7TH INTERNATIONAL CONFERENCE ON MERCURY AS A GLOBAL POLLUTANT 765 Part 2 Biogeochemistry RMZ-M&G 2004, 51 766 PART 2 BIOGEOCHEMISTRY RMZ-M&G 2004, 51 7TH INTERNATIONAL CONFERENCE ON MERCURY AS A GLOBAL POLLUTANT 767 Methyl Mercury and Sulfate-Reducing Bacteria in the Floating Macrophyte Rizosphere from an Amazonian Floodplain Lake, Bolivia DARIO ACHA1, VOLGA INIGUEZ1, MARC ROULET2, JEAN-REMY DAVEE GUIMARAES3, RUDDY LUNA1, LUCIA ALANOCA2,4 & SAMANTA SANCHEZ1 1Instituto de Biologia Molecular y Biotecnologia, UMSA, La Paz, Bolivia 2Institut de Recherche pour le Développement, HYBAM, UMR 154 LMTG, La Paz Bolivia, e-mail: roulet@cict,fr 3Laboratorio de Traçadores, Instituto de Biofisica, UFRJ, Rio de Janeiro, Brazil 4Laboratorio de Calidad Ambiental, Instituto de Ecologia, UMSA, La Paz, Bolivia Abstract: Mercury contamination in Amazonian rivers and lakes constitutes a significant threat to human and ecosystem health A fundamental step in its introduction to food webs and human exposure is its conversion from inorganic mercury to methyl mercury (MeHg), which is the most bioaccumulative and neurotoxic form of mercury Several studies have demonstrated that this transformation is mediated mainly by Sulfate-Re- ducing Bacteria (SRB) in aquatic environments Although links between sulfate reduc- tion and mercury methylation have been found in Amazonian sediments and floating macrophyte roots-associated periphyton, no SRB where yet detected This study used 16s rDNA molecular probes to identify six groups of SRB (Desulfotomaculum; Desulfobulbus; Desulfobacterium; Desulfobacter; Desulfococcus Desulfonema Desulfosarcina; Desulfovibrio Desulfomicrobium) on the rizosphere of four macrophytes (Polygonum densiflorum, Hymenachne donacifolia, Ludwigia heliminthorriza and Eichhornia crassipes) from La Granja lake in Beni river (upper Madeira) floodplain, Bolivia Methylation potential (labeled 203HgCl ), microbial activ- 2 ity (3H-leucine) and organic matter, total Hg and MeHg where also determined in root- associated periphyton Water physico-chemical characteristics (redox potential, conductivity, pH and dissolved oxygen concentration) were not significantly related to SRB community composition Even more, it is impressive that five groups of these anaerobic bacteria were detected in such oxidative (+113 - +350 mV) and aerobic (152 437 mg O L-1) environment 2 SRB composition seams to be more related to macrophyte species, but with different degrees of association to their roots Desulfobulbus was significantly more abundant on Hymenachne donacifolia Desulfotomáculum was also more abundant on Hymenachne donacifolia but was also abundant on Ludwigia heliminthorriza Desulfobacter was more abundant on Hymenachne donacifolia and Polygonum desnsiflorum and appeared to be highly associated to roots because it was almost not found in samples containing only periphyton However, most groups were found more frequently in periphyton-only samples Desulfococcus Desulfonema Desulfosarcina was always found on Hymenachne donacifolia and Polygonum desnsiflorum, while Desulfovibrio Desulfomicrobium is present in all samples and Desulfobacterium was never detected Mercury methylation potentials in periphyton varied greatly between Polygonum densiflorum (275-361%) and Eichhornia crassipes (<2%) according to SRB richness These data supports the association between SRBs and methylmercury production in Amazon macrophyte rizospheres by probing the existence of a diverse SRB community RMZ-M&G 2004, 51 768 PART 2 BIOGEOCHEMISTRY Results about variation on SRB composition related to macrophyte species and location are also consistent with other studies where differences in sulphate reduction and mer- cury methylation potentials appear to be related to macrophyte specie SRB ability to survive in this highly aerobic environment could be attributed to specific metabolic pathways to tolerate oxygen or to a microfilm type organization that creates more ap- propriate conditions to their development Key words: Amazon, lake, Floodplain, MeHg, SRB RMZ-M&G 2004, 51 7TH INTERNATIONAL CONFERENCE ON MERCURY AS A GLOBAL POLLUTANT 769 Hg partitioning between dissolved and solid organic matter in mor layer of Swedish forest soils STAFFAN AKERBLOM1, LAGE BRINGMARK1, MARKUS MEILI1 & KJELL JOHANSSON1 1Dept of Environmental Assessment, Swedish University of Agricultural Sciences, P O Box 7050, 750 07 Uppsala, Sweden, e-mail: staffan akerblom@ma slu se Abstract: The cycling of Hg in terrestrial ecosystems is of important concern based on indi- cations that concentrations of Hg are increasing despite declining deposition of Hg Toxicity of Hg is likely related to soil concentrations, which are determined by the bal- ance between input and output fluxes Therefore, factors controlling