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!
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