Saturday, September 3, 2016

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!