Grey oyster mushrooms (Pleurotus ostreatus) are cultivated all around the world as a choice food source but this fascinating multi-purpose fungus has a lot more to give. It can clean up contamination, help to create building materials and also likes to snack on the occasional small worm…
When mycologist Paul Stamets wrote his book Mycelium Running: How Mushrooms Can Help Save The World (2005), the grey oyster mushroom (Pleurotus ostreatus) was one species in particular he championed.
This prospective saviour of humanity had already had some of its potential flagged 90 years earlier, when it was first cultivated in Germany to provide a subsistence protein source during World War I (Falck, 1917). Not only is this mushroom tasty — and a convincing meat substitute — but its nutritional profile is impressive, packing up to 29% protein, 13% dietary fibre and a range of vitamins and minerals including the essential nutrient niacin, making it an ideal food in times of hardship (Jongman, Khaere and Loeto, 2018).
The genius of the grey oyster lies in its unfussiness and great adaptability. It is one of the easiest mushrooms to grow and can be cultivated on almost any cellulose-based substrate, including cheap and readily available agricultural byproducts such as sawdust and straw. Early efforts at growing them were soon expanded and full commercial production began in the US and Europe in the mid-1970s (Jongman, Khaere and Loeto, 2018). It is now the third most-cultivated species in the world behind the button/chestnut/portobello mushroom (Agaricus bisporus) and shiitake (Lentinula edodes) (Tridge Market Intelligence Team, 2020).
But there is so much more to this common woodland fungus than its nutritional and culinary appeal. What Stamets noted was its remarkable talent for mycoremediation — the ability of a fungus to remove chemical toxins, such as heavy metals, from the environment. Fungi have excellent metal-binding properties and tolerance thanks to the chitin in their cell walls, and can accumulate heavy metals in their fruiting bodies in high concentrations. Grey oyster's ability to adapt to different substrates and growing conditions, combined with its large biomass of mycelia, makes it a particularly practical, eco-friendly and cost-effective species for environmental clean-up (Kapahi & Sachdeva, 2017). Not only does it remove pollutants, however, it can also improve soil’s nutritional content.
The restorative impact of grey oyster on soil was something Stamets witnessed in a 1998 experiment using soil taken from a maintenance yard that had been heavily contaminated with diesel and oil. A pile of soil was seeded with grey oyster-cultured sawdust and after four weeks a huge flush of mushrooms had grown — an abundance typically only seen where nutrition is particularly rich. The contaminated soil had changed colour to light brown and no longer smelled of diesel and oil. The mushrooms attracted insects, which laid their eggs. Birds ate the larvae and brought seeds with them and the seeds grew into a variety of plant species, while other fungi also began to colonise the soil. After eight weeks the total petrol hydrocarbons in the soil had plummeted from 20,000 ppm to 200 ppm. “We felt we had witnessed a miracle,” Stamets wrote. “Life was flowering on a dead, toxic landscape.” (Stamets, 2005, pp. 91-92.)
Oyster species (Pleurotus spp) are natural saprophytes of wood and many of the bonds that hold plant material together are similar to the bonds found in petroleum products. This means their enzymes are well-adapted to decomposing these contaminants. The mycelium is able to break the hydrocarbon bonds and the principal byproducts produced are simply water and carbon dioxide (Stamets, 2005, p.88). The grey oyster’s ability to degrade hydrocarbons has put it on the front line of oil spill clean-ups, most famously during the COSCO-Busan oil spill of 2007, when 58,000 gallons of oil needed to be cleared from San Francisco Bay. More recently, grey oysters were deployed to help control contamination from ash following Californian wildfires in 2018, which included pollutants from many burned vehicles. Wattles — straw-filled fibre rolls designed to prevent erosion — were erected across hillsides to control toxic run-off but it was feared the measures would not be enough, so they were inoculated with grey oyster mycelium (Burlison, 2018).
The grey oyster mushroom is also a frontrunner in another emerging eco-friendly technology: mycelium construction materials, or “mycotecture”. Bricks and insulation panels can be created from mycelium and substrate, which is heat treated once it has formed a solid mass to prevent further growth. The biodegradable end product can trap more heat than fibreglass insulation, is fireproof, non-toxic and stronger pound for pound than concrete (Fisher, 2010). The grey oyster’s fast mycelium growth rate makes it the favoured species for this process.
Every hero is allowed their quirk and the grey oyster mushroom has a particularly unusual one for a woodland fungus: it’s carnivorous. This fact was uncovered by Barron and Thorn (1987), who observed that button mushrooms growers were often plagued by nematodes that ate the fruiting bodies — but strangely this was not a problem for grey oyster cultivators. They found that the fungi were able to stun the tiny worms with extracellular toxins and then penetrate the orifices of their bodies with their mycelium, before colonising and digesting them. It’s a property that also has ecological potential: could this fungus be able to control nematodes in other crops and avoid the use of pesticides?
Worms aren’t the only nuisance the grey oyster likes to consume. Later work from Barron (1988) revealed that this mushroom’s mycelium was the most effective out of 100 species tested for digesting bacteria (Pseudomonas and Agrobacterium), from which it extracts nitrogen and protein. In more recent antibacterial tests, grey oysters were found to strongly inhibit the common human pathogens E coli and Staph (Staphylococcus aureus) (Stamets, 2005).
Exploration of the nutraceutical properties of fungi is an ever-expanding field and this superstar mushroom has been a focus of research. Oysters species, including the grey oyster, contain significant levels of lovastatin, which is in common use as a drug to lower high cholesterol (Ramakrishnan et al, 2017). Various studies have also shown that extracts derived from grey oyster mushrooms have an anti-proliferative effect against cancer cell lines, without harming the normal cells, and offer “a reservoir of macromolecules” with health benefits, including those with anti-oxidant, anti-diabetic and immunomodulatory properties (Mishar et al., 2021). Grey oysters have also been shown to have a protective effect against acute renal failure in rats (Sirag, 2009).
So where can you find these fabulous fungi? While there are numerous oyster species and variants world wide — including blue, pink and yellow varieties — five can typically be foraged in the UK. The grey oyster is a hardy edible that appears all year round and, like the other oyster species, is a white-rot fungus generally found on dead deciduous hardwoods. Other species include the summer oyster (Pleurotus pulmonarius), the branching oyster (Pleurotus cornucopiae), the veiled oyster (Pleurotus dryinus) and the elm oyster (Hypsizygus ulmarius). The summer oyster, as its name suggests, prefers the warmer months while the branching oyster grows from summer to autumn, usually on elm and beech.
Are there any downsides to this mushroom? Only that you need to cook them before eating. Grey oyster mushrooms contain a haemolytic protein called ostreolysin, which can be toxic unless the mushrooms are cooked at temperatures exceeding 60 degrees centigrade/140 degrees Fahrenheit (Stamets, 2013).
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