Effects of long-term metal contamination on the structure and function of microbial communities in soils.



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Texas Tech University


Microbial communities are critical components of soils and are known to be important for a wide range of ecosystem-level processes. However, due in part to methodological limitations, much of the basic structure and activity of microbial communities in both pristine and anthropogenically disturbed soils remains unknown. One hundred years of mining and smelting activity at the Anaconda Smelter Site in Anaconda, Montana has caused high concentrations of metals to be deposited in surrounding areas, leading to significant degradation of the soils, loss of above-ground vegetation and toxicological effects on humans and wildlife. Different phytoremediation strategies were tested in situ within the 1.5 acre Dragstrip demonstration area, to assess the efficacy of different soil amendments (fertilizer formulations, organic matter, liming materials, and depth of soil plowing) for supporting plant growth. The success of plant-based remediation techniques is largely dependent on the health and stability of the soil, of which soil microbial communities play essential roles. While high concentrations of metals are known to negatively affect microbial activity, biomass, and enzyme function, amendment of soils during the remediation process may further modify microbial community structure and function in soils. Little is known about the effects of soil amendments on the structural and functional diversity of microbial communities in heavy metal contaminated soils. Additionally, a better understanding is needed of the effects of metals on microbial community structure and function following long-term in sutu exposure, and following contamination with increasing concentrations of metals. The following research attempts to characterize the effects of anthropogenic disturbance (i.e., soil metal contamination and/or different soil amendment strategies) on the structure and function of microbial communities in soils surrounding the Anaconda Smelter as follows: 1) Microbial communities within the six remediated Dragstrip demonstration plots and adjacent unremediated control plot were characterized using a combination of culture-based (Biolog) and non-culture based (DGGE) techniques to characterize the combined effects of soil metal contamination and amendment strategy on microbial community structural (community DGGE banding profiles) and functional diversity (community carbon substrate utilization profiles (SUPs)). 2) Microbial communities native to six smelter-impacted sites (representing a gradient of soil metal concentrations) and a non-impacted site (representing background levels of metals) were compared to determine the long-term effects of metal contamination on microbial community dynamics (microbial activity, biomass, structural diversity and functional diversity). 3) Soil native to two smelter-impacted sites and a non-impacted site (previously exposed to high, low or background concentrations of aerially-deposited metals, in situ, respectively) were artificially-amended with metal-salts in the laboratory to characterize the dose-response effects of increasing concentrations of metals on microbial community dynamics. Additionally, this research tested the hypothesis that soil metal contamination, acting as an extreme environmental stressor, will catalyze a shift in species diversity and abundance, causing initially unique communities to converge on a community with similar structure and function. Results from these studies show that several physiochemical soil characteristics (percent organic matter, soil pH, cation exchange capacity) significantly influence the bioavailability of metals in soils, and metal bioavailability in turn influenced the toxicity of metals to soil microbes. Not only did soil metals significantly decrease microbial activity and biomass, but they also caused significant shifts in community structure, indicating the potential for metal stress to shift species diversity and abundance. The effects of soil metal contamination on community SUPs was less pronounced, which may give evidence of functional redundancy within the enriched portion of the communities. Soil physiochemical profiles were influenced by soil remediation amendment strategies, and several physiochemical parameters (K, NH4organic matter, and cation exchange capacity) were correlated with shifts in microbial community structure, indicating that amendment strategy has the potential to modify microbial communities over time. Finally, while microbial communities were not observed to converge on a common community as a result metal stress, these studies have documented the potential for metal contamination to shape the structural and functional diversity of microbial communities in soils. Microbial community endpoints are increasingly being marketed as potentially good indicators of soil ecosystem health and stability. These studies have shown that microbial community activity, biomass, community structure, and community function are sensitive endpoints for monitoring microbial responses to metal stress. However, additional studies are necessary to truly understand the complexity of microbial community responses to long-term metal contamination.



Metal contamination, Microbial communities, Soil