2013). Once Hg has been introduced into the aquatic environment, a complex set of biologically mediated chemical re- actions within the anaerobic region of the aquatic sediments leads to the conversion of Hg to its biologically toxic form, mono- methylmercury (MMHg) (Randall et al., 2013), most commonly known as methylmercury. MMHg is of greatest concern because of its prevalence in the environment, its high neurotoxicity and ease bioaccumulation through food chains (Bloom, 1992; Fitzgerald and Clarkson, 1991). MMHg also adversely affects brain development, especially in fetuses and young children (Clarkson, 2002). In marine ecosystems, transformation of Hg to MMHg, is thought to occur in * This paper has been recommended for acceptance by Maria Cristina Fossi. * Corresponding author. E-mail address: joao.canario@tecnico.ulisboa.pt (J. Can ario). 1 Shared senior authorship. Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol http://dx.doi.org/10.1016/j.envpol.2017.03.075 0269-7491/© 2017 Elsevier Ltd. All rights reserved. Environmental Pollution xxx (2017) 1e11 Please cite this article in press as: Ces ario, R., et al., Evaluation of mercury methylation and methylmercury demethylation rates in vegetated and non-vegetated saltmarsh sediments from two Portuguese estuaries, Environmental Pollution (2017), http://dx.doi.org/10.1016/ j.envpol.2017.03.075