Development of metal tolerance in soil bacterial communities exposed to experimentally increased metal levels

Abstract

The development of metal tolerance in soil bacterial communities exposed to different heavy metals was examined under laboratory conditions. An agricultural soil amended with different Zn concentrations was studied most intensively, and measurements were made over a 28-month incubation period by means of the thymidine incorporation technique. Tolerance levels were not affected by metal concentrations lower than 2 mmol of Zn kg (dry weight) of soil ؊1 , but above this value, the level of Zn tolerance increased exponentially with the logarithm of the soil Zn concentration. An increased metal tolerance was detected after only 2 days of Zn exposure. Thereafter, stable tolerance values were observed at different sampling times for bacterial communities exposed to up to 8 mmol of Zn kg (dry weight) ؊1 , indicating no changes in tolerance with time. The tolerance of bacterial communities exposed to 32 mmol of Zn kg (dry weight) ؊1 increased rapidly within the second week of incubation, but then the values remained unchanged until the end of the experiment. Bacterial communities from soil contaminated with 16 mmol of Zn kg (dry weight) ؊1 showed an increase of the same magnitude, but the increase started later, after 4 months of incubation, and took place for a much longer period (more than 1 year). Cd, Cu, and Ni addition also resulted in metal-tolerant communities, and the level of tolerance increased with prolonged incubations of the soils. The bacterial community at the end of the incubation period also exhibited a lower pH optimum and an increased tolerance to low osmotic potential. The results suggest that the increase in metal tolerance of the community after adding metals can be attributed to an immediate effect due to the death of sensitive species and a later effect due to different competitive abilities and adaptation of surviving bacteria.

Copper tolerance in iron‐reducing bacteria: Implications for copper mobilization in sediments

“…By artificially amending a subset of Fe reducers from one site with Cu and comparing it to bacteria sharing an identical exposure history except for the Cu amendment, we were able to isolate the effect of Cu exposure on Cu tolerance. Our results are consistent with findings for aerobic soil bacteria, in which Cu tolerance was enhanced by previous Cu exposure [21]; similar studies with anaerobic bacteria have been performed, but only with methanogens [22]. This is the first report of Cu tolerance in Fe-reducing bacteria.…”

Bacterial diversity promotes community stability and functional resilience after perturbation

“…Atlas and colleagues (1991) showed that communities had lower genetic diversity following addition of chemical pollutants, but that the dominant populations within the community possessed enhanced physiological tolerances and substrate utilization capacities. Our results suggest that an adaptive tolerance response may have occurred in survivors (Díaz-Raviña and Bååth, 1996;Bååth et al, 1998), which is consistent with a lack of influence of copper on bacterial diversity.…”

Rapid Method for Assessing Pollution-Induced Community Tolerance in Contaminated Soil

“…Extraction from soil and cultivation of microbial populations can only be done for a very small part of the total soil microbial community [9]. Consequently, the outcome of the laboratory PICT experiments with soil microorganisms is valid only for the corresponding part of the community.…”

“…Doelman and colleagues [6,7] and Smit et al [8] have shown increased metal tolerance of terrestrial bacteria isolated from contaminated sites. Díaz-Ravina and Bååth [9] have shown increased metal tolerance of terrestrial bacterial communities to experimentally increased metal levels.…”