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Fungi as sources of chlorinated aromatics in the terrestrial environment
08/2008 Chlorinated compounds are generally perceived by the public as undesirable pollutants of anthropogenic origin. However, in our research we have discovered that higher fungi are a major source of chlorinated aromatic compounds in terrestrial environments. Higher fungi, which are well known for their macroscopic fruiting bodies, the mushrooms, play an ecologically important role as the key degraders of woody forest litter. As such, these organisms contribute significantly to the biomass. Each year approximately one ton of fungal biomass on a dry weight basis is produced per hectare in the litter layer of a typical northern European hardwood forest. The capacity to produce chlorinated compounds is ubiquitous among higher fungi. Approximately 50% of nearly 200 higher fungal strains tested in our lab were shown to produce clearly detectable levels of adsorbable organic halogens (AOX). Some of the most commonly occurring species belonging to the genera, Hypholoma, Mycena and Bjerkandera, produce AOX at exceptionally high concentrations ranging from 10 to 70 mg/kg in laboratory medium. If these fungi are grown on natural forest litter substrates, concentrations up to 193 mg/kg AOX have been recorded. In the genus Hypholoma, the specific yield of AOX accounts for 0.9 to 3% of the dry biomass weight which exceeds the yield observed in marine organisms. Approximately 100 halogenated metabolites have been identified from higher fungi. The overwhelming majority of these metabolites are chlorinated, although brominated and iodinated metabolites have also been observed. Some of the metabolites have structures which are identical or very similar to black-listed anthropogenic pollutants. Chloroform for example is produced by some of the litter degrading fungi. The most common halogenated metabolites are simple methyl ethers of chlorophenols such as tetrachloro-4-methoxyphenol (similar to pentachlorophenol) and 3,5-dichloro-p-anisyl alcohol. The latter compound is perhaps the most ubiquitous occurring organohalogen in terrestial environments. It is produced in large quantities by the ecologically important species Hypholoma fasciculare (e.g. the "Sulfur Tuft"), accounting for almost all of the AOX produced by this fungus. The chlorinated anisyl metabolite has been measured in the field samples of rotting wood under mushrooms of Hypholoma fasciculare at concentrations that range from 24 to 180 mg/kg. The annual production of the commonly occuring Hypholoma fasciculare is approximately 10 to 12 kg dry weight biomass per hectare in a typical northern European hardwood forest. Based on a specific AOX production of 0.9% per unit of dry biomass measured on woody substrates, we can estimate an AOX production by this ecologically important species of approximately 100 g AOX per hectare per year. The European Community contains 110 million hectares of forest. Making a conservative estimate that only 10% of these forests are habitats of Hypholoma fasciculare, than we can estimate an annual production of AOX of 1,100 tonnes of AOX per year in the European Community by this one mushroom species alone, corresponding to 3300 tonnes of the naturally-produced chlorophenol methyl ether. The chlorinated metabolites of the higher fungi cannot be written off as biological accidents. These compounds play important and purposeful physiological roles in wood degradation. Firstly, fungal organohalogens have been shown to participate in redox cycles to support lignin degrading enzymes. Secondly, these compounds have antibiotic properties which can fend off pathogens from invading the colony. And lastly, other research groups have demonstrated how some organohalogens function as synthons for biosynthesis of important fungal secondary metabolites. Little is known about the fate of the natural chlorinated fungal metabolites once these compounds have leaked out of the fungal colony into the greater environment. We have conducted studies evaluating the biodegradation of 3,5-dichloro-p-anisyl alcohol by indigenous microflora in forest soils. Under aerobic conditions, the metabolite is readily mineralized by microflora in all forest soils tested, all of the organically bound chlorine was recovered as inorganic chloride (table salt). The bacteria which were shown to be involved, Burkholderia cepacia, are also known from the literature as good degraders of anthropogenic organohalogens. Studies in anaerobic environments (environments lacking oxygen) were also considered since the highest AOX producing species, Hypholoma elongatum, is an important wetland fungal species growing in sphagnum moss. Under anaerobic conditions, 3,5-dichloro-p-anisyl alcohol is readily biotransformed to its demethylated counterpart yielding chlorophenolic compounds. The chlorophenols are reactive compounds which upon exposure to air can become incorporated into humus. Therefore, the higher fungal organohalogen metabolites are probably an important source of the chlorophenolic structures identified in natural humus by other research group. |
