Rapporteur Comments on the Bioremediation Session
Roger C. Prince
We have heard that bioremediation is an important and ethical approach to many environmental problems. Perhaps the best thing is that if successful, it is a permanent solution to the environmental problem. Environmental contaminants are of particular concern when they are bioavailable and are doing something to the environment. Almost by definition, bioremediation is likely to remedy this; successful bioremediation removes the biologically available material. Most of the competing technologies are not as final as this because they usually only move the problem. They may concentrate or reuse it, but the typical response is to pick it up and put it somewhere else. Even the more rigorous physical approaches such as thermal desorption and washing do not focus on the bioavailable material, and reducing the total contamination may not be as effective as bioremediation at removing this material. Bioremediation has the advantage that when the microbes have done what they can do to organic compounds, the organic compounds are usually essentially completely eliminated. That is not always true, but at least it is the major process that goes on in the bioremediation of organic compounds. Bioremediation is also relatively inexpensive, which means that the people who have to do it rather like it, and it does have at least some public support as an environmentally appropriate technology.
Corporate Research Laboratory, Exxon/Mobil Research & Engineering Co., Annandale, NJ
The US Environmental Protection Agency (EPA) has greatly supported bioremediation and has pushed its use in a variety of situations. Exxon probably would not have been allowed to use bioremediation in Alaska (Prince and Bragg 1997) without the EPA pushing strongly for it to be tried (EPA 1989), and I think their efforts have led to general public support for the technology.
When we think about marine spills, it is important to recognize that the US coastline is divided into jurisdictions with statutory groups that decide how they would handle spills (http://www.nrt.org). Bioremediation is typically included as a final polishing step for open shorelines, and it is unlikely to be the “frontline” approach except in remote locations. Most shorelines are too publicly essential for something as slow as current bioremediation to work.
Bioremediation is much more likely to be useful in places where time is not absolutely of the essence and where other processes such as physical removal of the oil with bulldozers are very difficult or too dangerous for work crews. It is also important that any clean-up strategy have a net environmental benefit (Baker 1995), which bioremediation can readily achieve because it is so noninvasive.
As we have heard from all of the speakers, there are many good reasons that bioremediation is valuable, both on land (NRC 1993; Prince 1998) and in the marine environment (Lee and deMora 1999; Lin and others 1999; Prince and others 1997, 1999b; Swannell and others 1999). A major issue for those of us who are, as it were, practitioners is to continue successful applications when necessary. One of the biggest issues we face is maintaining both public and regulatory support, and a major issue is that bioremediation tends to be slow. There is thus a real need to give responders and the general public some confidence that a bioremediation strategy is having the expected results. We need to be able to show that the approach is encouraging a real biological process that will lead to biodegradation and removal of the contaminant. There is a pressing need for interim measures of success, and several of us are working on this issue. We are working on portable instrumentation and tool kits that allow us to monitor the success of fertilizer application, and the initial stimulation of microbial activity (Prince and others 1999a), but there is an obvious opportunity for developing sensitive molecular probes that will assess microbial responses directly.
Dr. Lee addressed toxicity endpoints. We are somewhat saddled in the terrestrial environment with clean-up standards that set some sort of concentration level, typically a goal of x parts per million of a particular contaminant. An alternative, and perhaps more meaningful, endpoint would be a goal of some minimal level of toxicity in two or three appro-
priate routine tests (Mueller and others 1999; Potter and others 1999; Saterbak and others 1999). The EPA is also addressing monitoring the disappearance of genotoxicity with successful bioremediation at terrestrial sites (Brooks and others 1998; Hughes and others 1998), and there are obvious potential extensions of this work to marine sediments (Ho and others 1999). The US Geological Survey (USGS) is developing a toxicity identification evaluation protocol for sediments (Lebo and others 1999), with the goal of using it for monitoring remediation. Thus, although there is some work in this area, there is a real need for more research on this issue. One new approach is to use semipermeable membrane devices loaded with oils that mimic fish tissue; these can be exposed at the contaminated site, and subsequently analyzed for contaminants, in a more reproducible way than exposing living animals on-site (Macrae and Hall 1998; Parrott and others 1999; Utvik and Johnsen 1999). Developing modern molecular genetic tools for assaying toxicity may also revolutionize this area and have profound influences on how and when remediation activities should be conducted.
