Research

Fungal bioremediation has only been researched since the 1980s but the potential was quickly realised, resulting in many more experiments aimed at bringing the undoubted potential down to practical application. All of the organopollutants described in the earlier pages have been the subject experiments on fungal bioremediation, though with differing levels of success.

 

Polycyclic Aromatic Hydrocarbons.  The bioremediation of PAHs depends largely on the number of benzene rings in the structure.  The larger the number of benzene rings the less likely they are to be degraded.  Figs 1 & 2 show three- ring PAH degradation pathways that have been proposed.  High molecular weight PAHs (5 rings or more) are generally difficult to remove.  Pleurotus ostreatus has shown the greatest ability to degrade PAHs, with a reported 80% degradation in 35 days in one experiment, and 86% in 7 weeks in another, but the degradation was mainly of the lesser weight PAHs, with only 48% of the 5 ring structures degraded.  It is thought that the strong laccase and manganese-dependent peroxidase production by P. ostreatus is the reason behind its success rate, but different degradation pathways have been identified, so it is not yet clear.  It has also been shown that non-lignolytic fungi can also degrade PAHs, such as Cunninghamella elegans.
Fig. 1. A proposed degradation pathway for anthracene, a PAH comprised of three benzene rings. In the final step in this pathway the benzene dicarboxylic acid (phthalic acid) is opened to yield a straight-chain dicarboxylic acid which can be fully metabolised via the normal β-oxidation pathway for fatty acids and the Krebs cycle to carbon dioxide.

 

degradation pathway for phenanthrene
Fig. 2. Proposed fungal degradation pathway for phenanthrene.

Polychlorinated biphenyls. The aroclors (commercial PCB mixtures produced by the Monsanto Company) are the primary PCBs to be researched, and have been shown to be broken down by white rot fungi including Trametes versicolor and Phanerochaete chrysosporium, although the fungi show preferential degradation of types of Aroclor which are least chlorinated. The more persistent types have still not been successfully degraded by fungi.  Field studies have not been as successful as might be hopes because the introduced microorganisms do not compete well with the indigenous microorganism population. Recent research has focussed on ectomycorrhizal fungi which potentially degrade PCBs better and compete better with the existing populations of microorganisms in the environment.

 

Pesticides. Pentachlorophenol (PCP) has been shown to be detoxified by Phanerochaete species, via an enzymatic dechlorination.  In field trials, which are obviously of more interest than lab work, P. chrysosporium and P. sordida were shown to degrade PCP (in mixtures of creosote and other soil bound products) rapidly and extensively, giving a rate of about 80% conversion within 6 weeks.  Trametes versicolor has been suggested as a better degrader as it is more tolerant to high PCP levels, and so there are good possibilities for future work in this area. We have already mentioned the experiments that showed that incubation for a few weeks with spent Oyster Mushroom compost leads to destruction of pentachlorophenol (not just its adsorption), and that mycelium of Pleurotus pulmonarius dechlorinates PCP through a catabolic process that involves removal of the chlorine followed by opening of the benzene ring (view now).

DDT, aldrin, dieldrin, lindane and heptachlor have all been shown to be degraded most efficiently by Phanerochaete chrysosporium in the laboratory, but in field studies only lindane was significantly biodegraded.  However, there is evidence to show Trametes hirsutus has a greater potential in liquid cultures, further research will clarify whether it has the ability in field studies.  Endosulfan usually shows low degradation rates, and its primary metabolites are as toxic as the substance itself.  However, Phanerochaete chrysosporium can break it down to less toxic metabolites quite rapidly.  Organophosphates have only been researched to date using liquid cultures, but Trichoderma viride has tentatively shown it can transform some organophosphates into metabolites that are capable of further transformation.

 

Munitions. TNT is transformed to dinitrotoluene (DNT) by many microorganisms, but DNT is just as toxic and persistent.  However only white rot fungi have shown the ability to degrade DNT and mineralize it to CO2.   Successful experiments have found that Phanerochaete chrysosporium can degrade aqueous TNT to CO2, but in field studies P. chrysosporium is inhibited by high TNT levels.  A two step degradation process has been suggested, where more tolerant fungi are introduced to the site first, to initiate detoxification, then P. chrysosporium added later to enhance the breakdown.

