Seasonal mercury transformation and surficial sediment detoxification by bacteria of Marano and Grado lagoons

Estuarine, Coastal and Shelf Science 113 (2012) 105e115

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Seasonal mercury transformation and surficial sediment detoxification by bacteria of Marano and Grado lagoons Franco Baldi a, *, Michele Gallo a, Davide Marchetto a, Renato Fani b, Isabel Maida b, Milena Horvat c, Vesna Fajon c, Suzana Zizek c, Mark Hines d a

Dept. of Scienze Molecolari e Nanosistemi, Cà Foscari University of Venice, Dorsoduro 2137, 30123 Venice, Italy Dept. of Evolutionary Biology, Via Romana, 17, 50125 Firenze, Italy Dept. of Environmental Sciences, “Jozef Stefan” Institut, Jamova 39, 1000 Ljubljana, Slovenia d Dept of Biological Sciences, University of Massachusetts Lowell, Lowell, MA 01854, USA b c

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Article history: Received 24 March 2011 Accepted 9 February 2012 Available online 20 February 2012

Marano and Grado lagoons are polluted by mercury from the Isonzo River and a chlor-alkali plant, yet despite this contamination, clam cultivation is one of the main activities in the region. Four stations (MA, MB, MC and GD) were chosen for clam seeding and surficial sediments were monitored in autumn, winter and summer to determine the Hg detoxifying role of bacteria. Biotransformation of Hg species in surficial sediments of Marano and Grado lagoons was investigated while taking into consideration the speciation of organic matter in the biochemical classes of PRT (proteins), CHO (carbohydrates) and LIP (lipids), waterwashed cations and anions, bacterial biomass, Hg-resistant bacteria, some specific microbial activities such as sulfate reduction rates, Hg methylation rates, Hg-demethylation rates, and enzymatic ionic Hg reduction. MeHg in sediments was well correlated with PRT content, whereas total Hg in sediments correlated with numbers of Hg-resistant bacteria. Correlations of the latter with Hg-demethylation rates in autumn and winter suggested a direct role Hg-resistant bacteria in Hg detoxification by producing elemental Hg (Hg0) from ionic Hg and probably also from MeHg. MeHg-demethylation rates were w10 times higher than Hg methylation rates, were highest in summer and correlated with high sulfate reduction rates indicating that MeHg was probably degraded in summer by sulfate-reducing bacteria via an oxidative pathway. During the summer period, aerobic heterotrophic Hg-resistant bacteria decreased to