Saturday, August 27, 2016
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...
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/