The impact of increasing heavy metal stress on the diversity and structure of the bacterial and actinobacterial communities of metallophytic grassland soil

Abstract

A study was undertaken to investigate the bacterial community found in metallophytic grassland soil contaminated with Zn and Pb. We hypothesised that such communities would be tolerant of additional heavy metal stress due to phylogenetic and functional adaptation. In microcosm experiments, lasting 51 days, denaturing gradient gel electrophoresis (DGGE) analyses was used to compare the total bacterial and actinobacterial communities in non-amended soils and those to which additional Pb and Zn concentrations were added. There was a decrease in total bacterial diversity with each addition of Pb and Zn; in contrast, the actinobacterial community diversity remained unaffected. The community structures were analysed using multivariate analyses of the DGGE profiles. Total bacterial community profiles showed two distinct groups sharing less than 80% similarity, irrespective of Pb and Zn addition. The first contained profiles sampled during the first 7 days of the experiment; the second contained those sampled from day 10 onwards. Actinobacterial profiles from those that were non-amended showed a similar distribution to those of the total bacterial community. However, in soil amended with fivefold additional Pb and Zn, all the profiles shared more than 80% similarity. Raup and Crick analyses suggested that total bacterial soil communities were subject deterministic selection becoming significantly similar as the experiment progressed, but this was inhibited by the highest concentration of additional Pb and Zn. Actinobacterial communities showed a similar response but were less affected by elevated Pb and Zn concentrations. These data indicate that the diversity of the actinobacterial community was not negatively affected by additional heavy metal stress in contrast to total bacterial community diversity.