Lichens come in many colors, sizes, and forms. The properties are sometimes plant-like, but lichens are not plants. Lichens may have tiny Figure . Lichens are found in a variety of habitats. a. A fruticose lichen, growing in the soil. b. A foliose (“leafy”) lichen, growing on the bark of a tree in Oregon. c. A crustose lichen, growing on rocks leading to the breakdown of rock into soil.
Composition of a lichen
Ascomycetes are the fungal partners in all but about 20 of the approximately 15,000 species of lichens estimated to exist. Most of the visible body of a lichen consists of its fungus, but between the filaments of that fungus are cyanobacteria, green algae, or sometimes both Specialized fungal hyphae penetrate or envelop the photosynthetic cell walls within them and transfer nutrients directly to the fungal partner.
Note that although fungi penetrate the cell wall, they do not penetrate the plasma membrane. Biochemi-cal signals sent out by the fungus apparently direct its cyanobacterial or green algal component to prod-uce metabolic substances that it does not produce when growing independently of the fungus.The fungi in lichens are unable to grow normally without their photosynthetic partners, and the fungi protect their partners from strong light and desiccation.
When fungal components of lichens have been experimentally isolated from their photosynthetic partner, they survive, but grow very slowly.
Figure . Stained section of a lichen. This section shows fungal hyphae (purple) more densely packed into a protective layer on the top and, especially, the bottom layer of the lichen. The blue cells near the upper surface of the lichen are those of a green alga. These cells supply carbohydrate to the fungus.
Ecology of lichens
The durable construction of the fungus combined with the photosynthetic properties of its partner have enabled lichens to invade the harshest habitats—the tops of mountains, the far-thest northern and southern latitudes, and dry, bare rock faces in the desert. In harsh, exposed areas, lichens are often the first colonists, breaking down the rocks and setting the stage for the invasion of other organisms.
Lichens are often strikingly colored because of the presence of pigments that probably play a role in protecting the photosynthetic partner from the destructive action of the sun’s rays. These same pigments may be extracted from the lichens and used as natural dyes. The traditional method of manufacturing Scotland’s famous Harris tweed used fungal dyes.
Lichens vary in sensitivity to pollutants in the atmosphere, and some species are used as bioindicators of air quality. Their sensitivity results from their ability to absorb substances dissolved in rain and dew. Lichens are generally absent in and around cities because of automobile traffic and industrial activity, but some are adapted to these conditions. As pollution decreases, lichen populations tend to increase.