Fungi decompose nearly every dead organism on Earth, recycling organic matter back into soil that makes all new life possible. Louis Pasteur's 1857 yeast experiments revealed that fermentation — the same process behind beer and bread — is fungi's ancient superpower: absorbing energy by breaking down complex molecules.
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 Twitter and of course, we're down in the comments below. We'll see you next time.