or fungi (fuhn-jahy). Both are acceptable pronunciations.

But I say fungi because it's fungus. Not fun-jus.

Though fun-jus is also fun to say.

Fungi are a little bit like plants,

and more like animals than you might think.

They diverged from protists about a billion years ago,

and today scientists estimate that there are about 1.5 million

species of Fungi on the earth, though in a formal, taxonomic way,

we only know about 100,000 or so of them.

And those that we have met are wonderful, weird, and,

in some cases, deadly.

And the fact is, death is pretty much what fungi are all about.

Sure, there are the fun fungi, like the single-celled Saccharomyces,

also known as yeast.

Without them, we wouldn't have beer, wine or bread.

It's also true that fungi are responsible for all kinds of diseases,

from athlete's foot to potentially deadly histoplasmosis,

aka spelunker's lung, caused by fungus found

in bird and bat droppings.

Fungi can even make people crazy.

When the fungus Claviceps purpurea grows on grains used

to make bread and beer, it causes gangrene, nervous spasms,

burning sensations, hallucinations, and temporary insanity.

One compound in this fungus, lysergic acid,

is the raw material used to make LSD.

And finally there's the destruction that some fungi bring onto

other animals: More than 6 million bats in North America have died

since just 2007, due to a fungal disease called white nose syndrome.

And a fungus has been implicated in several extinctions

of amphibians and threatens many more,

perhaps as many as a third of all amphibians on Earth.

But none of this is what I mean when I talk about fungi and death.

While some members of the fungus family are total bummers,

all of them together perform perhaps the most vital function

in the global food web: They feast on the deceased remains

of almost all organisms on the planet.

And by doing that, they convert the organic matter that we're

all made of back into soil, from which new life will spring.

So, fungi: They thrive on death, and in the process,

make all life possible.

Aha! You Didn't expect to see me in the chair so soon!

But before we go any deeper into the kingdom fungi, I wanted

to make a toast to Louis Pasteur in the form of a Biolo-graphy.

By Pasteur's time, beer had been brewed for thousands of years

in cultures all over the world.

Some experts think it may have been the very reason that our

hunter-gather ancestors started farming and cobbled together

civilization in the first place.

But for all those millennia, no one understood how its most

important ingredient worked.

Until brewers could actually see what yeast were doing,

the magic of fermentation was... essentially magic.

Pasteur himself was never a big beer drinker, but part of his

academic duties in France required him to help find solutions to

problems for the local alcohol industry.

And as part of this work, in 1857, he began studying yeast under

a microscope and discovered that they were in fact living organisms.

In a series of experiments on the newfound creatures,

he found that in the absence of free oxygen, yeast were able to

obtain energy by decomposing substances that contained oxygen.

We now know that Pasteur was observing yeast undergoing the process

of anaerobic respiration, aka fermentation, breaking down the sugars

in grains like malted barley, and converting them into alcohol,

carbon dioxide and the range of flavors that we associate with beer.

Along the way, Pasteur also discovered that beer was often

contaminated by other bacteria and fungi.

The growth of these beer-spoiling microbes, he found, could be

thwarted for up to 90 days by keeping beer between

55 and 60 degrees Celsius for a short period of time.

Today, we call that heating process pasteurization, and it's used

in everything from milk, to canned foods, to syrups, to wines.

For our purposes, the thing to hold onto here is,

Pasteur discovered that yeasts decompose sugars to get energy.

And it turns out, most fungi spend most of their time decomposing

all kinds of organic matter.

Often the matter is dead when fungi get to it, but not always.

When a tree, or a person, or a deer keels over,

fungi move in and start the work of decomposition.

Same goes for that orange you forgot at the bottom of the fruit bowl.

If it weren't for this fungal function, plants, and the animals

that eat them, couldn't exist because the elements that

they take from the soil would never return.

Thankfully, the decomposition performed by fungi recycles

the nutrients for the enjoyment of plants and animals

as well as for other fungi.

All of this points to one of the main traits

that all fungi have in common.

From single-celled yeast to giant multicellular mushrooms, fungi,

like us, are heterotrophs.

But instead of eating, they absorb nutrition from their surroundings.

They do this mostly by secreting powerful enzymes that break down

complex molecules into smaller organic compounds,

which they use to feed, grow, and reproduce.

Most multi-cellular fungi contain networks of tiny,

tubular filaments called hyphae that grow through

and within whatever they're feasting on.

Unlike plant cell walls, which are made of cellulose,

the cell walls of fungi are strengthened by the nitrogenous

carbohydrate chitin, the same material found in the exoskeletons

of insects, spiders, and other arthropods.

The interwoven mass of hyphae that grows into the food source

is called the mycelium, and it's structured to maximize

its surface area, which as we've learned in both plants and animals

is the name of the game when it comes to absorbing stuff.

Mycelia are so densely packed that 1 cubic centimeter of rich soil

can contain enough hyphae to stretch out 1 kilometer

if you laid them end to end.

So as hyphae secrete the digestive enzymes, fungi use the food

to synthesize more proteins, and the hyphae continue to grow,

allowing the fungi to conquer new territory and grow even more.

