Final answer:
Lichens serve as indicators of air pollution due to their sensitivity to nitrogenous and sulfurous compounds. They form a symbiotic relationship between a fungus and an alga or cyanobacterium, playing vital ecological roles that include nitrogen fixation and soil formation. Option D.
Step-by-step explanation:
Lichens are well-known for their sensitivity to air pollutants, such as nitrogenous and sulfurous compounds, making them excellent indicators of air quality.
By observing the type of lichen present and their health, the U.S. Forest Service and National Park Service can assess the air quality in an environment, helping to monitor and manage the presence of pollutants.
Lichens have a dual nature consisting of a fungal partner intertwined with a photosynthetic algae or cyanobacterium, existing in a symbiotic relationship. This mutualism allows lichens to colonize and stabilize various environments, contribute to soil formation, fix atmospheric nitrogen, and serve as a food source for animals like caribou.
Moreover, due to their role as pioneer species in nutrient-poor environments and their long lifespans, lichens are an important part of many terrestrial ecosystems. Option D.