Chapter 2 EVALUATION OF MICROBIAL DIVERSITY OF DIFFERENT SOIL LAYERS AT A CONTAMINATED DIESEL SITE
EVALUATION OF MICROBIAL DIVERSITY OF DIFFERENT SOIL LAYERS AT A CONTAMINATED DIESEL SITE
In this study, we evaluated the hydrocarbon removal efficiency and microbial diversity of different soil layers. The soil layers with high counts of recoverable hydrocarbon degrading bacteria had the highest hydrocarbon removal rate compared to soil layers with low counts of hydrocarbon degrading bacteria. Removal efficiency was 48% in the topsoil compared to 31% and 11% in the 1.5 m and 1 m respectively. There was no significant difference between the Total Petroleum hydrocarbon (TPH) removal in the nutrient amended treatments and the controls at 1 m and 1.5 m soil layers. The respiration rate reflected the difference in the number of bacteria in each soil layer and the availability of nutrients. The high O2 consumption rate corresponded positively with the high TPH removal rate. Analysis of the microbial diversity in the different soil layers using functional diversity (community level physiological profile using Biolog) and genetic diversity using Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis (PCR-DGGE) of 16 SrDNA revealed differences in substrate utilisation patterns and DGGE profiles of 16 SrDNA fragments respectively. The microbial diversity as revealed by DNA fragments was reduced in the highly contaminated soil layer (1.5 m) compared to the topsoil and the soil layer at 1 m.
The effect of hydrocarbon contamination on soil microbial communities has been studied (Atlas et al., 1991; Wünsche et al., 1995; Lindstrom et al., 1999; MacNaughton et al., 1999; Stephen et al., 1999; Juck et al., 2000; Bundy et al., 2002). However, these studies only investigated the influence of hydrocarbons using mainly the topsoil. Information about the microbial diversity of different soil layers at a given site is lacking. Because oil contamination normally penetrates deeper than the top layer, it is important to understand the distribution of degrading populations with soil depth and how the distribution patterns influence the efficiency of biodegradation. The subsurface soil environment, though devoid of sufficient nutrients, oxygen and other factors, harbors an array of soil microorganisms that plays an important role in decomposition and the recycling of nutrients (Krumholz, 1998). It is widely presumed that the number of heterotrophic bacteria changes with increasing depth. This can be attributed to spatial and resources factors which can influence the microbial diversity of the soil (Zhou et al., 2002). Shallow subsurface micro-flora appears to be predominantly prokaryotic, appears to be specially adapted for growth and survival in nutrient poor conditions, includes strains that can function throughout a wide range of nutrient concentrations and may sometimes exert significant effect on groundwater chemistry (Ghiorse and Balkwill, 1983; Balkwill and Ghiorse, 1985; Bone and Balkwill, 1988; Ghiorse and Wilson, 1988; Balkwill et al., 1989). The availability of hydrocarbons in the vadose zone can alter the diversity of the heterotrophic community due to an increase in the carbon substrate. According to Atlas (1981), Leahy & Colwell (1990), the number of hydrocarbon bacteria and their relative abundance in the bacterial communities increases significantly in the presence of readily available hydrocarbons. Also the changes in hydrocarbon content in soil results in characteristic shifts of the substrate utilisation patterns by the microorganisms and that the altered pattern of substrate utilisation corresponds with similar changes in abundance of hydrocarbons in the soils (Wünsche et al., 1995). This is not surprising,and in accordance with the theories about gene accumulation and selection pressures, we can predict lower abundance of hydrocarbon degraders with depth as selection pressure and growth conditions in general lowers with depth. In this study, we investigated the hydrocarbon removal capacity and the microbial diversity of different soil layers after diesel contamination. The capacity of the soil layers in removing hydrocarbons was evaluated (using simple microbial assays), while microbial diversity was evaluated using functional diversity (community level physiological profiles using Biolog micro plates) and genetic diversity (PCR-Denaturing Gradient Gel Electrophoresis of 16SrRNA).
Chapter 1 : General Introduction
Chapter 2 : Evaluation of Microbial Diversity of Different Soil Layers at a Contaminated Diesel Site
Chapter 3 : Evaluation of Microbial Communities Colonizing Stone Ballasts at Diesel Depots
Chapter 4 : Soil Microbial Diversity: Influence of Geographic Location and Hydro- carbon Pollutants
Chapter 5 : Multiplanted and Monoculture Rhizoremediation of Polycyclic Aromatic Hydrocarbons (PAHS) from the Soil
Chapter 6 : Germination of Lepidiun Sativum as a Method of Evaluating the Removal of Polyaromatic Hydrocarbons (PAHS) from Contaminated Soil
Chapter 7 : The Use of Biological Activities to Monitor the Removal of Fuel Contaminants: Perspective for Monitoring Hydrocarbon Contamination.
Chapter 8 : Bioremediation of Petroleum Hydrocarbons through Landfarming: Are Simplicity and Cost-Effectiveness the Only Advantages?
Chapter 9 : Conclusions and Perspectives