February 4, 2015
Can Mushrooms decontaminate soils ?
Fungi, bacteria and plants break down their surrounding material; they absorb what is edible and neutralize what is harmful. Bioremediation is the planned decontamination of a natural environment by the action of these organisms: Its objective is to transform contaminants into water and carbon dioxide, ideally without side effects and at a lower cost than other methods. In this respect, fungi secrete enzymes, acids that modulate the environment at their convenience. They are more tolerant than bacteria to very high toxicity. Mycoremediation is the use of these skills to dispose of various noxious substances in variety of ways.
Fungi are unique in that they can break down lignin, the component that gives trees their rigidity, while also degrading hydrocarbons, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and pesticides into innocuous molecules (Adenipekun & Lawal, 2012).
The inoculation of targeted soil with mycelium is the most widely publicized method. For years, Paul Stamets has been preaching mycoremediation in books like "Mycelium Running" or conferences such as "The Future is Fungi" on Youtube. He has demonstrated how swiftly oyster mushrooms (Pleurotus ostreatus) on a cellulosic substrate can clean an oil patch on which it rests. This popular edible has thus become the emblematic oil buster.
Many species, armed with lignin peroxidase enzymes, display the same capacity: among them, ubiquitous shiitakes, button mushrooms, turkey tails, and others less heralded but more efficient fungi.
There are a few constraints of course. The mushrooms should be made to grow on lignocellulosic material, the natural source of energy for these species, before tackling related substances. Since fungi need to breathe as humans do, the range of their action can be restricted to the first oxygenated centimeters.The longer their cyclic chain, the more recalcitrant the PAH and the longer the process. The "heavy metals" remain intact even if they are removed by fungi: they are simply concentrated in their flesh. Hence the usual recommendation to foragers: avoid picking mushrooms near old mines, paved roads, dumps, all sites where lead, arsenic, nickel, mercury may be concealed. Finally, efficiency varies with species, sites and contaminants themselves: the proper species should be chosen for each specific task.
Biostimulation is based on the natural action of native microorganisms. It is one of the methods implemented at Lac-Mégantic, where a derailment happened July 6, 2013, killing 47 people and dumping 6,000 tons of oil. The decontamination program includes a biostimulation project. The project consist in activating bacteria, actinomycetes and fungi already present in the soil by improving growth conditions (oxygen, nutrients, moisture, acidity, ...).
The microorganisms involved have not been specifically identified locally and their respective role has not been determined. The combination will differ from one site to another depending on the organic matter and soil pH.
An alternate solution, ex-situ thermal separation, does not require fungi but consumes much more energy. Biostimulation has the advantage of being less expensive, without significant emissions of gas. However, it operates slowly and requires disposal grounds. Reclaimed land, though clean enough for most uses, will not be completely free of long chain PAHs and "heavy metals".
Plants rather than fungi have been known to decontaminate soils. The results obtained by phytoremediation may be improved when the symbiosis that binds fungi to plants is tapped. Dimitri Dagher, with the IRBV in Montréal, is experimenting with a willow, Salix purpurea, tied to such mycorrhizal species. Fungi are relegated to a supporting role: speed up the growth of vegetation with willows having the lead role.
The accumulation of "heavy metals" in fungi can be useful sometimes. It allows collecting undesirable substances and eventually disposing of them elsewhere. Several species accumulate heavy metals and other harmful substances. One species in particular, Gomphibius glutinosus, seems to benefit from the fallout around Chernobyl: it becomes radioactive while helping to clean the ground.
In the same vein, in 1991 in Chernobyl, the discovery of black mold covering the walls of the nuclear plant has sparked quite a stir: the prevalent explanation is that the melanin contained in some fungi plays a similar role to that of chlorophyll in plants by transforming gamma rays into chemical energy. Not for pickers.
Typically, fungi feed on bacteria and secrete around them antibacterial metabolites. Mycofiltration involves filtering bacteria and other undesirable substances from a stream. The simplest technique involves filling jute bags with mycelium inoculated straw and sawdust. Deposited across a low-flow stream, the bags form a barrier that filters water. The mycelium captures E. coli, phosphate, nitrite and other nutrients for cyanobacteria downstream. Wine-cap mushrooms (Stropharia rugoso-annulata) have given interesting results.
In many instances, the demonstration that mushrooms can clean up the environment is convincing. Up until now, results have mostly been obtained in the framework of pilot projects and laboratory experiments. Species and strains are millions, most of them still anonymous; little is known about their specific capabilities. The power of fungi in restoring our environment is for us to explore and harness.
(You are welcome to download. You will find books on this topic at the Mycoboutique)
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