Saturday, August 27, 2016
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...
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/
Saturday, August 20, 2016
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.
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, 4th 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.