About 120,000 species of fungi have been identified and described by taxonomists. That sounds like a lot, doesn't it? At least until one looks at estimates of the total existing species of fungi. An abstract in the online journal Microbiology Spectrum (2017) estimates that total at somewhere between 2.2 and 3.8 million species. Get to work, taxonomists!
If I ask you to picture a fungus, you might close your eyes and imagine a mushroom. Most of us are familiar with mushrooms as parasol-shaped food that we may (or may not) want sliced and cooked on our pizza. That would be one example, although it is only "the tip of the iceberg". If you are familiar with mycelium, you might know that it is the underground network of threadlike structures that an above ground mushroom is connected to. You might think of mycelium as being analogous to a plant's network of roots. And, you would be wrong. A more accurate representation would be to say that the mycelium is the mushroom. The part that shows above ground (mushroom) is somewhat comparable to a flower or fruit in the plant kingdom. The fruiting body of a fungus serves the purpose of reproducing itself via spores which are often produced within gills or pores on the underside of the cap.
Until technology became advanced enough to actually compare DNA from fungi to that from plants and animals, fungi were considered to be very unusual distant relatives of plants but, behold, DNA comparisons show that fungi are actually more closely related to (i.e. have more DNA in common with) animals than to plants.
What is the largest living organism known to exist today on planet Earth? If you said, "Blue whale", you made a common error. Blue whale is the largest animal. The largest living organism is a little bit less conspicuous, but it's effects are not. This one is a parasite and it kills trees. It is a fungus discovered in the Malheur National Forest in the US state of Oregon. At well over three square miles (as of 2015), the 'Armillaria' fungus, AKA 'Honey mushroom', AKA 'Shoestring mushroom', has been around an estimated 2,000 to 8,000 years. It is not unique to that location, but DNA studies show that that the whole infested area is a single huge organism. Scientists calculate that, if one could gather it all in one place, it could weigh at least 7,500 tons and maybe up to 35,000 tons. That's one humongous fungus!
The use of fungi as medicine is both ancient and new. Ancient in traditional use in some cultures, and new in the sense that new medicinal properties (of fungi) are being discovered at an accelerating pace. One pioneer in this journey is Tradd Cotter. Cotter has been developing a unique experimental technique to discover which fungi will inhibit which pathogens. He pairs them up in petri dishes with nutrients and lets them "slug it out". The really surprising result of these experiments is that fungi can often be trained, in a sense, to produce exudates (think fungal sweat) that will inhibit and even kill specific pathogens. These exudates can then be collected and used directly. Let that sink in. The fungal species are doing the "heavy lifting"! He just puts them together and observes. This writer suspects this may give rise to a novel way of rapidly developing new medicines. Can anyone think of ways that might be useful?
Fungi have also been found to be amazingly efficient at cleaning up some of the more difficult messes that humans make. Paul Stamets, a well known mycology author and researcher, created an artificial diesel spill in order to compare four different remediation methods. Four piles of soil were contaminated with diesel fuel and then treated, respectively, with enzymes, bacteria, Oyster mushroom mycelium, and the fourth was treated with nothing at all, as a control. After six weeks three of the piles showed no improvement. The pile inoculated with oyster mushrooms was covered with hundreds of pounds of healthy mushrooms. The PAH (polycyclic aromatic hydrocarbons) in the mushroom pile were reduced from 10,000 PPM to less than 200 PPM at sixteen weeks and the pile was teeming with diverse life.
Fungus species assay