Abstract

       Environmental pollution is a worldwide concern which is considered as a problem not only to public health but also to other life forms as well. Rapid urbanisation and industrialisation accompanied with improper waste management has given rise to industries on this waste management sector. Of the various technologies both physical and chemical methods available for treating environmental pollution, biological degradation is the most sorted remediation method for economic and ecological reasons. Among the many diverse microorganisms that have potential for degrading environmental pollutants, fungi are more versatile remediators and this review discuss the role of fungi on environmental pollutants. Fungi influence in balancing ecosystem from the activities associated with their lifecycle such as nutrient cycling, nutrient transport, decomposition and ability to transform or metabolise chemical compounds. Further they synthesise several extra and intra cellular enzymes that are involved in breakdown of xenobiotic compounds that could be used for achieving sustainable environment. In addition, a number of pollutants could be biodegraded by a single fungus which makes these organisms suitable for biological degradation.

Introduction

         All living and non-living things interact with each other to sustain their life. Nature provides shelter, food, clean air and water to all living things, and encompasses different vegetation. Improper waste management has affected various components of natural environment such as water, land, air, atmosphere, and oceans. Biological diversity present in different environments defends its own ecosystems for improving the sustainability environment management. But when pollutants such as hazardous chemicals, toxins, pesticides, phenols, various other organic and inorganic substances exceed their levels it starts to deteriorate the environment in which it is present. Fuel used as energy in various industries and automobiles is directly responsible for increase in the atmospheric carbon dioxide, resulting in current trend of global warming (Tortella and Diez, 2005). The use of chemicals in agriculture to sustain productivity has led to contamination of the environment with toxic pesticides and nutrient fertilizers that are known to change the course of biogeochemical cycles (Ogunseitan, 2003). The past production and improper disposal of environmentally persistent and toxic chemical by both the government and private sectors have generated legitimate public health concerns. Also clean-up of the environmental pollutants is economic burden to the society. The chemical and physical methods to restore the environment would be costly and might put stress on the environment even after removing the pollutant (Barr and Aust, 1994). Hence, the environmental pollution must be dealt with solutions not only effective for the present but also for the future generations.

         The number, diversity and complexity of chemicals being produced are overwhelming. The presence of organic compounds has become ubiquitous with our current lifestyle. More than 100,000 chemicals are produced commercially and yet the fate and impact of these compounds in the environments is available only for few. Soil and aqueous environment are natural and preferential sinks for contamination and the many pollutants could be bio-accumulated in the food chain which is a major concern for human and environmental health. Bioremediation is a process where the hazardous chemicals are turned into less toxic compounds by the living organisms. Most likely the indigenous organisms present in the environment are capable of degrading the pollutants and genetically engineered microorganisms could also be used. Bacteria are the most sorted organisms for bioremediation and recently fungi have received considerable attention not only because of their enzymes and metabolites but also their hyphae that can penetrate soil to reach pollutants (Rivilla and Collado, 2009).

Fungal biodegradation

         Environment contains numerous living organisms assembled in complex and varied communities. Among the higher organisms (plants, animals, etc.) and microorganisms (bacteria, virus, etc.), fungi are better known (Tortella and Diaz, 2005). Most of the fungi are microscopic organisms which can be cultured to be identified but biotrophs which are amenable to culturing in growth media go unnoticed. Therefore, many microorganisms including fungi still remain unidentified. About 63,500 fungal species have been identified with further 13,500 species associated with algae as lichens. Neither associated with plants nor animals and diverse species accounting in fungi they now merit for a kingdom of their own. Fungi could be classified based on their enzymatic machinery such as white rot fungi that decompose lignin, brown rot that decompose only cellulose, soft rot that attack cellulose on damp woods surface and so on. Interestingly, the metabolites produced by the fungi are similar to those produced form mammalian metabolism and these properties may help us in many ways of treating our body accumulated with organic pollutants.

         Biodegradation is the process of breakdown of organic matter by the microorganisms into simpler forms which occurs naturally in the environment while bioremediation is the use of either naturally occurring or deliberately introduced microorganisms to breakdown compounds which are pollutant. Microbial bioremediation strategies could be generally divided into three categories: (1) the target compound is used as a carbon source, (2) the target compound is enzymatically attacked but is not used as a carbon source (cometabolism), and (3) the target compound is not metabolized at all but is taken up and concentrated within the organism (bioaccumulation). Although fungi participate in all three strategies, they are often more proficient at co-metabolism and bioaccumulation than at using xenobiotics as sole carbon sources (Bennett et al., 2002).

         Variety of food crops are produced all over the world. Cereal crop residues such as wheat straw, paddy straw, corn straw etc. are used as animal fodder. Yet many crops that are produced are hard to biodegrade quickly and end up as agriculture waste. Numerous studies have done on these aspects of which fungal degradation of lignocellulosic material brings high quality of degraded biomass and several other enzymes and products of commercial interest in this process. A schematic representation showing biodegradation of cellulose, lignin and hemicellulose compounds from plant material is described in figure 1. Phanerocheate chrysosporium is a well-known white rot fungus and widely studied for lignin degradation (Syed and Yadav, 2012).

Fig.1: (A) Structure of a typical lignocellulosic residue, (B) Initial fungal attack on lignin hemicellulose matrix, (C) Degradation of all three polymers and fungal growth, (D) Nutritionally upgraded lignocellulose along with fungal proteins and simple sugars to be used as animal feed (Sharma and Arora, 2015). from various ecosystem.

