Shifts in diversity and microscale distribution of the adapted bacterial phenotypes due to Hg(II) spiking in soil

Abstract

In a previous experiment [Ranjard et al. (2000) FEMS Microbiol Ecol 31:107-115], the spatial heterogeneity of a mercury impact on soil bacterial community was revealed by an increase of mercury-resistant (HgR) bacterial numbers in the outer fraction and the sand fractions when compared to those in the silt fractions. The objectives of the present study were (i) to investigate whether mercury exposure affects the diversity and the distribution within the various fractions of the HgR populations and (ii) to evaluate the contribution of the HgR populations to the overall community adaptation. A total of 236 strains isolated before (104 isolates) and 30 days (132 isolates) after spiking were characterized by an amplified ribosomal DNA restriction analysis (ARDRA) and grouped into 30 different genotypes. Whereas almost the same numbers of different genotypes were observed at both sampling times when considering all microenvironments, important changes in their evenness were observed. At the microscale level, we also noticed a heterogeneous distribution of the genotypes. Partial 16S rDNA sequences of each genotype were determined to permit phylogenetic affiliation. Whereas Pseudomonas-like species were dominant in all microenvironments at T = 0, genotypes not detected before spiking such as genotypes closely related to Xanthomonas, Pseudoaminobacter, and Sphingomonas-like species became dominant at T = 30. Similarly, several genotypes close to Bacillus, Streptomyces, and Rhodococcus species were only detected in the sand fractions at T = 0 and could not be detected in these fractions at T = 30. Probes defined within the intergenic spacer between the rrs and rrl genes were designed for the dominant genotypes and were hybridized toward the RISA (Ribosomal Intergenic Spacer Analysis) profiles derived from the T = 0 and T = 30 bacterial communities associated to the unfractionated soil. Results showed that the culturable dominant HgR genotypes partially contributed to the adaptation of the whole bacterial community.