Constraints on the Use of Bioremediation in Wetlands

Irving A. Mendelssohn

INTRODUCTION

The objective of this brief presentation is to provide an independent assessment of the information gaps and research needs relating to the use of bioremediation in wetlands. The paper is organized as a series of questions and answers, which address the factors that limit or restrict the use of bioremediation in wetlands. It should be noted that within the context of this discussion, I equate bioremediation with biostimulation, the addition of nonmicrobial agents such as fertilizers and soil oxidants to stimulate the degradative capacities of naturally occurring microflora. It is now generally accepted that bioaugmentation, the application of oil-degrading bacteria to a contaminated site, is not useful in the wetland environment because of the abundance of indigenous hydrocarbon degraders in these carbon-rich systems.

IS BIOREMEDIATION APPLICABLE TO WETLANDS?

This basic question has not been adequately addressed. Although bioremediation has been demonstrated in laboratory, greenhouse, and some field trials, extensive field tests in a variety of different wetland types from coastal salt marsh to riparian forested wetlands have not been conducted.

Wetland Biogeochemistry Institute and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA



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OPPORTUNITIES FOR ENVIRONMENTAL APPLICATIONS OF MARINE BIOTECHNOLOGY: PROCEEDINGS OF THE OCTOBER 5-6, 1999, WORKSHOP Constraints on the Use of Bioremediation in Wetlands Irving A. Mendelssohn INTRODUCTION The objective of this brief presentation is to provide an independent assessment of the information gaps and research needs relating to the use of bioremediation in wetlands. The paper is organized as a series of questions and answers, which address the factors that limit or restrict the use of bioremediation in wetlands. It should be noted that within the context of this discussion, I equate bioremediation with biostimulation, the addition of nonmicrobial agents such as fertilizers and soil oxidants to stimulate the degradative capacities of naturally occurring microflora. It is now generally accepted that bioaugmentation, the application of oil-degrading bacteria to a contaminated site, is not useful in the wetland environment because of the abundance of indigenous hydrocarbon degraders in these carbon-rich systems. IS BIOREMEDIATION APPLICABLE TO WETLANDS? This basic question has not been adequately addressed. Although bioremediation has been demonstrated in laboratory, greenhouse, and some field trials, extensive field tests in a variety of different wetland types from coastal salt marsh to riparian forested wetlands have not been conducted. Wetland Biogeochemistry Institute and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA

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OPPORTUNITIES FOR ENVIRONMENTAL APPLICATIONS OF MARINE BIOTECHNOLOGY: PROCEEDINGS OF THE OCTOBER 5-6, 1999, WORKSHOP Additionally, within any given wetland type, environmental gradients in hydrology, salinity, and soil fertility exist that may constrain the realized efficiency and effectiveness of bioremediation. For example, areas of a wetland that are normally submerged may exhibit lower bioremediation effectiveness than sites experiencing daily inundation due to the more biochemically reduced conditions in the former. As a result, manipulative field experiments and controlled greenhouse studies are needed for a variety of wetland types before general conclusions can be made concerning the applicability of bioremediation in wetlands. Additionally, spills of opportunity should be used, whenever possible, to evaluate bioremediation under real-world conditions. Less emphasis should be placed on microcosm experiments because of the artificiality of this type of system. WHAT IS THE ROLE OF PHYTOREMEDIATION IN THE BIOREMEDIATION PROCESS? Because the application of bioremediation within a wetland environment generally occurs in the presence of wetland vegetation, we might ask what role the plants, per se, play in the degradation of the oil. Wetland vegetation could reduce oil concentrations in the soil directly by plant uptake as well as indirectly by maintaining a more suitable soil environment for microbial degradation of the oil. Traditional bioremediation agents such as fertilizers may act not only to directly stimulate microbial activity but also to increase plant growth and thereby indirectly affect plant-mediated controls on oil removal and degradation in the soil. Wetland plants may accelerate oil degradation by oxidizing the substrate by radial oxygen loss from roots and by root carbon leachates that may “kick start” the petroleum degraders into action. In the highly reduced soil of wetlands where oxygen may limit microbial activity, one might question whether bioremediation in the absence of plants will be effective. Thus, research to determine the role of phytoremediation in the bioremediation process is essential. We must answer the question: “Are plants necessary for significant bioremediation in wetlands?” HOW CAN BIOREMEDIATION BE MAXIMIZED IN WETLANDS? More research is needed to address applied questions relating to the type and mode of use of various bioremediation agents (biostimulants). Fertilization is arguably the primary bioremediation agent used to treat oil contamination in wetlands. A number of unanswered questions exist regarding nutrient amendments. For example, what nutrient most limits microbial degradation in wetlands—nitrogen or phosphorus? Does this limitation differ among different wetland types and even within a given

