Fungus Among Us Part 3

Note: This program first aired on September 3, 2016.

Last week we talked about fungus, and they ways it makes a living in this world. The mushrooms we see in the forest are just the tip of the fungal iceberg, the vast majority of fungal biomass in the forest is subterranean. These are the bundles of fungal fiber called mycelium, and if the mushroom’s job is reproduction of the fungus, the mycelium’s job is to nourish it.

There are three ways that fungi can get food from the environment; they can parasitize another fungus*, they can decay organic matter, or it can form a relationship with a plant in an “I’ll scratch your back if you scratch mine” symbiosis called mutalism. We covered parasitism and saprotrophism earlier, leaving mutalism for today.

It turns out that many of those mushrooms you see erupting from the forest floor are from fungal biomass that is in direct relationship with the trees that make up that forest. The individual hyphal filaments that make up the mycelium, or all that fungal biomass beneath the surface of the soil, get up close and personal with the tiny root hairs, or rootlets from the tree and form what is called a mycorrhizal relationship, “myco” referring to fungus, and “rrhizal” referring to roots. Trees and their fungus typically form what is called an ectomycorrhizal relationship—meaning the fungus only just barely infiltrates the upper layers of the rootlet tissue, squeezing in between the cells of the root outer cortex, forming a sheath.  The conjoining of the tree roots with the fungal mycelium effectively expands the tree’s root system by orders of magnitude, and even directly connects it to other trees of its species if the mycelium forms an ectomycorrhizal relationship with more than one individual. 

Typically a fungal species has only one suitable tree species it can pair with, though often trees can have many different fungal partners. Many of the trees I see daily are obligated to form relationships with fungal partners; they cannot grow without the assistance of the fungus. Those groups include pines, oak, beech and spruce. Other tree species are facultative, and can grow without a fungal partner but grow better with one. Examples include maples, juniper, willows and elms.

I said this is an I’ll scratch your back if you scratch mine kind of situation, both partners benefit from this trans species contact. As I noted, the tree gets a major extension of its root system, gaining what one source called “hundreds of thousands of kilometers’ of individual hyphae, collecting water and inorganic nutrients from the soil. Essential nutrients like phosphorous and nitrogen are limited in the terrestrial environment, and the fungus is able to aggregate these and make them more available to the tree than they would be otherwise. The fungus also is able to collect water from the soil, though there can also be instances when the plant gives moisture to the fungus as well. And in some cases the fungus produces plant growth hormones, stimulating tree root growth. What we know for sure the fungus gets out of the relationship is carbon, in the form of sugar. 10 to 15 % of the carbon fixed by the tree gets channeled to the mycorrhizal partner. Both players are able to trade something they are good at getting from the environment for something they need, to the benefit of everyone.

This kind of relationship isn’t limited to trees and mushrooms. Many herbaceous plants and agricultural crops form mycorrhizal relationships as well, relationships characterized by even deeper infiltration of the fungus into the plant tissue. And in a totally different part of the world, coral reefs, we see the mutualistic symbiosis of coral polyps and photosynthetic plankton, swapping carbon in the form of sugar in return for inorganic nutrients. In a great example of convergent evolution, many realms of life species have evolved to swap resources in order to increase their competitive fitness. That is the cool thing about evolution, when something works, it keeps popping up independently on the tree of life.

So many of those mushrooms you see in the woods this fall, are part of a legacy of remarkable biological cooperation.

* or plants or even animals!

 

References:

Same as the last two weeks!

Fungus Among Us Part 2

Note: This program first aired on August 27, 2016.

Last week we looked at mushrooms, and talked about how they are the reproductive structures of certain kinds of fungus. Mushrooms are simply the above ground spore making structures of an otherwise underground organism, one made of miles of bundles of filaments called mycelium. If the mushroom’s job is to make spores, then what is the job of the mycelium, all the fungal biomass we don’t see? Just like in other realms of life, different life stages have different jobs for the organism, and the mycelial job is feeding. Fungus are heterotrophs like you and I, they rely on food sources outside of their bodies (unlike plants—which create their own carbohydrates). As heterotrophs we eat food, ingesting the complex material which is then broken down into smaller more accessible biological molecules and absorbed directly into our bodies in our guts. Fungus don’t have mouths or guts, but they still digest food using the same process. They excrete the digestive enzymes onto the material they are eating, and once that material is broken down into smaller molecules, it can be absorbed by the fungal filament. For example neither humans nor fungus can absorb cellulose, one of the primary molecules of wood, and much plant material. When humans eat cellulose, we call it insoluble fiber, and though it has health benefits, we don’t digest it and don’t get any nutrition from it. When  the mycelium of certain fungi encounter cellulose, they are able to produce enzymes that break the cellulose down into the individual molecules of glucose it is made from. Fungi can then absorb the glucose, gaining nutrition from the cellulose. The ability to digest cellulose and lignin, the other primary constituent of wood, is one of fungi’s super powers.

