ABSTRACT
Large amounts of the petroleum products that are produced in the world end up entering the soil and the marine environment [1]. The fate of these extremely complex mixtures of hydrocarbons has been mostly connected with the action of microorganisms [2]. The expansion of petroleum development into new frontiers, such as deep offshore waters and ice-dominated Arctic environments, and the apparently inevitable spillages that occur during routine operations and as a consequence of accidents have maintained a high research interest in this field. Consequently, there is a growing industry dedicated to the biological cleanup of soils and groundwater that have been contaminated with all types of petroleum products [3]. However, the microbial cell capacity to utilize these hydrocarbons as a sole source of carbon and energy is highly dependent on the chemical nature of the compounds within the petroleum mixture and on environmental determinants. The soil environment consists, in varying proportions, of gas, liquid, and solid phases, each of which may differ considerably in composition. As a consequence, the physicochemical properties of soils vary greatly [4]. In turn, this affects the in situ microbiology and has profound implications for the biotechnologist whose aim is to enhance biodegradation rates in order to clean up contaminated sites. From the nutritional point of view, the solid-phase components of soil are varied but can be loosely grouped into organic and mineral species. The bulk of the former are present as humus, a complex, relatively recalcitrant mixture of polymers produced by chemical and microbial attack on plant material. With increasing depth, total organic carbon concentration decreases and the available carbon is highly recalcitrant. The mineral composition is dependent on the geology of the area. Moreover, soils are vertically stratified, and there are fundamental differences between depth horizons. The topsoil is the site of entry for organic carbon, oxygen, and water but may be subject to severe leaching of minerals and drying. The subsoil is oxygen-deficient and biologically quiescent but may contain leached minerals and a significant concentration of recalcitrant organic carbon.
