13.6 Using fungi to remediate toxic and recalcitrant wastes
Fungi are quite capable of growing on waxes, paints, leather goods and all forms of textiles, from the finest cotton to the heaviest canvas, and much of this degradative ability results from the activity of lignocellulose-degrading enzymes, particularly the panel of ligninolytic enzymes, described in Chapter 10:
- Breakdown of polysaccharide: cellulose section CLICK HERE to view the page;
- the section on Lignin degradation CLICK HERE to view the page);
- the section on Digestion of protein CLICK HERE to view the page);
- and processing agricultural, industrial and forest residues in Digestion of lignocellulosic residues in Section 17.22 (CLICK HERE to view the page).
A pest is an organism that is doing what it normally does, but in a place that we consider inappropriate. So a wood-degrading organism growing in the timbers of your house roof is a pest. However, the same organism could be considered a technological marvel if it were recruited to degrade some of our waste products. Overall, on biological and chemical grounds the more advanced fungi, especially the mushroom fungi, are the ideal candidates to degrade the waste vegetation that we produce through our agricultural activities.
On average, world agriculture currently loses 40% of its primary production to pests and diseases, and then throws away more than 70% of what’s left because the crop always represents so little of what is grown. Remember, the ‘crop’ may only be the seeds of the plant that is grown, or even only a portion of the seed, like its oil content. Just imagine how much of the coffee bush ends up in a jar of instant coffee. Typically, 80 to 90% of the total biomass of agricultural production is discarded as waste. Some agricultural wastes are polluted with pesticides. Other agricultural wastes are hazardous because they contain tannins and phenolics (toxic to plants and animals) as residues from extraction of oils, such as cotton, rape, olive, and palm oils, or fruit processing residues, like citrus wastes. These materials are hazardous because they contain compounds chemically similar to the complex phenolic compounds found in wood. Since the fungi can decompose the wood, they can also be used to degrade the environmental pollutants, both in soils and in liquid effluents. The latter including industrial waste water discharges such as those produced by the paper pulp industry, but also wastes contaminated with pesticides, such as chlorinated biphenyls, aromatic hydrocarbons, dieldrin and even the fungicide benomyl.
The advantage is that the fungi do not partially degrade these materials, leaving other possibly dangerous substances behind, rather they completely mineralise the pollutant so that its chemical constituents are returned to the atmosphere and soil as carbon dioxide, ammonia, chlorides and water. Laboratory tests have shown that the oyster mushroom (Pleurotus spp.) is particularly good at this sort of thing. The tests were conducted with pentachlorophenol (PCP), one of the chlorophenols that have been commonly used as disinfectants and preservatives around the world. They do a good job as pesticides but because most environmental microorganisms find them impossible to degrade they persist in the environment and remain toxic for many years. It is illegal to use PCP in most countries today, but it has been the most heavily used pesticide throughout the world. For example, in the United States during the 1980s, approximately 23 million kg y-1 was used, mainly as a wood preservative and since the 1960s large approximately 5 million kg y-1 was sprayed over vast areas of central China as a molluscicide to kill the snails that carry the schistosomiasis parasite. The chemical is very persistent and most of what has been released into the environment is still there. It is highly toxic, uncoupling oxidative phosphorylation by making cell membranes permeable to protons, and thereby dissipating transmembrane proton gradients. It is also cancer inducing and has been declared a priority pollutant for remediation treatment.
The conventional remediation strategy for PCP contaminated land is excavation and incineration or land filling. Such methods are expensive, obviously destructive to the environment and ineffective for anything other than highly localised ‘point source’ pollution. Bioremediation is a very promising alternative, using biological systems for the environmental clean up. The ability of a range of known wood decay or plant litter-decay fungi to remove PCP from a batch culture was compared. Fungi tested as mycelia were: Armillaria gallica, A. mellea, Ganoderma lucidum, Lentinula edodes, Phanerochaete chrysosporium, Pleurotus pulmonarius, a Polyporus sp., Coprinopsis cinerea and Volvariella volvacea, and the spent mushroom compost from farm beds growing the Oyster mushroom Pleurotus pulmonarius was also tested. All these fungi showed active breakdown and absorption of PCP removal mechanisms, though the tolerance level of the fungus towards PCP did not correlate with its degradative capacity. In a 7 day incubation, A. mellea mycelium showed the highest degradative capacity (13 mg PCP g-1 mycelium dry weight) and Pleurotus pulmonarius mycelium was second-highest with 10 mg PCP g-1 mycelium dry weight; the least effective was Polyporus with 1.5 mg PCP g-1 mycelium dry weight. On the other hand, the Pleurotus spent mushroom compost, harbouring both bacteria and fungi, had a degradative capacity of 19 mg PCP -1 dry weight in only 3 days exposure to PCP (Fig. 5)(Chiu et al., 1998).
|Fig. 5. Data showing that incubation for a few weeks with spent Oyster Mushroom compost leads to destruction of pentachlorophenol (not just its adsorption)(left hand panel), and that mycelium of Pleurotus pulmonarius dechlorinates pentachlorophenol (PCP) through a catabolic process that involves removal of the chlorine followed by opening of the benzene ring (right hand panel). For the left hand plot, absolute removal capacity of PCP by spent oyster mushroom substrate (that is, the substrate left after the last crop was harvested) was quantified by capillary electrophoresis. The right hand panel shows a mass selective gas chromatography (GC-MS) spectrum of the extract of the fungal biomass of Pleurotus pulmonarius mycelium after two days incubation in a medium containing 25 mg l-1 pentachlorophenol. The most prominent peak (retention time 13.53 min) is PCP, the next most prominent peak at 12.03 min is benzene-1,2-dicarboxylic acid (also called phthalic acid). Other peaks at longer retention times include fatty acids that result from opening the benzene ring and esterification of its straight-chain derivatives. For example, peaks at retention times of 15.73, 31.96 and 32.25 min have been identified as hexadecanoic acid (palmitic acid, C16H32O2) and its derivatives. Modified and redrawn from Chiu et al., 1998.|
GC-MS chromatograms revealed only residual PCP peaks in extracts of PCP-treated spent mushroom substrate extracts, a contrast with the mycelial incubations in which a variety of breakdown products were detectable. The spent compost left after oyster mushroom cultivation does two crucial things. It absorbs, immobilizes and concentrates PCP so it can be transported away from the contaminated site, and it also digests PCP completely. Mushroom cultivation is a common practice all over the world and the idea that hazardous waste materials could have their pollutants removed and produce a mushroom crop at the same time is exceedingly attractive.
But the idea is not free of problems. Oyster mushrooms can concentrate the metal cadmium (a common industrial contaminant) to such an extent that by eating less than an ounce (dry weight) of the most contaminated samples you would exceed the weekly limit tolerated by humans. Cadmium is so toxic that this situation could pose a public health hazard. There are no worries about conventionally cultivated oyster mushrooms. The point is that if the mushroom is grown on composts that might be mixed with industrial wastes (in remediation programs, for example), then it would be advisable to monitor the heavy metal contents before mushrooms are marketed for food. By far the most promising technique is use of the spent mushroom substrates remaining after harvesting mushroom crops. Ironically, these are often discarded as wastes themselves, but they are clearly able to offer an integrated approach combining soil conditioning with degradation of pollutants as an effective strategy for bioremediation in situ.
Updated December 17, 2016