Response of soil bacteria to Hg(II) in relation to soil characteristics and cell location

Abstract

The response of indigenous bacterial communities to the addition of inorganic mercury (50 μM of Hg(II)) was compared over a 30 day period in four soils of contrasting properties. Hg(II) impact was estimated by following population dynamics of viable heterotrophic bacteria (VH) and mercury resistant bacteria (Hg^R) by indirect enumeration in unfractionated soil and in the inner and outer parts of soil aggregates obtained by successive washings. Numbers of VH bacteria in unfractionated soils were not affected by mercury in any of the studied soils, whereas an increase in resistant bacteria was observed in all of them. The lag phase before the increase of Hg^R bacteria and the magnitude of the enrichment could be related to soil organic matter and clay contents suggesting that these factors govern mercury bioavailability. In fractionated soil, the increase of resistant populations was more pronounced in the outer part (representing mainly bacteria in macropores) than in the inner part of soil aggregates. This difference in response was clearer in soils with well developed aggregates. Qualitative analysis revealed that exposure to Hg caused a large increase in the proportion of Gram-negative strains among Hg^R bacteria in both soil parts.