نطق سوم

Lecture 3 - Fungi

Narrator: Listen to part of a lecture in an environmental science class. When you try to imagine a fungus, you’d probably picture a mushroom popping up out of the ground. And think that’s it. But a fungus like that…most of it actually lives underground.

And fungi in general are often an important active component of the soil. A fungus secretes enzymes into the soil, enzymes that break down, decompose organic material in the soil. So the fungus can absorb this material and get nutrition.

But to me, what’s most interesting about this process is how it may enable fungi to help clean up environmental pollution in the soil. And that’s thanks in part to a substance in their cell walls called Chitin.

Now a lot of people think fungi are related to plants, but they are not. Believe it or not, the only other place chitin is found in abundance is in the exoskeletons of insects, crabs and such. So in this sense, fungi are more associated with insects than with any plant. Strange, huh?

And the chitin in the cell walls of a filamentous fungus…a filament, of course, is a long thread-like structure, cells joined end to end. Filamentous fungi grow in soil and in decaying vegetation. And as their name implies they exist as filaments. And although regarded as microorganisms, filaments from a single fungus can fan out to occupy many square meters or even several square kilometers of forest floor.

Their vast surface area allows them to break down and take in huge amounts of nutrients, but beyond that, the filaments also pull out of the soil a great deal of the pollution that might be in there, especially heavy metals.

And here is where chitin comes in, like some other substances in fungal cell walls, chitin forms strong chemical bonds to heavy metals in the environment, in a process we call adsorption.

Now, don’t confuse this with absorption, where a substance is absorbed into a cell, into the interior of a cell. I mean, that is happening here too. But adsorption means binding to the outer surface of the cell. And a filamentous fungus can adsorb toxic heavy metals, bind them to the surface of its enormous network of filaments, and thereby detoxify a large soil ecosystem. The heavy metals are still there, but instead of leaching into the water system and contaminating the water underground, large amounts of these metals may remain bound to the chitin, to the cell walls of filamentous fungi in the soil, and thus remain chemically inactive for as long as 30 years, perhaps longer.

In fact, we can actually use the cell walls of filamentous fungi as a filter, even after the fungi are dead.

For example, the pharmaceutical industry grows filamentous fungi in large quantities in the lab, like to produce the antibiotic penicillin, the drug company grows the fungus penicillium, and after the penicillin is extracted, these dead penicillium filaments, we can use the chitin in their cell walls to make industrial filters to adsorb heavy metals. We can put these filters into waste pipes from industrial processes, and use the filters to trap heavy metals, like mercury and zinc.

Later, we can chemically extract the heavy metals and reuse the filter over and over.

Now going back to the absorption of toxic metals into the body of the fungus, let’s turn our attention to mushrooms.

Like other fungi, mushrooms can absorb large quantities of heavy metals. In fact, they may contain up to two and a half times the concentration of toxic metals found in the soil they grow in. So mushrooms, at least what we see above ground…we can potentially harvest them and then once for all safely dispose of the pollutants contained within them.

In fact, to clean up, especially the groundwater system, permanently, harvesting mushrooms is probably the best way to go. For some reason, this hasn’t happened yet as far as I know, but I can easily envision cultivating mushrooms for the sole purpose of detoxifying a large underground ecosystem.