Fungi interact with other organisms in many ways, both harmful and beneficial. Saprobic fungi play critical roles in the recycling of elements required by living organisms. Lichens are mutualistic associations of a fungus with algae or cyanobacteria. Mycorrhizae are associations of fungi and the roots of plants; they are essential for the survival of most plant species.

Question 1

Fungi break down organic matter, especially the cellulose and lignin of plant walls, thus returning the carbon in dead plants back into a form that can be used again by other organisms. Fungi also break down the keratin of animals, which is resistant to bacterial decay. Without fungi, these materials would build up in the environment and be a global sink for carbon.

Question 2

Lichens represent a mutualistic association between a fungus and a photosynthetic partner (a unicellular alga and/or a cyanobacterium). The fungus obtains fixed carbon (organic compounds) from its photosynthetic partner and provides it with minerals and water. Both the fungus and the photosynthetic partner benefit and can grow in harsh environments (such as on rocks or the bark of trees) where neither organism would survive well, if at all, on its own.

Question 3

As with lichen associations, mycorrhizal fungi form a mutualistic partnership with a photosynthetic species, in this case a multicellular plant. Many plants depend on these mycorrhizal associations for survival. The plant provides organic compounds (the products of photosynthesis) to the fungus, and the fungus provides minerals and water from the soil to the plant. The high surface area-to-volume ratio of the fungal hyphae greatly increases the plant’s ability to absorb water and minerals. Also, the hyphae act like a sponge, holding water around the roots.