A soil microscale study to reveal the heterogeneity of Hg(II) impact on indigenous bacteria by quantification of adapted phenotypes and analysis of community DNA fingerprints

Authors

  • Lionel Ranjard,

    1. Laboratoire d'Ecologie Microbienne, UMR CNRS 5557, Université Claude Bernard, Lyon I, F-69622 Villeurbanne Cedex, France
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  • Sylvie Nazaret,

    1. Laboratoire d'Ecologie Microbienne, UMR CNRS 5557, Université Claude Bernard, Lyon I, F-69622 Villeurbanne Cedex, France
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  • François Gourbière,

    1. Laboratoire d'Ecologie Microbienne, UMR CNRS 5557, Université Claude Bernard, Lyon I, F-69622 Villeurbanne Cedex, France
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  • Jean Thioulouse,

    1. Laboratoire de Biométrie, UMR CNRS 5558, Université Claude Bernard, Lyon I, F-69622 Villeurbanne Cedex, France
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  • Philippe Linet,

    1. Service Central d'Analyses du CNRS, Chemin du Canal, BP 22, 69390 Vernaison, France
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  • Agnès Richaume

    Corresponding author
    1. Laboratoire d'Ecologie Microbienne, UMR CNRS 5557, Université Claude Bernard, Lyon I, F-69622 Villeurbanne Cedex, France
      *Corresponding author. Present address: UMR-CNRS 5557 - Ecologie Microbienne, Université Claude Bernard Lyon I, Bât 741, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne cedex, France. Tel.: +33 (4)72431380; Fax: +33 (4)72431223 richaume@cismsun.univ-lyon1.fr
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*Corresponding author. Present address: UMR-CNRS 5557 - Ecologie Microbienne, Université Claude Bernard Lyon I, Bât 741, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne cedex, France. Tel.: +33 (4)72431380; Fax: +33 (4)72431223 richaume@cismsun.univ-lyon1.fr

Abstract

The short term impact of 50 μM Hg(II) on soil bacterial community structure was evaluated in different microenvironments of a silt loam soil in order to determine the contribution of bacteria located in these microenvironments to the overall bacterial response to mercury spiking. Microenvironments and associated bacteria, designated as bacterial pools, were obtained by successive soil washes to separate the outer fraction, containing loosely associated bacteria, and the inner fraction, containing bacteria retained into aggregates, followed by a physical fractionation of the inner fraction to separate aggregates according to their size (size fractions). Indirect enumerations of viable heterotrophic (VH) and resistant (HgR) bacteria were performed before and 30 days after mercury spiking. A ribosomal intergenic spacer analysis (RISA), combined with multivariate analysis, was used to compare modifications at the community level in the unfractionated soil and in the microenvironments. The spatial heterogeneity of the mercury impact was revealed by a higher increase of HgR numbers in the outer fraction and in the coarse size fractions. Furthermore, shifts in RISA patterns of total community DNA indicated changes in the composition of the dominant bacterial populations in response to Hg(II) stress in the outer and in the clay size fractions. The heterogeneity of metal impact on indigenous bacteria, observed at a microscale level, is related to both the physical and chemical characteristics of the soil microenvironments governing mercury bioavailability and to the bacterial composition present before spiking.

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