Listen to part of a lecture in an environmental science class.
Professor: Today we're going to talk about a fascinating organism called lichen. Lichen is not a single organism; it's a symbiotic partnership between a fungus and an alga or a cyanobacterium. The fungus provides structure and protection, while the alga produces food through photosynthesis. This relationship allows lichen to survive in extreme environments, from Arctic tundra to desert rocks. One remarkable ability of lichen is its role as a bioindicator. Because lichen absorbs nutrients and water directly from the air, it is highly sensitive to air pollution, especially sulfur dioxide. When pollution levels rise, lichen populations decline, and certain species disappear entirely. Scientists use the presence or absence of lichen species to assess air quality in urban and rural areas. However, lichen grows very slowly, often less than a millimeter per year, so recovery after pollution control can take decades. This makes lichen a valuable but fragile tool for monitoring environmental health. Now, let's consider some specific examples...
❓ 题目 What is the main reason lichen is considered a useful bioindicator?