In environmental biology, which of the following organisms is widely used as an indicator of air pollution, because it is very sensitive to pollutants such as sulphur dioxide and disappears when the air quality is poor?

Introduction / Context:
Certain organisms are very sensitive to environmental changes and can be used as biological indicators of pollution. In the case of air pollution, some species die out or fail to grow when concentrations of harmful gases become too high. By observing the presence or absence of these indicator organisms, scientists and environmentalists can infer the quality of air in a region. This question asks which group of organisms is commonly used as an indicator of air pollution.

Given Data / Assumptions:
- The question focuses on “air pollution indicator.”
- Options include algae, fungi, bacteria, and lichens.
- Lichens are known to be especially sensitive to air pollutants like sulphur dioxide.
- We assume standard environmental science knowledge used in school and competitive exams.

Concept / Approach:
Lichens are symbiotic associations between fungi and algae (or cyanobacteria) that grow slowly on rocks, tree bark, and other surfaces. They absorb most of their nutrients directly from the air and rainwater. Because they lack protective structures like roots or cuticles, lichens are highly sensitive to airborne pollutants, particularly sulphur dioxide from industrial emissions and vehicle exhausts. When air is clean, many lichen species flourish; when air pollution increases, lichens decrease or disappear. Therefore, the presence or absence of lichens is a good natural indicator of air quality. Algae, fungi, and bacteria include many species that can tolerate polluted conditions and are not specific indicators of air pollution in the way lichens are.

Step-by-Step Solution:
Step 1: Identify that the question is about organisms used specifically as indicators of air pollution. Step 2: Recall that lichens are well known for being sensitive to sulphur dioxide and other air pollutants. Step 3: Recognise that in heavily polluted urban or industrial areas, lichens are often absent from tree trunks and rocks. Step 4: Note that algae, fungi, and bacteria occur in many environments and are not uniquely associated with clean or polluted air. Step 5: Conclude that lichens are the correct choice as air pollution indicators.

Verification / Alternative check:
Environmental science textbooks commonly state that lichens are bioindicators of air quality. They mention that in clean mountain or rural areas, many colourful lichen species can be seen growing on tree bark and rocks, while in polluted cities, lichen diversity is much reduced. Some monitoring programmes classify air quality zones based on the types of lichens present. In contrast, algae are more often used as indicators of water pollution, and bacteria and fungi are associated with a wide range of habitats including polluted ones. This consistent emphasis supports choosing lichens as the air pollution indicator.

Why Other Options Are Wrong:
Algae, while useful as indicators of water quality (for example, algal blooms), are not specifically used as standard indicators of air pollution.
Fungi, many of which are decomposers, can survive in a variety of conditions and are not particularly sensitive indicators of air quality.
Bacteria are extremely diverse and include species that thrive even in highly polluted environments, so their presence alone does not indicate clean air.

Common Pitfalls:
Students sometimes confuse indicators of water pollution with indicators of air pollution and may mistakenly choose algae because of their role in eutrophication. To avoid this, remember the word pair “lichen = air” and “algae = water.” Lichens are literally living air quality meters on tree trunks and rocks; their absence in cities is a warning sign of polluted air. Keeping this pairing in mind will help you correctly answer indicator based questions in environmental science.

Final Answer:
The organism widely used as an indicator of air pollution is Lichens, symbiotic associations of fungi and algae on rocks and tree bark.