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leakage of underground storage tanks or are added directly to agricultural soils as the "inactive" ingredients of pesticides. Many pesticide formulations contain 7-14% BTEX by weight as carriers. Benzene in particular is a known carcinogen and is soluble in water at concentrations far greater than the drinking water standard (1 ^g liter-1). For BTEX compounds, the principal concern is their ready migration away from source areas.

Polycyclic aromatic hydrocarbons are the contaminants of concern at manufacturing petroleum plants and EPA Superfund sites across the United States (Table 17.9). Of the PAHs, the high-molecular-weight compounds (those with four or more rings) are of most concern with respect to health risks since a number of these compounds are known carcinogens. PAHs in general are characterized by very low solubility in water.

In addition, these compounds readily form nonaqueous phases in soil and sediment and also bind tightly to soil constituents such as SOM. Despite the limited water solubility of PAHs, PAH-contaminated sediment can lead to teratogenicity and toxicity of the water. Contaminated sediments represent a continuing source of contamination in the aquatic food chain.

Despite the high potential for bioremediation as an effective technology, its use is limited by the depth of understanding of biodegradation processes and inexperience in managing these processes in the field. This includes aspects of cometab-olism, inoculation, evolution of biodegradation capabilities, monitoring and process control, measures of effectiveness, and genetic engineering.

concluding comments on microbial ecology

From the foregoing discussion it is evident that humans can and have managed microorganisms for our own benefit. However, much of our discussion has centered on "unnatural" systems in which we manipulate the organism environment to accomplish the desired outcome. For example, culturing all of the organisms in a gram of soil and using them in a fermentation of grape juice would produce undrinkable wine, but inoculating the fermentation with specific species of microorganisms produces a quality product. In composting for gardens or commercial production, the operation is usually controlled at least with respect to ingredients, aeration, and moisture.

But what are the options and benefits with organisms and their management in "natural" environments? In natural environments, the mass of organisms and species composition (structure) are governed by the environment, including habitat, moisture, and temperature. Organisms form trophic levels with various modes of interaction and coexistence, from symbiotic to predatory. The community of soil organisms may increase or decrease and their composition may change principally due to influences of abiotic microclimate and natural input of metaboli-cally available substrates (plant litter, leachates, and exudates). The complexity of the system dictates that introduced organisms must find a niche to survive and be persistent. In general, organisms introduced into a soil system do not usually survive more than a period of days to weeks. The reason for poor survival rates could be due to nutrient competition, susceptibility to predation and chemical attack due to differences in cell chemistry from culturing, or simply nutrient deficiencies.

In natural systems, organisms can be structured compartmentally to be close to other organism for symbiosis, away from other organisms for protection, and in proximity to nutrients and water. An example of organism symbiosis is fungi breaking down the macromolecule cellulose into smaller more "digestible" compounds that can be utilized by other bacteria and fungi that cannot utilize cellulose directly. In addition, an organism may need a specific growth factor or vitamin that may be produced by specific bacteria, thus by growing in a mixed culture (soil) the first organism receives "nutritional symbiosis." There is, however, a down side to living among a myriad of different organisms, that is, some are predators and some will resort to predation to survive. Predation is probably the biggest factor in organism composition changes over time. When carbon inputs from litter reach the soil, bacteria increase in numbers, and then bacterial-feeding protozoa increase and a new short equilibrium will be reached in community composition. This cyclic flux in population structure drives the complex food web system of the soil and dictates nutrient availability to plants. From this perspective, perhaps it is the soil microflora who is doing the "managing" in many ecosystems.

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