A combination method to study microbial communities and activities in zinc contaminated soil

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Abstract

Zinc (Zn) plays a special role in soil ecology and fertility because it can support the growth of soil organisms or inhibit their growth depending on its concentrations. In this work, the effects of different concentrations of Zn on soil microbial communities and activities were analyzed by loading five different doses of Zn (160–6000 μg g−1) into a wheat surface soil. The microbial metabolic process revealed a significant bimodal pattern at high concentrations of Zn (>1920 μg g−1). This phenomenon suggested that soil microorganisms were very sensitive to zincous poisoning. A variety of soil quality properties were also measured and assessed. The results showed slower bacterial growth in soil cultures polluted with high levels of Zn. In addition, two kinds of fungi were identified by morphology and glomalin-related soil protein content in the Zn-contaminated soil. The growth of the first kind was inhibited with increase in Zn concentration. By contrast, the second kind could survive and continue to grow with increasing doses of Zn at 160–1920 μg g−1 and its growth began to decline with further increase in Zn concentration. Finally, the fungus could not survive at very high (6000 μg g−1) Zn concentration. In this work, we conclude that soil microbial communities and activities can adapt to Zn pollution to a certain extent.

Introduction

Concentrations of toxic metals in the soil are a major soil ecological and toxicological concern because of their hazardous effects on plants and human beings [1], [2]. As a nutrient, zinc (Zn) can not only accelerate microorganism growth but can also be toxic if its concentration is high. Its toxicological characteristics cannot be ignored especially when it is at or above 1165 μg g−1 [3], [4]. Some reports demonstrated that certain microorganisms have a whole battery of responses to cope with toxic metal stress in order to survive in toxic metal-contaminated ecosystems. Species with the least efficient detoxification systems will disappear from the ecosystem [5]. When toxic metal pollution is so severe that there are consistent detrimental effects on metabolism, microorganisms are subjected to selective pressures and will increase their resistance to toxic metal [6], [7], [8]. Although a vast number of literature has reported the impact of metal toxicity on plant communities and populations [9], [10], the ecotoxicity of metal on soil microbial populations still remains a wide concern.

Soil microbial characteristics are frequently assessed through traditional methods such as counting the number of microbial communities [11], functional activities including respiration and mineralization [12], carbon dioxide evolution [13], and microbial biomass [14], [15]. However, most of these classical methods involve the destruction of the original soil samples and the study conditions are very different from the field environment [16].

In addition, quantitative and representative recovery of microorganisms from the natural environment is essential in understanding the ecosystem function. Recently microcalorimetry has become a useful technique to assess metal toxicity to soil microorganisms. The salient advantage of microcalorimetry is that it can quantify microbial activity continuously and real-time and the incubation conditions are very similar to actual soil environment [17], [18], [19], [20]. To our knowledge, research on real-time monitoring of microbial activity, soil biology and ecotoxicology are still scarce.

In this work, the effect of Zn on soil microbial communities and activities is assessed and evaluated by using microcosms. Soils could be homogenized to evenly distribute with both microbial populations and toxicants; thereby reducing spatial variability. This allowed us more effectively to assess the Zn poisonous effect on microbial community [21]. The soil microbial metabolic process was then monitored by a microcalorimeter. Other chemical and biological indicators including soil organic matter (OM), total organic carbon (TOC), carbon/nitrogen ratio (C/N), dissolved oxygen (DO), colony forming unit (CFU), fungal morphology and glomalin-related soil protein (GRSP) were also simultaneously studied. In this work, we report the profound changes in the ecology of Zn-contaminated agricultural soil microorganisms using a combination of biological and chemical parameters. Finally, the correlations of these parameters were assessed by the principal component and correlation analyses and the results demonstrated that they were indeed closely correlated.

Section snippets

Reagents

Ammonium sulfate and glucose were purchased from Shanghai Yuelong Chemical Factory (Shanghai, P.R. China). ZnSO4·7H2O was obtained from Beijing Sinopharm Chemical Factory (Beijing, P.R. China). All reagents of analytical reagent grade or above were used as received.

Description of soils

Soil samples were collected in February, 2005 from a wheat surface soil at a depth of 5–10 cm after removing the top surface layer in Wuhan city (30.58°N and 114.27°E), Hubei province, China, a place with a typical subtropical climate

Metabolic heat curves of soil microorganisms

The metabolic process and activities of soil microbial communities exposed to different levels of Zn and were measured. Cell growth was exponential during the log phase of the growth as depicted in Fig. 1. If the heat output power P0 and Pt are the heat output powers at time = 0 and time = t, respectively, then Pt = P0 exp (kt) or ln Pt = ln P0 + kt. All the power–time curves displayed a typical process of microbial metabolic activity [30]. The growth rate constant (k) can be obtained from the rise part of

Conclusion

In summary, this study successfully evaluates the effect of Zn on soil microbial communities and activities by PCA of multiple parameters and microcalorimetry. The thermokinetic parameters obtained from the metabolic power–time curves can be used quantitatively to indicate the toxic effect of Zn to soil microbial activity. Our work reveals the ecotoxicity of Zn at high concentrations in soil solution. The thermokinetic parameters obtained by microcalorimetry are in good agreement with the

Acknowledgements

We express our sincere thanks to Prof. Shixue Zheng of the State Key Lab of Agricultural Microbiology, Huazhong Agricultural University for his valuable technical assistance in plate counting. This research was supported in part by grants from the Governmental International Science and Technology Cooperation Project (2006/3139 and 2006-262/32-35), National Natural Science Foundation of China (40425001 and 40673065), the Specialized Research Fund for the Doctoral Program of Higher Education

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