Bioremediation Capabilities of White Rot Fungi
Throughout the past century, industrial, military, and farming activities have released many organopollutants into the environment. Some of these (polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCBs), trinitrotoluene (TNT), and DDT) are persistent in the environment and have potential toxic effects. Removing these organopollutants from the soil in an ecologically responsible, safe, and cost-effective way is a top concern for land management agencies. Bioremediation using various microbial organisms is one way to do this. Through intensive study of lignolytic fungi, it has been determined that these organisms produce extracellular enzymes with very low substrate specificity. This makes them suitable for degradation of many different compounds, notably organopollutants with structural similarities to lignin (PAH, PCBs, TNT, DDT). The three main lignin-modifying enzymes (LMEs) are lignin peroxidase, manganese dependent peroxidase, and laccase. White-rot fungi contains all three enzymes and is therefore able to breakdown and mineralize several environmental pollutants into nontoxic forms. This takes place through the generation of radical species that cause the complete biodegradation of lignin polymers. This process and others like it have been extensively studied in the laboratory, showing great potential for the bioremediation capabilities of white-rot fungi. However, more research needs to be done to determine the applicability and practicality of utilizing this organism in contaminated field sites. This paper discusses the role of the lignolytic enzymes, primarily laccase, in the growing field of bioremediation.
Introduction
Background
One of the major environmental problems facing the world today is the contamination of soil, water, and air by toxic chemicals. Eighty billion pounds of hazardous organopollutants are produced annually in the United States and only 10% of these are disposed of safely (Reddy and Mathew, 2001). Certain hazardous compounds, such as polycyclic aromatic hydrocarbons (PAH), pentachlorophenols (PCP), polychlorinated biphenyls (PCB), 1,1,1-trichloro-2, 2-bis(4-chlorophenyl) ethane (DDT), 2 benzene, toluene, ethylbenzene, and xylene (BTEX), and trinitrotoluene (TNT) are persistent in the environment and are known to have carcinogenic and/or mutanogenic effects. It has cost approximately $1 trillion to decontaminate toxic waste sites in the United States using traditional waste disposal methods such as incineration and landfilling (Reddy and Mathew, 2001). Due to the magnitude of this problem and the lack of a reasonable solution, a rapid, cost-effective, ecologically responsible method of cleanup is greatly needed. One growing mechanism of decontamination that may fit these requirements is bioremediation. Utilizing microorganisms to degrade toxic organopollutants is an efficient, economical approach that has been successful in laboratory studies. The next step is to apply these techniques in situ under field conditions on a large scale. This paper will review the research thus far on the potential use of microorganisms (especially white-rot fungi) in degrading some of the top pollutants throughout the world and the mechanisms involved in this process.
Characterizing organopollutants
There are several classes of chemicals that have been targeted by the USEPA as priority pollutants due to their toxic effects on the environment and human health. Six of these include PAH, PCP, PCB, DDT, BTEX, and TNT. PAHs are recalcitrant environmental contaminants that are generated from the burning of fossil fuels, coal mining, oil drilling, and wood burning (Lau et al, 2003; Verdin et al, 2004). They are not water soluble, favoring sorption to soil organic matter. Because of this they have a tendency to bioaccumulate in natural systems. They consist of 2 or more fused aromatic rings in linear, angular, or clustered arrangements. Clustered and angularly arranged ring 3 structures are more stable than linear arrangements, making them less biodegradable (Reddy and Mathew, 2001). Compounds with more than 3 rings are typically extremely toxic to microorganisms and, therefore, very difficult to break down into mineralizable substrates (Lau et al, 2003). PCPs have been used as wide spectrum pesticides and wood preservatives throughout the world. They are currently banned in most countries, however soil contamination continues to be a problem. PCP is toxic to most organisms at concentrations of 50ppm but some contaminated sites have concentrations greater than 1600ppm, making them very difficult to biodegrade (Aust et al, 2004). PCPs are also hydrophobic with low water solubility, increasing their persistence in the environment. DDT is an organochloride insecticide that was banned in the United States over 30 years ago. This chemical is persistent in the environment, however, biomagnifiying through the food chain. Toxic effects and population declines at higher trophic levels due to DDT have been recorded, with some studies finding stable residues in air, water, soil, sediment, fish and birds more than 10 years after it was banned (Breivik et al, 2004). PCBs were used extensively until 1993 in dielectric and hydraulic fluids, flame retardants, plasticizers, solvent extenders, textiles and printing (Reddy and Mathew, 2001). Estimates of total production of this volatile, bioaccumulative toxin range from 1.3-2 million tons worldwide (Breivik et al, 2004). BTEX compounds are components of gasoline and aviation fuels that are carcinogenic and neurotoxic to most organisms (Levin et al, 2003). They enter the environment primarily through leaking into soil, sediment, or water from underground storage tanks and pipelines. Lastly, TNT contamination is a major problem at many military complexes, with over 900,000 kg produced annually in the United States alone. It is toxic to most organisms at 50ppm but some sites have 4 concentrations of 4,000-12,000ppm (Boopathy, 2000). Currently, incineration is the most effective and common remediation practice, but this is extremely costly, in terms of dollars and energy used. All of these chemical compounds pose a significant threat to the health and vitality of the earth system and a sustainable method of detoxification is key.