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

Thr short term impact of 50 mu M Hg(II) on soil bacterial community structu re was evaluated in different microenvironments of a silt loam soil in orde r to determine the contribution of bacteria located in these microenvironme nts to the overall bacterial response to mercury spiking. Microenvironments and associated bacteria, designated as bacterial pools, were obtained by s uccessive soil washes to separate the outer fraction, containing loosely as sociated bacteria; and the inner fraction, containing bacteria retained int o aggregates, followed by a physical fractionation of the inner fraction to separate aggregates according to their size (size fractions). Indirect enu merations of viable heterotrophic (VH) and resistant (Hg-R) bacteria were p erformed before and 30 days after mercury spiking. A ribosomal intergenic s pacer analysis (RISA), combined with multivariate analysis, was used to com pare 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 Hg-R numbers in the outer fraction and in the coarse size fractions. Furthermore, shifts in RISA patterns of total c ommunity DNA indicated changes in the composition of the dominant bacterial populations in response to Hg(TT) stress in the outer and in the clay size fractions. The heterogeneity of metal impact on indigenous bacteria, obser ved at a microscale level, is related to both the physical and chemical cha racteristics of the soil microenvironments governing mercury bioavailabilit y and to the bacterial composition present before spiking. (C) 2000 Federat ion of European Microbiological Societies. Published by Elsevier Science B. V. All rights reserved.