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OPPORTUNITIES FOR ENVIRONMENTAL APPLICATIONS OF MARINE BIOTECHNOLOGY: PROCEEDINGS OF THE OCTOBER 5-6, 1999, WORKSHOP wetland type? We cannot assume that results from salt marsh bioremediation trials are applicable to oil degradation in freshwater marshes. Can micronutrients become limiting to microbial degradation after macronutrient limitations have been alleviated? What forms of the nutrients should be used—urea, NH4Cl, NH4NO3? Is slow-release fertilizer more effective than soluble forms? Are multiple or split applications more beneficial than single applications? What are the best application methods? Research concerning the use of nonfertilizer agents such as soil oxidants, surfactants, and dispersants is also required. As previously mentioned, the lack of oxygen in wetlands may be a primary factor constraining maximum oil degradation. The effectiveness of oxidants such as calcium peroxide, nitrate, and manganese oxide, to name a few, in stimulating oil biodegradation in wetlands requires further research. In addition, the use of surfactants and/or dispersants either alone or in combination with fertilizers and soil oxidants to maximize biodegradation needs investigation. The simultaneous use of multiple agents should be considered. WHAT ARE THE MECHANISMS CONTROLLING EFFECTIVE BIOREMEDIATION? Basic research that elucidates the primary biotic and abiotic factors controlling oil degradation in wetlands is a first step in developing more effective bioremediation methodologies. How do nutrients, oxygen, temperature, and their interactions limit bioremediation? What is the relationship between the plant and microbial responses relative to oil biodegradation? The effect of the plant rhizosphere, that zone in the soil affected by plant roots, on microbial activity and oil degradation needs considerable more research. The differential effect of various plant species on oil degradation has not been investigated. For example, the question of whether there are plant species-specific differences in capacities to accelerate oil degradation, tolerances to oil, soil oxidative capacity, root architecture and distribution, root exudate release, and rhizosphere development should be addressed. The wetland environment can be complex, with abiotic factors such as salinity, inundation, and pollutants other than oil, which affect the potential for bioremediation. Thus, the effects of multiple and interacting environmental stressors on oil biodegradation requires investigation. HOW CAN BIOREMEDIATION BE INTEGRATED WITH HABITAT RESTORATION? A major concern in oil spill response is the integration of oil cleanup and habitat restoration. Can we employ methods that simultaneously ac-

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OPPORTUNITIES FOR ENVIRONMENTAL APPLICATIONS OF MARINE BIOTECHNOLOGY: PROCEEDINGS OF THE OCTOBER 5-6, 1999, WORKSHOP celerate oil degradation and promote habitat restoration? One methodology that has this potential is phytoremediation. The use of marsh plantings both to speed the recovery of the habitat and to accelerate oil degradation by phytoremediation is very appealing. Even if plantings are not needed because the original vegetation survived the spill, the application of fertilizer to increase plant growth rates and vegetative reproduction will accelerate habitat restoration and likely accelerate biodegradation. WHAT IS THE ROLE OF BIOREMEDIATION IN OIL SPILL RESPONSE? Immediately after an oil spill, mechanical cleanup is usually the first method employed to remove the bulk oil, if indeed the oil is removed at all. Bioremediation is not a first-response choice because of its inability to degrade large volumes of oil, but rather it is useful as a finishing technique to remove residual oil. Therefore, bioremediation is not the complete answer to oil spill cleanup, but instead is one of a number of methods that may be employed at various stages of the oil spill response to cleanse the environment. Bioremediation, if it is used at all, should be part of an integrated oil spill cleanup response. WHAT ARE THE NEGATIVE IMPACTS OF BIOREMEDIATION AND HOW CAN THEY BE MITIGATED? The impact, if any, of the application of bioremediation agents like fertilizers or soil oxidants to the environment must be assessed. This information would allow natural resource trustee agencies and oil spill responders to make informed decisions concerning the potential tradeoffs between using a bioremediation agent and allowing the oil to degrade naturally. In addition and if possible, this information can be used to alleviate concerns that the public may have concerning bioremediation. Questions such as the following must be answered: Would large-scale fertilization result in significant coastal eutrophication and harmful algal blooms? Are there ecotoxicological effects of these agents on biota? If negative impacts are identified, how can they be mitigated? The widespread use of bioremediation will likely not be accepted until the potential environmental impacts are adequately addressed. WHAT IS THE ROLE OF GOVERNMENT AND SOCIETY IN LIMITING THE IMPLEMENTATION OF BIOREMEDIATION? Federal and state trustee agencies, local government, oil spill responders, oil and gas industry representatives, and the public directly or indi-

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OPPORTUNITIES FOR ENVIRONMENTAL APPLICATIONS OF MARINE BIOTECHNOLOGY: PROCEEDINGS OF THE OCTOBER 5-6, 1999, WORKSHOP rectly determine the widespread acceptance of bioremediation as a cleanup tool for oil spills. Research directed to answering many of the general and specific questions posed above is essential to provide the information required to determine 1) whether bioremediation is effective in oil spill remediation in wetlands, 2) under what conditions it can be maximized and therefore when it should be used, and 3) whether negative environmental impacts can result and what can be done to avoid or reduce these impacts. Once this information is available, and assuming that it is favorable to bioremediation, trustee agencies, interested parties, and the public will likely be much more accepting of this procedure. HOW CAN WE BETTER INFORM USER GROUPS? The acceptance of bioremediation by regulatory agencies and the public requires dissemination of the information gained from bioremediation research to both of these groups. Organizations such as the American Petroleum Institute and the National Research Council could be instrumental in this regard. IS FUNDING FOR RESEARCH IN BIOREMEDIATION ADEQUATE? To answer the questions above, a concerted research effort in bioremediation is needed. This will require additional sources of funding to those presently available. SUMMARY Bioremediation shows potential as an oil spill remediation technique for wetlands. However, considerably more information is needed before this potential can be realized and the effectiveness of bioremediation can be maximized. Funding to support research to address bioremediation in the wetland environment and the dissemination of this information to user groups are essential if we are to see the widespread acceptance of this methodology for oil spill remediation in wetlands.