Effects of Cd and Pb on soil microbial community structure and activities. Environ Sci Pollut Res Int

Department of Environmental Sciences, University of Peshawar, 25120, Peshawar, Pakistan.
Environmental Science and Pollution Research (Impact Factor: 2.83). 04/2009; 17(2):288-96. DOI: 10.1007/s11356-009-0134-4
Source: PubMed


Soil contamination with heavy metals occurs as a result of both anthropogenic and natural activities. Heavy metals could have long-term hazardous impacts on the health of soil ecosystems and adverse influences on soil biological processes. Soil enzymatic activities are recognized as sensors towards any natural and anthropogenic disturbance occurring in the soil ecosystem. Similarly, microbial biomass carbon (MBC) is also considered as one of the important soil biological activities frequently influenced by heavy metal contamination. The polymerase chain reaction-denaturing gradient gel electrophoresis (DGGE) has recently been used to investigate changes in soil microbial community composition in response to environmental stresses. Soil microbial community structure and activities are difficult to elucidate using single monitoring approach; therefore, for a better insight and complete depiction of the soil microbial situation, different approaches need to be used. This study was conducted in a greenhouse for a period of 12 weeks to evaluate the changes in indigenous microbial community structure and activities in the soil amended with different application rates of Cd, Pb, and Cd/Pb mix. In a field environment, soil is contaminated with single or mixed heavy metals; so that, in this research, we used the selected metals in both single and mixed forms at different application rates and investigated their toxic effects on microbial community structure and activities, using soil enzyme assays, plate counting, and advanced molecular DGGE technique. Soil microbial activities, including acid phosphatase (ACP), urease (URE), and MBC, and microbial community structure were studied.
A soil sample (0-20 cm) with an unknown history of heavy metal contamination was collected and amended with Cd, Pb, and Cd/Pb mix using the CdSO(4) and Pb(NO(3))(2) solutions at different application rates. The amended soils were incubated in the greenhouse at 25 +/- 4 degrees C and 60% water-holding capacity for 12 weeks. During the incubation period, samples were collected from each pot at 0, 2, 9, and 12 weeks for enzyme assays, MBC, numeration of microbes, and DNA extraction. Fumigation-extraction method was used to measure the MBC, while plate counting techniques were used to numerate viable heterotrophic bacteria, fungi, and actinomycetes. Soil DNAs were extracted from the samples and used for DGGE analysis.
ACP, URE, and MBC activities of microbial community were significantly lower (p < 0.05) in the metal-amended samples than those in the control. The enzyme inhibition extent was obvious between different incubation periods and varied as the incubation proceeded, and the highest rate was detected in the samples after 2 weeks. However, the lowest values of ACP and URE activities (35.6% and 36.6% of the control, respectively) were found in the Cd(3)/Pb(3)-treated sample after 2 weeks. Similarly, MBC was strongly decreased in both Cd/Pb-amended samples and highest reduction (52.4%) was detected for Cd(3)/Pb(3) treatment. The number of bacteria and actinomycetes were significantly decreased in the heavy metal-amended samples compared to the control, while fungal cells were not significantly different (from 2.3% to 23.87%). In this study, the DGGE profile indicated that the high dose of metal amendment caused a greater change in the number of bands. DGGE banding patterns confirmed that the addition of metals had a significant impact on microbial community structure.
In soil ecosystem, heavy metals exhibit toxicological effects on soil microbes which may lead to the decrease of their numbers and activities. This study demonstrated that toxicological effects of heavy metals on soil microbial community structure and activities depend largely on the type and concentration of metal and incubation time. The inhibition extent varied widely among different incubation periods for these enzymes. Furthermore, the rapid inhibition in microbial activities such as ACP, URE, and MBC were observed in the 2 weeks, which should be related to the fact that the microbes were suddenly exposed to heavy metals. The increased inhibition of soil microbial activities is likely to be related to tolerance and adaptation of the microbial community, concentration of pollutants, and mechanisms of heavy metals. The DGGE profile has shown that the structure of the bacterial community changed in amended heavy metal samples. In this research, the microbial community structure was highly affected, consistent with the lower microbial activities in different levels of heavy metals. Furthermore, a great community change in this study, particularly at a high level of contamination, was probably a result of metal toxicity and also unavailability of nutrients because no nutrients were supplied during the whole incubation period.
The added concentrations of heavy metals have changed the soil microbial community structure and activities. The highest inhibitory effects on soil microbial activities were observed at 2 weeks of incubation. The bacteria were more sensitive than actinomycetes and fungi. The DGGE profile indicated that bacterial community structure was changed in the Cd/Pb-amended samples, particularly at high concentrations.
The investigation of soil microbial community structure and activities together could give more reliable and accurate information about the toxic effects of heavy metals on soil health.

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    • "Cadmium (Cd) is a heavy metal that can be added to soil by phosphate fertilizers, pesticides, limestones, and industrial effluents which can contaminate groundwater and be absorbed by the plant, causing damage to the environment and human being (Arruda and Azevedo 2009; Saier and Trevors 2010. In soils, Cd affects microbe population and plant growth even in low concentrations (Gratão et al. 2005, 2008, 2012; Mobin and Khan 2007; Wahid and Ghani 2008; Khan et al. 2010; Tezotto et al. 2012; Li et al. 2013). In plants, Cd affects several processes such as water transportation, mitochondria oxidative phosphorylation, photosynthesis and chlorophyll content , and ultrastructural changes (Vitória et al. 2006; Gratão et al. 2009; Gallego et al. 2012). "

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    • "Microbial community had been well shown to be sensitive to increase in heavy metal concentrations in soils (Giller et al. 1998). Reduction in microbial abundance and diversity had been often reported, either in short-term lab spiked incubation (Gao et al. 2010; Harris-Hellal et al. 2009; Khan et al. 2010) or under long-term exposure to toxic metals in fields (Li et al. 2006; Wakelin et al. 2010). Heavy metal contamination in soil could affect the rates of microbial-mediated biogeochemical processes (Liu et al. 2012a, 2014), resulting in changes in GHG production and emission. "
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    • "Metals and metalloids such as cadmium (Cd), lead (Pb), mercury (Hg), and zinc (Zn) are called heavy metals because of their high densities (Oves et al. 2012), while arsenic (As) is included in this list because of similar properties (Chen et al. 1999). Essential and nonessential trace elements , when exceed the threshold limits, can cause different physiological, morphological, and genetical anomalies including reduced growth, mutagenic effects, and increased mortality (Khan et al. 2010a; Li et al. 2010; Luo et al. 2011). Food crops are one of the important parts of our diet, and they may contain a number of essential and toxic metals (Yang et al. 2011; Waqas et al. 2015) depending on growing media characteristics. "
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