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/
