QUESTIONS AND ANSWERS

Why do organic pollutants tend to degrade slowly in situ? 
Jerry Sims, Univ. of Illinois

Q:  Most commonly, students asked about the general utility in using microbes (be they naturally occurring or "engineered") as a clean up tool in environmental spills. Is the utility of either class of microbe governed by cost, technical capability, potential risk, or something else?

A:  The idea of collecting superior performing microorganisms, usually bacteria, or engineering better ones for deployment in the environment has been around for many years. This was done routinely in the case of inoculating soybeans with Bradyrhizobium japonicum all over the US (the soybean was an introduced species, and native soil populations contained few of the appropriate bacteria to promote nitrogen fixation). Years of research in this area showed that as soils began to harbor the soybean symbionts, it became increasingly more difficult to displace the indigenous bacteria with the introduced ones, presumably because the indigenous forms were better fit for that particular site. To some extent, this has been true of attempts to introduce organisms that degrade pollutants. That, however, does not mean inocula are never used. Remediation of sites contaminated with some chlorinated solvents can be enhanced by the introduction of degraders, probably due to a lack of indigenous degraders. Another angle is the use of an overpowering inoculum, continuously grown on site and introduced typically with some sort of irrigation system. This allows development of enormous populations of an organism that may not really be competitive for that site. Supposedly, the organism will die off rather quickly once introduction ceases.

As you might guess, it is not only costly to engineer organisms, there are regulatory hurdles for their introduction. Early engineered organisms meant to be general purpose degraders tended to be miserable failures, which was attributed to their lack of fitness and competition from indigenous forms.  Even without burdening an organism with excess DNA, simply maintaining cultures in the lab creates selection pressure that may reduce the organism's fitness in the environment. Thus, the prospects for coming up with inocula that will be superior to indigenous forms are not always that good.

In addition to the use of pure cultures of microorganisms, various consortia may be used, and it is also possible to bring environmental samples, such as soil or water treatment plant samples to another site as an inoculum.

Going back to your question, the use of microbial inocula may be limited by cost, technical capability (usually not), potential risk (regulatory concern), and something else (difficulties of introducing a new microorganism into the environment).

Q:  How are the products of the breakdown process monitored (to ensure that these are not merely a different category of toxin)? How routine or quantitatively standardized is this monitoring?

A:  Remediation is under the jurisdiction of environmental regulatory concerns, state and federal. Whatever plan is filed must be approved by some such regulator. Approval is given on a case by case basis, and even the monitoring regime used is subject to regulatory approval. For many, if not most organic contaminants, an approved method exists. These methods are usually based on gas chromatography-mass spectrometry or liquid chromatography-mass spectrometry methods, and they often have their own EPA "number." Various quality assurance protocols will be required.

As to whether the degradation products are toxins, that is another question. If a degradation product happens to be a compound that is also being commercially synthesized and used for some purpose, there is generally some sort of opinion in place about its toxicity. So, if your are dealing with a simple parent compound, its degradation products are probably simple compounds that have already been studied, and an opinion is available about toxicity. If you propose a remediation strategy that produces a product known to be toxic, the regulator is unlikely to approach your plan.

For many novel compounds, such as pharmaceuticals and pesticides, one must obtain a registration before these materials can be sold. In the process of registration, questions about toxicity of degradation products may be raised, or not, depending on the negotiations that have been made. As a case in point, back when I worked for a chemical company, there were EPA guidelines that required some minimal toxicology studies to be performed on metabolites that exceeded some concentration threshold. If the parent compound was an herbicide, that might consist of showing the metabolite was no longer herbicidally active. It was also possible that EPA might require extensive toxicology work if they were concerned, and may even ask for toxicology studies on metabolites that did not reach the concentration trigger. These kinds of questions are not usually handled in a black and white fashion. There is a certain amount of judgment (or possibly, lack thereof) used on the part of the regulator.

There has been increased concern over degradation products of pesticides, and as a result, USGS, who routinely monitors surface and groundwater for pesticides and fertilizer components, also looks for degradation products. Their methods are generally standardized.

Also, these things are subject to being revisited. If additional data arises that casts doubt on the safety of a degradation product, regulators become concerned. The fumigant, 1,3-dichloropropene became restricted because of the discovery of the toxicity of a contaminant. The herbicide 2,4,5-T had the same problem (it contained a dioxin). Similar issues may arise with breakdown products.

Q:  Why does atrazine persist at high levels in the environment if it can be so "easily" degraded?

A:  That is really at the heart of the seminar I gave. In pure culture, atrazine can be completely degraded to carbon dioxide and ammonium within a few days. However, biodegradability does not ensure degradation in situ. For atrazine, there are general and specific reasons for the compound to hang around.

General: Like any compound that enters the environment primarily via the soil, atrazine may spend considerable residence in the soil before it is washed out and enters some other compartment (surface or subsurface water). If one assumes that around 1,000-10,000 atrazine degraders are present per gram of soil (this is the range most commonly reported in the literature), and the compound is present at a concentration too low to drive significant growth of the organisms, atrazine will have to find its way to these few organisms through a maze of some 10⁸ pores per gram.  Diffusion will be limited by sorption and, when the soil is not completely saturated with water, discontinuity of the pores as well. Thus, lack of bioavailability will contribute to persistence.

Specific: Many organisms that degrade atrazine yield little or no energy from it. The compound is largely a nitrogen source. Degradation of complex nitrogenous organic compounds tends to be carefully regulated in cells to prevent wasting biomolecules that were very expensive (energetically-speaking) to make. Atrazine degradation involves enzymes that may have been recruited from purine, pyrimidine (or similar compound) degradation and thus is regulated by such things as ammonium, and probably other signals of nitrogen sufficiency. In most soils, there is sufficient ammonium or nitrate to inhibit atrazine degradation. In addition, atrazine is used mostly for production of corn, which requires considerable nitrogen fertilization, thus atrazine degradation is likely expressed at a relatively low, constitutive level at many sites.