Like us, fungi are heterotrophic and digest other organisms to survive. Unlike us, they do this digestion outside of their bodies, then absorb the nutrients. Consider the structure of a hyphal filament and what you know about surface area to volume ratio. How does the structure of the mycelium allow for more efficient uptake of water and nutrients?
Because fungi must absorb their food, they tend to live on or in whatever it is they are eating. If a mushroom is growing out of a log, it is likely that it is eating that log or something contained within it. This structure and diversity is what makes them such important decomposers. However, fungi play many other important ecological roles.
As you saw in Lab Microfungi - Slimes, Molds, and Microscopic True Fungi, not all fungi are decomposers and parasites. Members of the Glomeromycota form a mutualistic relationship with plant roots called endomycorrhizae, penetrating inside the cell wall.
Some members of both the Ascomycota and Basidiomycota (as well as at least one zygomycete, Endogone) also form mycorrhizal relationships with plants, but the interface between the plant and the mycelium is a bit different. Instead of penetrating inside the cell wall, the fungal hyphae surround the plant cells and make a sheath on the exterior of the root. This type of relationship is called being ectomycorrhizal (ecto- meaning outer), because the hyphae are outside of the cells.
View specimens of fruiting bodies of ectomycorrhizal fungi (such as chanterelles, boletes, Russula, etc…). Compare and contrast these fungi with the glomeromycetes you saw in Lab Microfungi - Slimes, Molds, and Microscopic True Fungi.
In the diagram above, both ectomycorrhizal and endomycorrhizal fungi are shown interacting with the roots of a tree. In the root cross section, label which structures represent each type of mycorrhizal relationship. Next, show the direction of the flow of sugars with arrows. Do the same for the flow of water and nutrients.
A lichen is a mutualistic relationship between several organisms. There is always a fungal partner, the mycobiont, and this is usually an ascomycete. There are a few basidiolichens, but they look very different from ascolichens.
The mycobiont is heterotrophic and so must eat sugars produced by other organisms. The sugars it consumes are harvested from the photosynthetic partner, the photobiont, which is either green algae, cyanobacteria, or less commonly, both (these lichens are called tripartite lichens)!
The photobiont produces glucose through photosynthesis, which the mycobiont harvests and converts into mannitol, a form of sugar that only the fungus can use. The photobiont only really gets to eat when the mycobiont is focused on maintaining the lichen thallus from desiccation (drying out) and sun damage. Though this relationship is technically a mutualism, it has some very parasitic components.
What does the photobiont get out of this relationship?
Lichens are incredibly diverse, but we can begin by classifying them into three different growth forms:
- Crustose lichens are firmly attached to the substrate and usually have to be chipped off. These lie flat, like a crust.
- Foliose lichens have a distinct upper and lower surface, like a leaf (think ‘foliar’).
- Fruticose lichens are shrub-like and usually attach to the substrate at a single point, like a plant with roots.
Observe the diversity of lichens on display. Draw a lichen from each of the three forms below. Label any ascocarps (or basidiocarps) that you find.