 

Bleach Plant effluents. Trametes versicolor has been shown to be the most effective fungus to combat this lignin- and chlorine-containing effluent.  Phanerochaete chrysosporium is also effective, making the analysis of enzymes that decolour harder, as it was initially assumed the high amount of laccase produced by T. versicolor decolourised the effluent.  As P. chrysosporium does not produce significant amounts of laccase, this has been ruled out, and further studies are needed to clarify the exact method of degradation.

 

Dyes.  Batch culture research with white rot fungi has shown that they can decolourise most of the major chromophore groups of synthetic dyes. Phanerochaete chrysosporium can decolourise azo-dyes at a very useful rate, with studies recently reporting a near 100% decolourisation, with the dye being broken down into a quinone and a phenyl diazine, which are then further broken down into nitrogen and a phenyl compound.  Further experiments have used Trametes versicolor, Pleurotus ostreatus and Pycnoporus cinnabarinus with success varying on the type of dye used, as some are resistant to some of the lignin metabolising enzymes. 
 

However even though some of the fungi do not degrade in the environment as well as they do in the laboratory, the majority of them at least partially degrade the substances, which in turn allow better degradation by indigenous bacterial populations.

Agricultural waste degradation

There is another aspect to fungal bioremediation that does not involve degrading organopollutants.  Instead it degrades agricultural waste.   Lignocellulose waste is produced in vast quantities by the agricultural industry.  For example, sugar cane bagasse, which is the residue from sugar-cane milling, consisting of the crushed stalks from which the juice has been expressed and is made of approximately 50% cellulose, 25% hemicelluloses, and 25% lignin.  It is abundant and is therefore a cheap source of carbon if the carbon can be harnessed from the substrate. Fungi, with their lignolytic enzymes are ideal for this purpose, and can be used to convert the sugars present in the bagasse into useful products.  A major area of interest is in the production of xylitol, a sugar alcohol which has the same sweetness as sucrose yet is metabolised in humans without the need for insulin, making it an ideal sweetening agent for diabetic patients. Fermentation of bagasse by some Candida species results in an efficient production of this sugar alcohol.

Other prospects with fungi include making low cost proteins using the waste as a carbon source to grow mushrooms such as Pleurotus ostreatus and Agaricus bisporus, which are common edible mushrooms.  Also, by-products from certain fungi are important for biochemical industries, and so a cheap source of carbon is important.  Here, the fungi are used in a fermentation process which harnesses the products of the fungi.  This is good for waste that cannot be converted into anything else due to the content of other substrates apart from lignocellulose.

For example coffee pulp is a one of the most abundant agricultural wastes with around 3 x 106 tons produced worldwide annually.  However, due to its high caffeine and phenolic content it is difficult to manage and cannot be used for animal feed in the way many other agricultural waste products are.  But with treatment from fungi such as Aspergillus species, an end product is created which has a higher amino acid content than the pulp, without the harmful substrates, making it a far more efficient feed for animals.

Obviously there is broad potential for fungi in this industry, and further uses include composting with fungi which uses waste material which cannot be used for anything else.

For more specific information about the scale of agricultural waste production and the opportunities for fungal remediation of those wastes, consult ther following paper:

Sánchez, C. (2009). Lignocellulosic residues: biodegradation and bioconversion by fungi. Biotechnology Advances, 27: 185-194. DOI: http://dx.doi.org/10.1016/j.biotechadv.2008.11.001.

Further information about bioremediation can be found in the new textbook 21st Century Guidebook to Fungi by David Moore, Geoffrey D. Robson & Anthony P.J. Trinci. Published 2011 by Cambridge University Press: ISBN: 9780521186957. URL: ttp://www.cambridge.org/gb/knowledge/isbn/item6026594/?site_locale=en_GB. View Amazon page.

Updated December 15, 2016