Symbiotic Relationships

Two important symbioses involve fungi: the mycorrhizae that occur on the roots of almost all vascular plants and the lichens that have evolved entirely different body forms from those of their symbionts.

Mycorrhizae

Fungi and the roots of almost all vascular plants form mutualistic associations calledmycorrhizae (singular, mycorrhiza). The fungus gets its energy from the plant, and the plant acquires an efficient nutrient absorbing mechanism—the actively growing hyphae that penetrate regions of the soil untapped by root hairs. Phosphate uptake especially is increased when mycorrhizae are present.

Two general types of mycorrhizae occur, differentiated by whether the hyphae livewithin the cortical cells of the roots or remain outside the cells: endomycorrhizae(endo = within; myco = fungus; rhizae = roots) and ectomycorrhizae (ecto = outside). Zygomycete taxa are components of most endomycorrhizae while basidiomycetes and a few ascomycetes form ectomycorrhizae.

Lichens

The symbiotic relationship of fungi with either algae or cyanobacteria produces a body—a lichen—so distinctly different from either of its symbionts that it is treated as a separate organism. The fungal hyphae give the lichen thallus its characteristic shape, and the cells of its photosynthetic partner are dispersed among them. While the algal or cyanobacterial member can live independently, the fungus cannot, so the fungus in essence is a parasite on the photosynthesizer in the lichen thallus. The fungus, however, provides a “home” for the photosynthetic cells as well as absorbing water and nutrients that the photobiont uses. This makes the symbiosis mutualistic as much as parasitic in the view of some biologists.

Life is becoming precarious for lichens in many urban environments today. Many lichens are intolerant of air pollutants. They have no means of getting rid of the elements, toxic or useful, which they absorb. Sulfur is particularly toxic to many, and sulfur dioxide released from burning coal has eliminated many susceptible species from cities. Lichens can be used as biomonitors—and warnings—of the quality of the air we breathe.