There is also a very pressing need to deal with the mixed contaminants found in dredged materials from harbors and estuaries (NRC 1997). Dr. Young described anaerobic processes that might target such contaminants. A biological technology for cleaning contaminated sediments would be very useful, but it must accommodate the fact that the anaerobic conditions that are slowing down the degradation of some contaminants (e.g., hydrocarbons) are also immobilizing others (e.g., metals). The complete degradation of extensively halogenated compounds such as polychlorinated biphenyls (Bedard and others 1998; Wu and others 1999) requires initial anaerobic reductive dehalogenation followed by aerobic degradation. If one made currently anaerobic-contaminated sediments aerobic, one might well speed up the degradation of some organic contaminants, but at the potential expense of mobilizing currently immobilized metals and slowing reductive degradation processes. The issue of handling such mixed contamination requires much more research, and modern molecular tools may have an important role to play once the fundamental microbiological processes are understood. The area of anaerobic degradation of organic pollutants is so new that it is quite likely that basic research in this area will open new avenues for bioprocessing.
There are, also, some surprises when considering the spectrum of applications of bioremediation. In Europe there is now concern over the large volumes of vegetable oils that are shipped by sea (Mudge 1995). On several beaches, for example, mats of vegetable oil have polymerized on the beach. One might have thought that vegetable oil would be readily biodegradable and a very easy target for bioremediation. However, un-
der some circumstances, it is proving to be a long-lived contaminant. Thus, there are many areas in bioremediation as a response for marine spills where we are still being surprised.
Bioremediation is basically aiming to stimulate natural processes. Someone asked yesterday, “So, you mean if you just waited, it would happen anyway?” and the answer is essentially, “Yes.” With our current knowledge of bioremediation, we are only speeding up the natural process; and if we are lucky, we stimulate it up to five-fold.
The other side of this issue is that there are some situations where the natural rate of biodegradation of a contaminant is fast enough that there is no pressing need to stimulate it. There is quite a bit of research in this area, because of course such an approach might be even cheaper than bioremediation. Relying on natural attenuation is being accepted as an appropriate response for some terrestrial spills (Chapelle 1999; Lahvis and others 1999; Lu and others 1999; McNab and Dooher 1998; Stapleton and Sayler 1998), usually with the proviso that the site be monitored to ensure that the contaminant is indeed degrading and not migrating. There are obvious opportunities for using modern molecular probes in studying and quantifying the phenomena associated with natural attenuation, and work for terrestrial applications is well under way (Stapleton and Sayler 1998).
As we heard in Dr. Portier's presentation, it is often more important to clean up the source of chronic contamination than to clean the contaminated site. If the point source is removed, natural attenuation may well remedy the contaminated area that was being affected by the source.
So, a part of what we heard was the need to continue and steadily improve the successful applications of bioremediation. However, another important avenue for research and development is to move bioremediation to the next level of effectiveness and speed. There is a general optimism in the field that we ought to be able to do radically better in stimulating natural processes without causing any significant harm—We need to find ways of getting things to happen much faster. In the laboratory, one can get many contaminants to disappear with dramatically rapid rate constants, but we are not within two orders of magnitude of these rates in the field. We have to understand better what it is that is limiting the biodegradation of some of our contaminants.
The presentations principally dealt with organic compounds, and the great thing about organic compounds is that eventually they are converted to CO2 and water and to all intents and purposes disappear. Inorganic contaminants can only be moved or collected; and although there are several technologies for handling inorganic contaminants in wastewater (Krishnan and others 1993), including several biological ones (Keasling and others 1998; Kefala and others 1999), there are not yet any
proven biotechnologies for dealing with metals and metalloids in sediments, shorelines, and marshes.
One potential new approach is phytoremediation—the use of plants to remedy environmental problems (Salt and others 1995). Although there is interest in using plants to stimulate oil biodegradation (Carman and others 1998), most work appears to be focused on using plants to accumulate metals and metalloids (Raskin and others 1994).
Thus, in summary, we heard that bioremediation in the marine environment is an important option for dealing with spills and a potential option for dealing with contaminated sediments. Progress in developing these technologies will come from a number of fronts, and modern molecular approaches must be integrated into this ongoing work. Terrestrial applications of bioremediation have received more attention than marine ones because of the far greater need, and gene-probe and molecular taxonomy approaches are beginning to move from the laboratory to the field. Some of these will be directly transferable to the marine environment, but there will undoubtedly be a need to develop saline-specific techniques. The next few years promise to be an exciting time for such developments.
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