Chemical degradation

         Large amount of xenobiotics has been generated from various sectors such as industries of oil, paper, textile, defence, aerospace, medical, dye etc. including agriculture that resulted in environmental contamination. Polycyclic aromatic hydrocarbons (PAHs) are generated as products from the combustion of fossil fuels such as petroleum, coal tar, and shale oil. PAHs are persistent in environment that can act as carcinogens and also affect reproductive, neurologic and immune systems in animals. Diverse fungi are capable of PAH mineralization and notably the filamentous fungus (nonligninolytic fungus) Cunninghamella elegans, a typical soil fungus, has been studied extensively for its ability to transform PAHs. Other filamentous fungi have also demonstrated the ability to degrade PAHs. Cyclothyrium sp., Penicillium simplicissimum and Psilocybe sp, transformed PAHs compounds such as pyrene, anthracene, phenanthrene and benzo[a]pyrene (Da Silva et al., 2003). Polyporus sp. S133 showed bioremediation potential for crude oil with 93% degradation rate in the soil and could be used to remediate soil contaminated with crude oil (Kristanti et al., 2011). The white-rot fungus Phlebia brevispora was shown to degrade polychlorophenols (PCBs), an important group of phenols which have been used as fungicides, herbicides, insecticides, and in the synthesis of other pesticides.

          Phenolic compounds such as nonylphenols (NP) and bisphenol A (BPA) are known as endocrine disrupting compounds and are persistent in aquatic environment. Fungi such as Phanerochaete chrysosporium, Pleurotus ostreatus, Trametes versicolor and Bjerkandera sp. BOL13 have shown capability to degrade these phenolic compounds. Munitions waste disposal is a serious problem for the military. There is not only risk of detonation but also the constituent compounds that are released from the explosives and underground disposal sites may migrate to the soil and ground water upon contact. In addition, the non-explosive components also cause problems to the environmental toxicity (Pointing, 2001). 2,4,6- trinitrotoluene (TNT) is present in bombs and shells, dinitrotoluene (DNT) is used as an energetic additive in propellants and RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine) are hazardous military explosives and fungi such as Aspergillus, Coniothyrium, Paecilomycetes, Penicillium, and Trichoderma sp. have shown the potential for biodegradation of these compounds.

Metal remediation

          Contamination of soil by toxic metals has become a serious problem; researchers worldwide are investigating new sustainable methods to remediate such environmental contamination. Heavy metal and non- degradable chemical contamination of soil and water is a major environmental threat because of their long-term persistence and their diffusion into underground water resources. Most of the common filamentous fungi can absorb heavy metals (Zn, Cd, Pb, Fe, Ni, Ag, Th, Ra & U) and the process is termed as bioaccumulation / biosorption. Many fungi with different characteristics possess metal binding potential making them an economical and sustainable option for the removal and recovery of heavy metals. Several metals are remediated by fungi by biosorption and accumulating within their hypae. The white-rot basidiomycete Phanerochaete chrysosporium, the macrofungus Ganoderma carnosum and Aspergillus parasiticus are few fungi species known for their efficiency in metal remediation.

Conclusion

         There has been an increasing awareness of natural energy conservation and recycling of waste materials from various parts of the world. Chemical methods have several disadvantages in degradation of pollutants but bioremediation using biocatalytic reactions of microorganisms are safe, environment friendly and economical. Fungi as alone or can associate with other microorganisms to control levels of pollutants remediating the contaminated environment. Fungi have been shown to possess immense potential for various activities that can be harnessed to transform it to our benefits such as enzymes, antibiotics and other bioactive compounds. Most of the fungal research is conducted only in laboratory scale and condition but needs to be practically demonstrated in field trails by upscaling relevant fungal species.

References

Barr, D. P. and Aust, S. D. (1994). Mechanisms white rot fungi use to degrade pollutants. Environ. Sci. Technol. 28 (2), 78A-87A.

Bennet, J. W., Wunch, K. G. and Faison, B. D. (2002). Use of fungi in biodegradation. Environmental Microbiology. 2nd edition, ASM Press Washington DC.

Da Silva, M., Cerniglia, C. E., Pothuluri, J. V., Canhos, V. P. and Esposito, E. (2003). Screening filamentous fungi isolated from estuarine sediments for the ability to oxidize polycyclic aromatic hydrocarbons. World J. Microbiol. Biotechnol. 19 (4), 399 - 405.

Kristanti, R. A., Hadibarata, T., Toyama, T., Tanaka, Y. and  Mori, K. (2011). Bioremediation of crude oil by white rot fungi Polyporus sp. S133. J. Microbiol. Biotechnol. 21 (9), 995 - 1000.

Ogunseitan, O. A. (2003). Biotechnology and industrial ecology: new challenges for a changing global environment. Afr. J. Biotechnol. 2 (12), 596 - 601.

Pinedo-Rivilla, C., Aleu, J. and Collado, I. G. (2009). Pollutants biodegradation by fungi. Curr. Org. Chem. 13 (12), 1194-1214.

Sharma, R. K. and Arora, D. S. (2015). Fungal degradation of lignocellulosic residues: an aspect of improved nutritive quality. Crit. Rev. Microbiol. 41(1), 52 - 60.

Syed, K. and Yadav, J. S. (2012). P450 monooxygenases (P450ome) of the model white rot fungus Phanerochaete chrysosporium. Crit. Rev. Microbiol. 38 (4), 339 - 363.

Tortella, G. R., Diez, M. C. and Durán, N. (2005). Fungal diversity and use in decomposition of environmental pollutants. Crit. Rev. Microbiol. 31(4), 197 - 212.