As a result, fungi can get crazy big. Record-holding big.

A single honey mushroom in the Blue Mountains of Oregon

is thought to occupy some 2,386 acres.

By area, the largest organism on the planet.

Now there are all kinds of crazy ways that fungi are classified,

but probably the easiest and most useful is organizing them by how

they interact with other organisms.

The straight-up decomposers that break down dead stuff,

the mutualists, which form beneficial relationships

with other organisms, especially plants,

and then there are the predators, and the parasites.

Decomposer fungi secrete enzymes that break down and absorb

nutrients from nonliving organic material, such as that tree

that nobody heard fall in the forest.

In fact, the ability of fungi to break down lignin,

which is what makes wood woody, and break it into glucose

and other simple sugars is crucial for the cycle of life.

They're pretty much the only organism that can do that.

They can even decompose proteins into component amino acids.

Basically, all the black bits in the soil in your backyard

are tiny fragments of former plants digested by fungi.

Mutualist fungi are a smaller group.

Many have specialized hyphae called haustoria that tangle

themselves with plant roots for the benefit of both organisms.

These guys help plants absorb nutrients, especially phosphates,

by breaking them down more efficiently

than the roots can themselves.

In turn, the fungi send out their hyphae into the plant's root

tissue and withdraws a finder's fee, basically,

in the form of energy-rich sugars.

These mutualistic relationships are known as mycorrhizae,

from the Greek words "mykes," or fungus and "rhizon" or root.

Mycorrhizae are enormously important in natural ecosystems,

as well as in agriculture.

Almost all vascular plants, in fact, have fungi attached

to their roots and rely on them for essential nutrients.

Growers of barley, the main ingredient in beer,

will even inoculate barley seed beds with specific

mycorrhizal fungi to help promote growth.

Other fungi aren't nearly so kind to their hosts.

Predatory fungi actively capture prey with their hyphae,

the soil fungus Arthrobotrys uses modified hoops on its filaments

to snare nematodes and absorb their inner tissue.

Then there are the parasites, those fungi that feed on living

organisms without killing them, at least for a while.

Take one of my personal favorites:

the zombie ant fungus, or Ophiocordyceps.

It shoots spores into an ant, where their hyphae grow into its body

and absorb nutrients from non-essential ant organs.

When the fungus is ready to reproduce, it invades the ant's brain

and directs it to march to a cool, moist location in the forest

where its so-called fruiting spores erupt

through the ant's head to spread even more spores.

And just to prove that even fungi have superheroes,

in 2012 scientists discovered that these zombie spores

have themselves been targeted by another parasitic fungus.

Not a lot is known about this ant-saving fungus,

other than it sterilizes many of the zombie spores

through a process likened to chemical castration.

That is so messed up. Weird!

Alright now, since I brought that up,

we should talk briefly about fungus sex.

Fungi reproduce any way they can, either sexually or asexually.

Some species even do it both ways.

But whichever way they choose, most propagate themselves

by producing enormous numbers of spores, much like we saw

in nonvascular plants and the simplest

of vascular plants, the ferns.

But, and this is a big but, sexual reproduction in fungi

isn't like sex in any other organism we've studied so far.

The concepts of male and female don't apply here. At all.

Some fungi reproduce on their own.

Others can reproduce with any other individual

that happens to be around.

And still others can only mate with a member of a different

so-called mating type: they're not different sexes,

they just have different molecular mechanisms that

either make them compatible or not.

Sometimes these types are called plus or minus,

and other times 1 and 2.

In any case, it's still considered sexual reproduction,

because each parent contributes genetic information

when they make with the spore-making.

It all starts with this beautiful chemical mating dance,

as the mycelium from one fungus sends out pheromones that are picked

up and bound to receptors by another willing and able partner.

This binding compels each mycelium to send its hyphae toward the other.

When they meet, they fuse the cytoplasm of their cells,

a stage of reproduction called plasmogamy.

Sometime between hours and centuries later,

yes, it can literally take hundreds of years for fungi to have sex,

this union leads to the production of spores

that each fungus is then able to disperse.

Certain types of fungi, including the tasty morel,

produce spores in sac-like asci contained in

fruiting bodies known as ascocarps.

That is the part you pick when you're wandering through the forest.

Some fungi shoot their spores off into the breeze,

other spores float away on the water.

More enterprising spores will hitch a ride on passing critters,

hopefully to be dropped off somewhere where there's plenty of

nutrients to absorb, so they too can grow, send out sexual pheromones

when their time comes and let their hyphae do the tango.

Finally, for some fungi, sexual reproduction just isn't all

it's cracked up to be.

They'd rather just get it on with themselves.

Some of these grow filamentous structures

that produce spores by mitosis.

These structures are visible, and they're called molds,

the stuff on the orange in the bottom of the fruit bowl

or the heel of the piece of bread that you left for a roommate

who decided to leave it for the other roommate

who thought that you'd rather have it.

In the unicellular yeast, the asexual reproduction occurs by

old-fashioned cell division or the formation of buds