Colorful lichens on the black Bidahochi basalt. Photo by Marge Post In many areas of Petrified Forest National Park, large collections of exposed rock surfaces are covered by lichens, especially the shaded north side of a rock. A lichen is actually a simple community of at least two mutually-dependent organisms: fungi and green algae. When both organisms are dependent on the other, they are said to be symbiotic. Green algae uses the photosynthesis process to produce food for the fungi, while the fungi protects the algae from the elements and extracts nutrients from soil and rock. The lichen structure is more elaborate and durable than either fungi or algae alone. Lichens are well adapted to arid climates. They can continue food production at any temperature above freezing.
Lichens can absorb more than their own weight of water and can absorb emphemeral water, such as dew, almost directly into their cells. The water does not need to go through roots and stems as it does in vascular plants. Many other plants benefit from the presence of lichens. The green algae component of lichens can transform nitrogen in the air, which is unusable to most organisms, into a form which is essential for life. This is especially important in arid climates where lack of nitrogen is known to limit productivity. Because lichens take everything they need from the air, they are dependent on good air quality. Scientists turn to lichens as indicators of air quality. It is truly amazing how nature, over time, can establish symbiotic relationships for the benefit of the whole ecosystem.