Impact of Heavy Metals on Bacterial Community Structure and Microbial Processes

The deleterious effects of heavy metals on microbe-mediated processes have been discussed in detail by several researchers (Baath 1989; Giller et al. 1998). Generally, decreases in carbon mineralization and fixation, nitrogen transformation, soil enzyme activities, and litter decomposition can be observed. Other typical effects of heavy metal contamination are a decrease in the number of microbes (CFU), microbial biomass, or an increase in the frequency of heavy metal resistant bacteria (Pennanen et al. 1996; Müller et al. 2001).

However, measuring these parameters is not a suitable approach for determining changes in the entire structures of soil communities exposed to pollutants. Since many of the microbiological and biochemical techniques used to study the effects of heavy metals on soil bacteria are cultivation dependent, they do not provide detailed information on noncultivable bacteria, thus neglecting the major part of the soil microbial community. Consequently, soil microbial communities are treated as a black box. These limitations have been overcome by recent advances in molecular fingerprinting methods. These fingerprinting techniques, which are based on analyses of signature biomarkers such as phospholipid fatty acids or nucleic acids, have been used in numerous studies and have indicated significant changes in the microbial community in response to heavy metal stress. Moreover, these methods have allowed the bacterial community to be monitored during the remediation process (Kelly 1998; Macnaughton et al. 1999). These studies have mainly increased our knowledge of sensitive bacterial populations that are negatively affected by heavy metals, but it should also be noted that heavy metals favor the development of tolerant species that can survive and adapt due to their genetic characteristics.

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