There are lots of fungi out there, but we’ve been trying to limit our discussion to mushrooms—so do all mushrooms eat wood? No, in fact there are three different modes of nutrition for the mushrooms that we see out in the woods. The first, and least common, so we’ll get it out of the way, is parasitism. There are fungi that parasitize other fungi! Parasitism is a symbiosis that is typically thought to benefit one partner and have a negative impact on the other. The parasitic fungus benefits by stealing nutrition from the parasitized fungus. In our region the most common parasitic fungus you will see is lobster mushroom, which is a fungus that doesn’t make its own mushroom, but hijacks the mushroom of other species. I’ve talked about Lobster mushroom before on the show.

The second mode of nutrition for mushrooms is to be a decomposer or saprotroph. This is the default or ancestral mode of fungal nutrition, the ability to excrete digestive enzymes into the environment and break down complex organic molecules into simple (and thus absorbable) organic molecules is the hall mark of this kingdom of life. If it weren’t for fungi (and many bacteria as well), we would be overwhelmed with dead organic matter, and in fact life would stop because it would run out of raw materials. Fungi are the recyclers of the biological world, they process millions of tons of organic waste a year, turning dead material back into building blocks like carbon dioxide and individual mineral nutrients that can be used again by plants to make more food. The balance between the carbon taken out of the atmosphere by plants and the carbon put back into the atmosphere by animals, bacteria and fungi is what keeps climate relatively stable*, at least until plate tectonics changes atmospheric circulation** and weather patterns change, and throw that balance out of whack, driving extinction and more importantly evolution. Heady stuff for those little mushrooms along the trail.

I said there are three modes of nutrition for fungi, but we are out of time for today, so we will look at the third, and if you are a plant, most interesting mode, next week. 

*Before the geologists get mad at me--Yes, the rock cycle plays a really important part in this as well--carbon going into and coming out of geological sinks like limestone...

**And influences the rock cycle by exposing or burying carboniferous rocks...

 

References:

  

All the same ones as last week plus:

About external digestion: http://bugs.bio.usyd.edu.au/learning/resources/Mycology/Feeding/extracellDigestion.shtml

Still one of my favorite theories out there, even if it is now being challenged: www.scientificamerican.com/article/mushroom-evolution-breaks-down-lignin-slows-coal-formation/

The challenge: http://news.stanford.edu/2016/01/22/coal-formation-pangea-012216/

Fungus Among Us

Note: This program first aired on August 20, 2016.

There’s fungus among us. Though it has been a dry summer, in the past few weeks, right after each heavy rain, on the trails I run I see mushrooms pushing their way up out of the forest floor. Russulas and Lactariuses, coral fungus and boletes, an occasional amanita and delicious chanterelles. And those are just the groups I can identify with relative ease.

Mushrooms are the reproductive structures of certain kinds of fungus, ascomycetes and basidiomycetes. There are many other kinds of fungus out there as well, but they don’t make mushrooms (think mold and yeast and a bunch of stuff that is essentially invisible to human eyes).  As the reproductive structures of ascomyctes and basidiomycetes, they emerge when environmental conditions favor fungal growth. The timing of these appearances gives a clue as to what those favorable conditions are. It has been a dry summer, dry enough that all of the organic matter that makes up the upper layers of the forest floor is dry, and we’ve experienced a few small forest fires. Scary stuff in our dense, low fire frequency eastern forest.  The mushrooms we see emerge after damp weather are the result of the action of billions of fungal filaments below ground in the soil. These filaments, called mycelium make up the bulk of fungal biomass, at least in terms of the mushrooms we see in the forest. The mushrooms are truly only the tip of the ice berg.

Mycelium are made up of even smaller individual filaments called hyphae, and grow through the soil in the forest feeding on organic matter. Like the fine hair like roots of plants, these microscopic fillaments don’t do well when the soil is very dry, their movement and metabolism are aided by the water that makes the soil damp. Hence, a nice flush of rain  that wets the forest soil results in a boom of mycelial activity, and it is when compatible mycelium meet up underground that a mushroom results. Rapid increases in mycelium increases the chance of these meetings, hence mushrooms appearing overnight after wet weather. The mushroom’s only job is to create spores, which can result from sexual reproduction between those two compatible mycelia, and are a dispersal mechanism for fungus. Tiny and airborn, spores can travel great distances on air currents, and if they land in the right spot, can germinate and form a new hyphal strand. If that strand of hyphae finds what it needs it continues to grow and becomes multistranded mycelium. If it runs into another mycelium from the same species, and they are compatible mating types, they will merge and share their genetic information, and build a mushroom from this conjoined mycelium. In special cells in the mushroom (typically on the gills underneath the cap) meiosis will occur and the spores that are formed will contain a mix of genetic information unique from either parent.

