Hook, Line, and Sinker: Rise of the Killer Mushrooms

By Nathan Smith

Pleurotus ostreatus, or the oyster mushroom, is a common edible mushroom. As much at home in a stir fry or a soup, one would not expect this culinary baseline to be anywhere other than at the bottom of the food chain. Surprisingly this is not the case, as the oyster mushroom is a stealthy and efficient predator of nematodes.

Pleurotus ostreatus-- a killer in disguise?

Pleurotus ostreatus— a killer in disguise? (Credit Jean-Pol GRANDMONT)

Nematodes, a type of worm, are infamous agricultural pests. The damage caused by these miniature beasts has been estimated at $US80 billion per year, though this is believed to be a severe underestimate as many growers are unaware of them. They even present a threat to the cultivated mushroom industry, being a renowned pest of button mushrooms, so how is it that they fall prey to the oyster mushroom?

The answer is one of ingenuity on the part of the fungus. Unable to chase the nematodes, mushrooms are notoriously sessile, it instead lays a trap. It secretes a toxin which, upon contact with a nematode, proceeds to immobilise the worm in as little as 30 seconds. Fungal hyphae, attracted to the (still alive) nematode through host leakage products released by immobilisation, penetrate one or more of the nematode’s orifices and proceed to digest it.

The unrelated fungus Arthrobotrys also hunts nematodes, but through a completely different mechanism. Instead of stunning its host, it captures it in a hyphal lasso. Known as a constriction ring, this consists of a hypha fused with itself to form a three-celled ring about 20-30 microns in diameter. If, and when, a nematode enters the ring, it triggers the three cells to expand rapidly (within 1/10th of a second) and trap the nematode.

Killer fungi aren’t just of academic interest either. The hunting abilities of fungi, particularly the oyster mushroom, make them potential effective and green bio-control agents. Indeed, initial tests have found the oyster mushroom effective at controlling the Sugar Beet Nematode (Heterodera schachtii), through field tests have yet to be carried out.

It appears that fungi aren’t just passive members of the woodland scenery but rather edible guardians protecting against the nematode threat.

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Harmful algal blooms; the monster in the shallows

By Alex Steeples

Many of us will have been there, sat on a sunny beach unable to go into the sea, due to the presence of a polite sign warning you of toxic algae. To many this seems illogical; what harm can some floating green specks or tangle of sea weedy mush do? Especially when there are great white sharks and box jellyfish lurking in the deep.

Harmful algal bloom (HAB) is a non-specific term used to refer to any sudden increase in the amount of algae that is deemed to be detrimental to the environment. This harm can be either through the production of harmful toxins, primarily neurotoxins such as brevetoxin and domoic acid; or through the large increase in algal biomass reducing water oxygen content and affecting the food web.

HABs occur due to a sudden increase in the nutrient content of the water, which allows for rapid growth. These increases, particularly in nitrogen and phosphorous, are often associated with specific seasonal changes, meaning many areas suffer from repeated periodic algal blooms.

Neurotoxin producing algal species such as Karenia brevis, primarily show their effects through the killing of large quantities of fish, which later wash up on shore. Higher mammals may also be killed, or suffer severe illness, if they ingest toxins via a vector such as fish or sea grass. The consumption of contaminated fish was associated with death of over 100 bottlenose dolphins off the coast of Florida in 2004. In the case of humans, whilst fatalities are rare, shellfish poisoning often occurs. This results from the ingestion of shellfish, primarily mussels and clams, which accumulate the toxin.

Although HABs have a wide ranging ecological impact, they also have important socio-economic effects. HABs can cause the closure of fisheries, and sea side resorts for the duration of the bloom, leading to loss of income and, in some cases, livelihood.

So next time you see that sign warning you of algae, pay attention. After all, not all dangers lurk in the deep.