Field studies have
shown that many organic contaminants degrade naturally by microorganisms present
in soil and groundwater. This naturally occurring bioremediation, known as natural
attenuation, has been accepted as a passive remediation process for contaminated
soil and groundwater. When the natural degradation is insufficient to meet remediation
goals protective of human health and the environment, there may be attempts
to increase the degradation rate by adding nutrients to stimulate the growth
of the native microbial populations.
Active bioremediation, which includes adding specialty microbes, may mimic the naturally occurring process, but it has the advantage of taking the guesswork out of nutrient-stimulated bioremediation. For example, the degradation rate of the target compound is known and the specialty microbes can be delivered in populations higher than a stimulated natural system can achieve. In addition, adding specialty microbes can be faster and less expensive than attempts to stimulate indigenous microbes.
Basis of Cometabolism
Cometabolic aerobic oxidation is the microbial breakdown of a contaminant in which the contaminant is oxidized incidentially by an enzyme produced during the metabolism of another compound. In other words, a cometabolite is an enzyme produced by microbiological metabolism that aids in the degradation of a contaminant. Cometabolism in Pseudomonas bacteria is possible because they have genetic coding available for metabolic pathways, which produce enzymes. The development and preservation of these genetic traits may be indirect evidence that the cometabolism provides some advantage to the bacteria.
This characteristic of Pseudo-monads provides an effective mechanism for in situ bioremediation of a wide range of organic contaminants. Pseudomonas bacteria are well known for their ability to break down a wide range of chemicals. Their natural ability to tolerate and degrade contaminants can be enhanced by selective regeneration in the laboratory. At times the combination of several strains of microbes in a synergistic consortium can induce further microbial capabilities. Pseudomonads represent the most widely studied bacterial genera for investigations involving the degradation of complex hydrocarbons. Pseudomonads are gram negative aerobic rods that are ubiquitous in nature. Early studies in 1926 in the Netherlands by den Doorende Jong demonstrated that many different organic compounds supported the growth of Pseudomonads.
These early studies led to many investigations in countless laboratories around the world into the complex biochemical pathways that were discovered to be present in these bacteria. As our ability to understand the genetic basis for this metabolism grew in the 1970s more laboratories devoted time to understanding the genes encoding for this metabolism. Since the 1960s certain investigators (I. C. Gunsalus, D. T. Gibson, H. J. Knackmuss, B. W. Holloway, A. M. Chabrabarty, R. H.Olsen, K. N. Timmis, J. C. Spain, L.P. Wackett and G. J. Zylstra) have been associated with studies involving the molecular basis for the degradative metabolism of the Pseudomonads. Through their scientific efforts to understand the molecular basis of the degradative metabolism of Pseudomonads, the successful application of the strains into the environment has been possible.
One of the advantages of Pseudomonads is their aerobic respiration. The fact that aerobic oxidation is possible broadens the scope of applications. Pseudomonads are suitable for use in not only aerobic soil and groundwater applications, but have been used in applications open to the atmosphere and in combination with air sparging as well.
Site Closure by Cometabolism
A case study shows that while the naturally occurring microbial population may have partially degraded contaminants, the degradation was slow and incomplete. Using specialty microbes, the degradation was completed in an efficient time frame. At a former chemical mixing company, contamination by a wide range of volatile organic chemicals was found in soil and groundwater. The contamination resulted from the accumulation of years of incidental spills. The volume of contaminated soil within the source area was estimated to be 2,200 tons. The soil contaminants were mainly tetrachlororethene (PCE) and trichloroethene (TCE), and 1,1,1-trichloroethane (1,1,1-TCA) (see Table 1).