If that is the job of the mushroom, what is the job of all that mycelium in the forest soil? We’ll answer that question next week.

References:

Mostly books this time around: 

David Arora, Mushrooms Demystified

George Barron, Mushrooms of Northeast North America

Lawrence Millman, Fascinating Fungi of New England

Elizabeth Noore-Landecker, Fundamentals of the Fungi, 4^th ed.

James, Timothy (2007). "Analysis of mating type locus organization and synteny in mushroom fungi: Beyond model species". In J. Heitman; J. W. Kronstad; J. W. Taylor; L. A. Casselton. Sex in Fungi: Molecular Determination and Evolutionary Implications. Washington DC: ASM Press. pp. 317–331.

Lobster Mushrooms

Note: This show first aired on August 30, 2014.

Last week I was in western Maine, enjoying a last blast of summer vacation. Hiking through the mixed woods around Flagstaff Lake, I was amazed by the abundance of fungus growing in the woods. We saw several different coral fungi, puffballs, earth tongues, amanitas, slime molds and many different kinds of mushrooms. Late summer and fall are peak times for fungal fruiting activity, as long as there is sufficient moisture and we were clearly there at the right time. Fungi are very ephemeral, responding to temperature and humidity, and the species you see around you fruiting on the forest floor change from week to week and even day to day.

The species that made the biggest impact on me however, was the lobster mushroom, otherwise known as Hypomyces lactiflorum. They were everywhere, huge and bright orange and erupting out of the duff and leaf litter. They are called lobster mushrooms due to their orange color, looking something like the orange of lobster meat, or cooked lobster shells, and they are reportedly edible, though I doubt they bear any culinary resemblance to lobster.

The lobster mushroom’s claim to fame or dirty secret is that it isn’t really a mushroom at all, not in the conventional sense anyway. What we see and call a lobster mushroom is actually a mushroom of one of two common genera that has been parasitized by the Hypomyces lactiflorum fungus. Hypomyces has no shape of its own, its mycelium (or generic fungal fibers found in soil everywhere in the forest) live in the soil among the mycelium of its host species. When the host species fruits, the term for creating a mushroom, Hypomyces invades and uses the host’s mushroom for its own purposes.  And what is the purpose of a mushroom? It is the reproductive structure of many kinds of fungi. When the mycelium of two separate fungal individuals of the species meet in the soil, they exchange genetic material and the resulting mushroom is simply the above ground tissue where the spores  (which are analogous in many respects to plant seeds) are formed and then released into the air, which is the medium in which they are dispersed. For some reason Hypomyces can’t make a mushroom on its own, so it uses someone else’s. If you look closely at a lobster mushroom, you will see the tiny little sacks that are the fruiting structures of Hypomyces, they cover the surface of the lobster mushroom, and that surface area is what Hypomyces is after. The host fungus also undoubtedly provides nutrition to the parasite, so this is a case of a fungus eating another fungus as well as the story of a reproductive service unwillingly provided by one fungus to another.

The victims of this attack are fungi in the Lactarius and Russula genera. They are both very common in the forest and include hundreds of species. Both form mycorrhizal or mutually beneficial relationships with most trees species as well. In the northeast, Hypomyces typically parasitizes Russula brevipes and Lactarius piperatus.

We are coming into peak mushroom season. I encourage you to get out on a fungal foray of your own. Go for a walk in the woods and take your time, bring a field guide and a camera and spend the afternoon exploring the diversity that can pop up overnight, and disappear just as quickly. And it must be said, you have to be careful. Just because lobster mushroom is named after a food, that doesn’t necessarily mean it is edible. Remember it’s a parasite, and you can’t identify the host once its been parasitized. There are people out there who can teach you about wildcrafting edible mushrooms, I am not one of them. So please, due diligence is in order. Fungal appreciation is a life long pursuit, I am enjoying the state of “beginners mind” I find myself in, where the discovery and identification of a common species brings excitement and joy. I hope this fall, you will join me in this simple pleasure.

References:

George Barron’s Mushrooms of the Northeast North America, Lone Pine Press 1999, one of my favorites, nice photos.

David Arora Mushrooms Demystified Ten Speed Press 1979, 1986 The definitive mushroom book for most of us! Pacific Northwest centric, but still useful. Dense, and lacking lots of pretty pictures (most are black and white). Clearly kooky author.

Lawrence Millman Fascinating Fungi of New England Kollath and Stensaas Publishing 2011

Tom Volk’s webpage on Hypomyces:

http://botit.botany.wisc.edu/toms_fungi/aug2001.html