the Hg turnover in forest soils need to be quantified The leaching of Hg and its association with organic matter is used in this study to describe partitioning between Hg in solid organic matter and Hg in dissolved organic phase Field studies were conducted with the aim of assessing to what extent the association of Hg to organic matter differs between dissolved and solid phases of forest soils, and between sites of high and low Hg deposition Despite differences in quality of organic matter the partitioning factor between dissolved and solid organic Hg is assumed to be comparable between different climatic and geographical regions Percolating water was collected from zero-tension lysimeters installed below the or- ganic so called mor layer and analyzed for dissolved organic carbon (DOC) and total Hg every second week during 2003 Solid phase chemistry data was used from previous studies Two spruce forest sites were chosen, one in the climatically harsh Northern Sweden and one in Southern Sweden, where regional Hg loads are high Ratios between Hg and organic matter were calculated for dissolved and solid organic matter separately Hg found to be correlated to organic matter in both dissolved and solid phases Ratios between Hg and organic matter were similar in solution and in solids from a given site Between sites, however, the ratios differed and were higher at the site with a higher atmospheric Hg deposition during the past decades These results suggest that Hg leaching from organic soils is mostly governed by organic matter fluxes Hg content of organic matter in solutions can be deduced from Hg levels in solid organic matter Future estimates for mass balance modelling will be strongly dependent on organic matter dynamics and factors that may affect mobilisation of or- ganic matter from organic top layers of forest soils Key words: organic matter, forest soil, Hg-cycling RMZ-M&G 2004, 51 770 PART 2 BIOGEOCHEMISTRY Mercury species benthic flux measurements in coastal environments as influenced by the biological activity DAVID AMOUROUX1, MATHILDE MONPERRUS1, DAVID POINT1, EMMANUEL TESSIER1, LAURENT CHAUVAUD2, GÉRARD THOUZEAU2, FRED JEAN2, AUDE LEYNAERT2, JACQUES CLAVIER2, JACQUES GRALL2, ERWAN AMICE2 1Laboratoire de Chimie Analytique Bio-Inorganique et Environnement, CNRS UMR 5034, Université de Pau et des Pays de lAdour, Hélioparc, 64053 Pau, France; E-mail: david3amouroux@univ-pau3fr3 2Laboratoire des Sciences de lEnvironnement Marin, Institut Universitaire Européen de la Mer (IUEM), CNRS UMR 6539 Université de Bretagne Occidentale, Plouzané, France3 Abstract: In this paper, field experiments performed in the Bay of Brest on the French At- lantic Coast are presented as a contribution to the French PNEC program Experiments were done in shallow waters using benthic chambers to determine benthic fluxes of mercury species together with major biological activities Simultaneous determination of mercury compounds was performed by speciated isotope dilution mass spectrometry (SIDMS) allowing excellent precision and low detection limits for in situ benthic cham- bers experiments The results demonstrate that benthic fluxes of mercury species in such coastal environment are probably related to biological processes involved in sur- face sediments with significant time variability They suggest that sediment-water ex- changes of mercury species are highly dynamic and driven by transient redox change at the sediment water interface Key words: Inorganic mercury, methyl mercury, coastal environment, sediment-water inter- face, benthic fluxes INTRODUCTION characterised by extensive beds of free-liv- ing red calcareous algae (maerl) Maerl beds In this paper, we present field experiments are fragile ecosystems occurring in shallow performed within the Bay of Brest coastal waters from the tropics to the arctic (Britanny), a semi-enclosed macrotidal en- They are an important habitat for a large va- vironment connected to the North Atlantic riety of marine organisms living amongst or Ocean (Oct 2002) Experiments were per- attached to its thalli, or burrowing in the un- formed in shallow waters (6-8m depth) us- derlying dead maerl/fine sediment matrix ing benthic chambers to both monitor benthic fluxes of mercury (Hg) species, such as in- Benthic Chambers Experiments: organic mercury (IHg) and monomethyl Two different stations have been investi- mercury (MMHg), and major benthic bio- gated: one located in a rather pristine area logical activity Two stations were investi- (Rozegat, M1) and one exposed to anthro- gated on a natural benthic ecosystem pogenic inputs (Kerialou, M2) For each in- RMZ-M&G 2004, 51 7TH INTERNATIONAL CONFERENCE ON MERCURY AS A GLOBAL POLLUTANT 771 vestigated station, two series of 3 replicate experiment'