Fungi: Death Becomes Them – CrashCourse Biology #39

Fungi: Death Becomes Them – CrashCourse Biology #39

August 31, 2019 100 By Ewald Bahringer


Hello and welcome to the wonderful
world of fungi (fuhn-gahy), 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 that get pinched off
into separate organisms. Since some species of yeast,
like our beer-making friend, Saccharomyces cerevisiae,
convert sugars into alcohol, brewers create conditions that
encourage high rates of yeast production, like giving
them lots of sugar and oxygen, since more yeast means more alcohol. So, yeah, fungi!
They feast on death, and they can make us go
insane and turn ants into unholy zombies of the night. But because of their hard
work and strange ways, they make possible stuff
like agriculture and beer and everything else
worth living for. So thanks to the fungus.
And also thanks to you for watching this episode
of Crash Course Biology. And of course, thanks to the people
who helped me put it together. They’re awesome. Thank you guys! There’s a table of contents over
there if you want to click on it and go review any of the stuff that
you want to reinforce in your brain. And if you have questions
or comments or ideas for us, we’re on Facebook and
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