Influence of xenobiotic contaminants on landfill soil microbial activity and diversity

Abstract

Landfills are often the final recipient of a range of environmentally important contaminants such as hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). In this study the influence of these contaminants on microbial activity and diversity was assessed in a municipal solid waste (MSW) landfill placed in Torrejón de Ardoz (Madrid, Spain).

Soil samples were collected from four selected areas (T2, T2B, T8 and T9) in which the amount of total hydrocarbons, PAHs and PCBs were measured. Soil biomass, substrate induced respiration (SIR) and physiological profiles of soil samples were also determined and used as indicators of total microbial activity.

Highest concentration of total hydrocarbons was detected in T2 and T9 samples, with both PCBs and benzopyrene being detected in T9 sample. Results corresponding to microbial estimation (viable bacteria and fungi, and SIR) and microbiological enzyme activities showed that highest values corresponded to areas with the lowest concentration of hydrocarbons (T2B and T8). It is noticeable that in such areas was detected the lowest concentration of the pollutants PAHs and PCBs. A negative significant correlation between soil hydrocarbons concentration and SIR, total bacteria and fungi counts and most of the enzyme activities determined was established. DGGE analysis was also carried out to determine the microbial communities’ structure in the soil samples, establishing different profiles of Bacteria and Archaea communities in each analysed area. Through the statistical analysis a significant negative correlation was only found for Bacteria domain when Shannon index and hydrocarbon concentration were correlated. In addition, a bacterial 16S rRNA gene based clone library was prepared from each soil. From the clones analysed in the samples, the majority corresponded to Proteobacteria, followed by Acidobacteria and Actinobacteria. It is important to remark that the most polluted sample (T9) showed the lowest microbial diversity only formed by six phyla being Proteobacteria and Acidobacteria the most representative.

Introduction

In Spain the most habitual practice for elimination of urban solid wastes has been its storage in municipal solid waste landfills. Although many old landfills are now sealed, these landfills continue to be potentially sources of significant environmental contamination. Moreover, the generation of contaminated leachate remains as inevitable consequence of the practice of waste disposal in landfills (Banar et al., 2006), with groundwater pollution being the most significant concern arising from leachate migration (El-Fadel et al., 1997). Degradation of the old landfill material is a slow process, lasting over 30 years.

Among the most hazardous compounds accumulated in soil and leachate, heavy metals, nitroaromatic compounds (NACs,) polycyclic aromatics (PAHs), and pesticides are the most dangerous. These compounds originated from household and industrial wastes can be found in most municipal landfills. Depending on their composition and on soil characteristics, these products may cause dramatic changes in aquifer geochemistry and landfill microbiology (Röling et al., 2001, Sastre et al., 2003).

Through knowledge of microbial community structure in polluted landfills, the capabilities of the microbial populations and their effect on the environment may be used as tools for predicting and monitoring natural degradation (Jain et al., 2005). Although bacteria capable of degrading pollutants usually play central roles in bioremediation, other organisms (i.e. fungi, protozoa and plants) can also affect the process (Demmerová et al., 2005). Techniques used to study landfill microbiology include traditional and emerging genetic molecular tools (Arias et al., 2005). The genetic diversity of microbial communities may now be monitored using profiling techniques such as denaturing gradient gel electrophoresis (DGGE). This technique allows the analysis of many samples simultaneously, and provides information relating to microbial communities. The construction of clone libraries and the sequencing of 16S rRNA gene sequences is another molecular technique frequently used to obtain phylogenetic information of the microorganisms present in a sample.

The aim of the present work was to study how the presence of different pollutants, including hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) affect the microbial activity and diversity in a municipal solid waste landfill at Torrejón de Ardoz (Madrid), Spain.